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Micronta Radio Shack SI-100 Signal Injector, 1978
Not so long ago, when your radio, amplifier, tape recorder or record player, in fact almost anything with a loudspeaker or headphone socket, went on the Fritz you simply took it to your local radio or TV repair shop to get it fixed. Nowadays you chuck it away, give it to a charity shop or flog it at a car boot sale or on ebay. (And don’t forget to say it's probably okay but you don’t know if it works or not because you haven’t got the right batteries, leads, tapes, records etc. to test it…).
The sad fact is the item in question can probably be fixed. In many cases a competent service engineer, using a very basic test instrument like this, can quickly locate the general area of the fault, if not the actual faulty part. It’s a signal injector and the one featured here is the Micronta SI-100, sold by Radio Shack (aka Tandy in the UK) in the late 70s
Signal injectors are not complicated but feel free to skip this paragraph if you glaze over at the mention of teccy stuff. Essentially it’s a simple oscillator circuit, specifically a blocking oscillator that produces a range of audio and radio frequency signals. This particular one uses a single transistor plus a few other common components, including a small coil, a couple of capacitors, some resistors and a diode, mostly to stop it getting zapped if it accidentally comes into contact with very high voltages. In addition to the main or fundamental frequency (around 300kHz) it generates lots of harmonics and sub-harmonics that include low frequency signals in audible range (20Hz – 20kHz), up to around 30MHz and beyond, which is into the Short Wave and VHF/FM radio bands. That broad spectrum of frequencies means that it can be used to inject signals into a wide variety of devices, like radios, which have circuit elements that operate over narrow bands of frequencies.
That's enough of the techno-guff, all you really need to know is signal injectors are used to determine if there’s any life in an electronic device, by prodding strategic points on the circuit board with the probe and listening for a tone from the loudspeaker. If a tone is heard it should be possible to backtrack to the part of the circuit where it disappears, which is the place to start looking for the fault. Incidentally another simple diagnostic tool, called a Signal Tracer, is often used in conjunction with an injector. This works the other way around and a small amplifier, built into a hand-held probe, and connected to an earpiece, is used to find the point where the signal cuts out.
Either way these handy little widgets can significantly reduce the amount of time and effort it takes to track down straightforward faults in analogue audio and radio devices. They also helped to make a generation half-assed service engineers look like they knew what they were doing…
Two AA cells power the Micronta Signal Injector and because they are used intermittently they can last for months if not years. It is very well made and with virtually nothing to go wrong (but it’s easy to fix if it does). To use it all you have to do is connect the crocodile clip with the black wire (normally stowed in a small compartment next to the probe) to the faulty item’s chassis or negative power connection; touch the probe on a known point in the signal path on the circuit board and press the red button on the side. This fires up the oscillator and to let you know it’s working there’s a red LED on the top. In some cases you don’t even need to make a physical connection; the injector puts out weak radio frequency signals and you can often hear a tone just by holding it close to the aerials or antennas on some radios.
I honestly cannot remember how or when I acquired this one, or even if paid for it – over the years I have acquired most of my test gear by accident, as swapsies or impulse purchases -- but it has been in my possession for at least 25 years ago and in semi regular use ever since. In all that time it has never let me down, though the croc clip had to be reconnected a couple of times. It looks almost as good as new and I am confident that it will continue to work, and prove useful, for as long as I keep it fed with AA batteries, and there are dead radios, tape recorders and amplifiers that need fixing.
What Happened To It?
The SI-100 made its first appearance in Radio Shack’s 1978 US catalogue priced at $3.95; it went on sale in the UK’s Tandy subsidiary a year or so later for £2.79. This was the first of a new generation of Korean-made injectors using a silicon transistor, LED indicator and a new slim case. This replaced a long series of Japanese-made models, dating back to the 1960s, housed in cylindrical cases. It made its final appearance in 1983, replaced by a more sophisticated logic probe for troubleshooting faults in digital devices, though the new model featured a ‘tone’ output, which could be used for testing analogue audio circuits.
Signal injectors can still be found in many old-school service engineers’ toolboxes, though these days I fear they get little use. Relatively few consumer products can be economically repaired – compared with the cost of a replacement – but in any case the extensive use of digital circuitry and near microscopic surface mounted components (SMCs) make repairs, using conventional workshop tools nigh-on impossible. It’s also worth mentioning that modern electronic devices are inherently more reliable. Even when they do go wrong, even if it’s only after a year or two, there’s usually a new model on the market with a few more pointless bells and whistles, slightly different cosmetics, updated software or simply a lower price that makes an older product instantly less desirable
I cannot believe there’s much of a collectors market for old signal injectors like this one though I did come across a SI-100, sold on ebay in the US, for what looks like a ridiculous amount (£50). I put that down to it being in as-new condition, in its original unopened packaging and like as not, bought by someone collecting vintage Radio Shack products. Even so it is still a practical test instrument, and genuinely useful to electronics enthusiast and repairers or restorers of ancient analogue technology. £10 or so for one in good working order seems like a fair price, though, rather than wait for one to come up on ebay a keen tinkerer can easily put one together for next to nothing using a few simple parts from the spares box and readily available circuits on the web.
First seen: 1978
Original Price: $3.95 (£2.79 UK 1979)
Value Today: £10 (0117)
Features: single transistor blocking oscillator, centre frequency approx 300kHz with AF, IF & RF harmonics, LED power on indicator, steel probe, crocodile clip earth lead with cable stowage
Power req. 2 x 1.5v AA cells
Dimensions: 102 x 41 x 29mm
Made (assembled) in: Korea
Hen's Teeth (10 rarest): 8
Nuclear Enterprises PDM1 Doserate Meter, 1983
Although nuclear or 'ionising' radiation is invisible, has no taste or smell and is beyond the normal range of our senses, it is not too difficult to detect. There are several methods, apart from growing an additional head or zombieism, and the best known is the ubiquitous Geiger Counter. Other method include things like Cloud Chambers and scintillation detectors, which contain exotic crystals that produce brief flashes of light when struck by radioactive particles. However, one of the simplest type of detector is the Ionisation Chamber. You probably have several in your home in the form of 'free air' ionisation chambers. These are the little metal cylinders used in most types of domestic smoke detector. They are open to the atmosphere and inside there's a tiny radioactive source. This emits a stream of particles and if it senses that the flow has been interrupted, by passing clouds of smoke or noxious gasses, an electronic circuit sets off the alarm.
Another type of ionisation chamber can be found inside this Nuclear Enterprises PDM1 Portable Doserate Meter, it’s around 50 times larger than the ones in most smoke detectors, and this time it is sealed and filled with a gas at low pressure. It is designed to detect radiation, rather than smoke, and instruments like this are used throughout the nuclear power industry, in research laboratories, hospital nuclear medicine and radiography departments and for environmental monitoring. If fact you’ll find them wherever there’s a possibility of encountering potentially harmful levels of X-Rays, Gamma rays and Beta particles, which are the main types of radioactivity proven to cause long-term damage to human tissue, with prolonged or uncontrolled exposure.
Inside the PDM1’s ionisation chamber there is a pair of electrodes carrying an electric charge. When a radioactive particle enters the chamber it interacts with or ‘ionises’ gas molecules, releasing electrons that head towards the electrodes – attracted by the electric field – and from there the tiny charges can be measured using relatively straightforward electronic circuits. This type of Ionisation chamber is not especially sensitive and it doesn’t respond to low-level sources or Alpha radiation but it is very good at detecting and accurately measuring radiation in terms of exposure or dosage.
It looks quite complicated but it is actually very easy to use and everything the user needs to know is shown on that large analogue meter. The rotary switch on left is responsible for switching it on, checking the batteries, zeroing the meter and putting it into measuring mode. There’s a small knob in the middle for the meter’s zero adjustment and the switch on the right sets the type of measurement (dose or doserate) and the measuring range.
Broadly speaking dose is an indication of how much radioactivity you are being exposed to at any one time. Nowadays dose is measured in Sieverts but a Sievert is a helluva lot of radioactivity and in practice it is more convenient to express it in terms of microsieverts (uSv). Doserate is a measure of radioactive exposure over time, in micro or milliSieverts per hour (uSv/hr). The Dose ranges on the PDM1 are 30 and 300 uSv, and for Doserate it’s 30 and 300uSv/hr and 3, 30 and 300 mSv/hr. What the readings mean, and when it’s time to run is another matter, but be assured that if ever your job entails using an instrument like this, you’ll know exactly what to do when you see that needle move…
There’s only one other item of interest on the outside and that’s a sliding panel on the base. When open this exposes a sheet of thin metalised plastic film, and behind that is another metal film covering one end of the ionisation chamber. The purpose of the panel is to block Beta particles, which the instrument will detect, but they skew readings of X-Rays and Gamma rays, which is what the device is calibrated to measure.
The PDM1 was made in the UK (Scotland) in the early 1980s by Nuclear Industries, then a division of Thorn EMI. It’s a little larger than a standard house brick and probably cost hundreds of pounds when new. Precisely how much it cost is difficult to say, it’s not the sort of thing you have found in an 80's Argos catalogue… Around a third of the case is taken up by the ionisation chamber, which is mounted underneath the meter. There’s a large compartment, beneath the carry handle, for the batteries; it takes five, four 9 volt PP3s, used to generate the high voltage field for the ionising chamber, and a 9 volt PP9, which drives the electronics. The sealed module beneath the battery compartment is a high-gain, low noise amplifier that’s connected directly to the Ionisation chamber. It is very sensitive and the designers have gone to a lot of trouble to shield it against interference from other electrical and electronic devices, which could cause spurious readings. No expense was spared in its design and construction, so you can take it as read that it’s rugged and capable of withstanding a lot of harsh treatment. The only really fragile components are those thin metalised plastic membranes covering the ionisation chamber and if the inner one is punctured it’s basically kaput.
According to the stallholder at the Surrey antiques fair where I bought this PDM1 (one of a pair of meters he had – the other, a simple PDR1 rate meter will appear here soon) it was part of a lot of instruments sold off by a company involved in the digging of the Channel Tunnel. It sounds quite plausible and there’s no doubt that the extracted materials would have been routinely monitored. However this one appears to have been removed from service quite early on in its career, judging by its unusually clean appearance and a ‘Not to be used sticker’ on the meter. In other circumstances it might have been a risky purchase but since the two meters only cost me £20, and the film covering the ionising chamber seemed to be intact, it wasn’t a huge gamble. The meters alone were worth the asking price. It turned out that the PDM1 had been decommissioned for good reason, someone forgot to change the batteries and there was a rather nasty mess inside the battery compartment. Fortunately the damage was minimal and mostly confined to the battery terminals, which had to be replaced. The corrosive fluid that leaked from the batteries had dried out and cleaned up quite easily. It didn’t even stain the thick layer of powder coat protecting the alloy case and apart from the battery terminals the only other casualty was a foam insert meant to stop the batteries rattling around.
There was a minor problem testing the unit as PP9 batteries are now obsolete. They are still available but very expensive and typically sell online for over £7.00, far too much to spend on what may have turned out to be a doorstop. Luckily they’re really easy to replicate with a cheap 6 x AA cell battery holder costing £1.50. I still wasn’t holding my breath; this particular instrument is well over 30 years old but I needn’t have worried and it fired up first time, responding well to a particularly ‘lively’ travel alarm clock with radium-painted luminous hands and face. The readings may not be that meaningful as by now it is well out of calibration but it definitely detects radioactivity, and long-term readings suggest that it may even be sensitive enough to respond to normal background radiation, and over time, could let you know if there’s an unexpected increase.
What Happened To It?
The roots of Nuclear Enterprises dates back to the 1950s as Netsensors, a company making instruments for the aerospace industry but after a number of takeovers and mergers it was sold to EMI in the mid 1960s, and became part of the Thorn EMI group in the late 70s. By then Nuclear Enterprises was heavily involved in radioactive measurement and instrumentation, and doing quite well by all accounts, but the division was sold off in 1987 in a management buyout. I worked for the consumer side of Thorns in the late 70s and I was aware that the company was having a tough time. By the mid 80s they were selling off a lot of their smaller subsidiaries, so it may have been a bit of a fire sale. Anyway, Nuclear Enterprises continued in the field of nuclear detection and instrumentation and in 2002 it was acquired by the French company FGP Sensors, at which point the NE brand and identity seems to have disappeared from view.
Back to the here and now and technology has moved on. Modern instruments are smaller, more responsive, have many more features and almost certainly cheaper than this old beast so it’s probably outlived its usefulness. Nevertheless, it is entirely possible that there are still a few PDM1s of a similar vintage still in service and provided they’re regularly calibrated and well looked after they can go on for a very long time. Outside of their natural homes, in the lab or in the field, they’re not a lot of use to the average citizen and the enthusiast and collector’s market is quite small, so don’t expect to turn a quick profit if you ever find one going cheap at your local car boot sale. On the other hand, if you’re of a cautious disposition, concerned about the next (and probably last, world war…) or living next door to a flaky nuclear power station or weapons facility, it might be worth having a working one tucked away, just in case the balloon goes up. It might be a long wait, though, so don’t forget to remove the batteries…
First seen 1983
Original Price £?
Value Today £10 (1116)
Features Down-pointing ionisation chamber with sliding beta shield, (100sq cm detection area), skin & depth dose/doserate measurement, (30 – 300 uSv, 30 – 300uSv/h & 3 – 300mSv/h, 9cm analogue meter display, battery condition indication, set zero adjustment
Power req. 1 x 9v PP9 & 4 x 9v PP3
Dimensions: 245 x 125 x 170mm
Made (assembled) in: UK
Hen's Teeth (10 rarest): 8
Smiths SR/D366 Automotive Instrument Tester, 1965?
These days when something goes wrong with your car’s engine or electrics one of the first things most mechanics reach for is not a spanner or screwdriver, but an electronic box of tricks called an OBD scanner. OBD or On Board Diagnostics was developed in the early 90s and since then it has become an industry standard, adopted by virtually all major car manufacturers. It uses the vehicle’s computer or engine management system to log and report on faults and changes in performance. On most cars you will find a 16-pin OBD connector mounted close to the steering wheel, under or behind the dashboard.
OBD scanners and code readers are now readily available on the web for under £20 so just about anyone can play at being a car mechanic. Whilst a lot of the information they display will be meaningless to most people, there are plenty of websites that can help to decode and interpret fault codes. OBD scanners can’t necessarily fix faults, but knowledge is power and having advance warning of problems can save motorists a few bob on costly garage bills.
Back in the BODs before OBDs (bad old days, before on-board diagnostics…), fault-finding was largely down to the knowledge and experience of skilled mechanics. However, in the 1960s there were stirrings of what was to come, in the shape of devices like this Smiths SR/D366 Automotive Electrical Instrument Tester. Obviously it is nowhere near as sophisticated as a modern OBD reader and as the name suggests it is mainly designed to test car instruments, but it was part of a growing trend in the automotive industry towards taking the guesswork out of fault detection and diagnosis.
The SR/D366’s basic test functions are for fuel and temperature gauges but as part of that, and before the checks are carried out, it can also be used to assess battery voltage and voltage stabiliser operation. It’s designed to work with the most common types of gauges manufactured by Smiths, and other companies. According to the instruction label inside the lid these include fuel gauges that use ‘Bi-metal’ or ‘Segment’ senders, and temperature gauges with ‘Thermal Segment’, ‘Semiconductor’ and ‘Bi-Metal’ sensors. The instruments under test are identified by code prefixes, and these are selected using the rotary switch on the right side of the top panel. The tester connects to the instrument or wiring using a pair of leads, terminated in crocodile clips. The operation of the instrument is then shown on the small meter, which appears to be based on the distinctive ‘quadrant’ family of gauges made by Smiths and fitted to many British cars built in 60s.
Under the bonnet, as it were, it is really simple and not too dissimilar to a regular multimeter. The circuit consists of a meter connected, via the selector switch, to a bank of 15 precision wire wound resistors. The instruction sheet is very easy to follow with advice on how to tell if the problem lies with the instrument under test, the wiring or the car’s voltage stabiliser. This is a really well made piece of kit, from Smiths Motor Accessory Division’s Service Department, located at 50 Oxgate Lane in London. It is housed in a smart little plywood box that is quite capable of standing up to the sort of rough treatment it would be expected to receive in a typical garage workshop.
A large boot sale in Sussex was where I found this one. The stallholder started the haggling process at £5.00 and moments later a mutually acceptable £3.00 changed hands. It seems like an absolute bargain now, but at the time it was a bit of a gamble as the box and its fittings all looked a bit rough and there was no way of knowing if it worked or not. Luckily getting it into a presentable condition wasn’t too difficult, though it did entail a complete strip down, sanding the case and bringing the tired wood back to life with liberal applications of a good quality wax polish. There was an accumulation of grime on the inside of the meter so this had to be taken apart, which again wasn’t too difficult. The meter uses a bimetal strip instead of a more common moving coil or moving magnet mechanism. When the strip is heated, by a current passing through a small coil wound around it, it flexes and a simple lever arrangement connects it to the needle. The pointer moves very slowly, and is unaffected by vibration, which is why it was the meter technology of choice for instruments like fuel gauges that would otherwise be jumping around every time the vehicle goes around a corner, or over a bump in the road. The meter on this one was in good shape and the needle moved when connected to a bench power supply. There’s not much that can go wrong with a bunch of resistors and a rotary switch so it is safe to assume that it’s still in working order, but until I acquire a British made car from the 1960’s fitted with Smith’s instruments that will have to be taken as read.
By the way, the manufacturing date of 1965 is only a semi-educated guess. There’s nothing about the SR/D366 in the history section of the Smith’s website, or indeed any references to it on the web, other than a tiny handful of archived sales listings on auction sites. The meter, however, is very clearly related to those shown fitted to vehicles in mid 1960’s catalogues and motoring magazines and is it reasonable to suppose that an instrument to test them would have been developed at around the same time.
What Happened To It?
Smiths dates back to the early 1850s and started out as a family run business making watches and clocks. During the early years of the twentieth century Smiths expanded into parts and accessories for the emerging automotive industry; one thing led to another and by the beginning of World War One they were also making aircraft instruments. At the outbreak of Second World War the Motor Accessories Division was split off from the rest of Smiths Industries and in 2000 the parent company merged with the TI Group. The Smiths success story continues and nowadays it has fingers in numerous pies concerned with industrial electronics, interconnections, measurement and instrumentation.
The timeline for the SR/D366 is less clear. It’s not too surprising, though, and given its specialist nature it is likely that relatively few of them were ever made. It may be relevant that this one has the number 143 written in pencil on the inside of the box. It was probably only ever marketed through trade catalogues and magazines but how much it originally cost remains a mystery. I’m not even going to take a guess, but it won’t have been cheap and as usual clarification and corrections are most welcome. I suspect it lasted from my estimated manufacturing date of the mid sixties to the late seventies or thereabouts. By then it would have become largely obsolete thanks to the demise of the UK car industry through fierce competition from overseas companies and the appearance of more flexible and sophisticated test instruments. Doubtless a few units have survived in the hands of collectors and restorers, but they don’t come onto the market very often, which again suggests that it could be quite a rare item. As far as I determine only couple have appeared on ebay in the last few years and they went for £50 and £70; the only other one I have seen mentioned was sold by a specialist auction house for £150, so you know what to do if you ever spot another one in the wild!
First seen 1965?
Original Price £?
Value Today £40 (0816)
Features Battery test, fuel gauge (segment/thermal), temp gauge (Bi-Metal/Semiconductor/Thermal), Voltage Stabiliser
Power req. n/a (powered by vehicle under test)
Dimensions: 182 x 118 x 118mm
Made (assembled) in: UK
Hen's Teeth (10 rarest): 8
Dawe Transistor Stroboflash 1209B, 1967
How often have you wanted to know exactly how fast is something spinning or vibrating, or experienced a sudden desire to indulge in a spot of disco dancing? Good news! Here’s something that can help satisify both urges. It’s a Dawe Instruments Ltd Transistor Stroboflash. It's a stroboscope, aka a strobe’, a box of tricks that emits flashes of light, which on this one, can be varied between around 10 and 180,000 times a minute.
It was originally designed for scientific and industrial measuring and analysis, rather than a disco lighting effect, and even though that is something it does quite well, it is a bit over-qualified for such a mundane task. Once you get past 2500 or so flashes per minute (fpm), it appears like a continuous light, taking all the funkiness out of your dance floor moves, but shine it on something that spins or vibrates really quickly – anything from a motor spindle to a loudspeaker cone -- by carefully adjusting the speed the flashes will appear to make it stand still, and that can be quite useful. For example it can be used to examine the item whilst it is in motion, accurately measure how fast it is moving, and whether or not that movement is steady.
The Dawe Stroboflash can also be coupled to a camera, to take high-speed photographs, or it can be triggered externally but what makes it especially useful, though, is the fact that it is very accurate. When the fine speed control is used to make a moving object appear stationary the flash rate can be read off the large meter built into the top of the unit. Accuracy is maintained by a simple mains frequency calibration feature. The AC mains supply is used as the reference as in most countries the frequency is carefully regulated to within 0.2% of the nominal 50Hz (or 60Hz).
The business end of the Stroboflash is a white light Xenon flash tube, mounted in front of a large parabolic reflector. Some versions use a neon flashtube, which gives out a less intense red light, presumably for applications where that colour is more effective. The controls are very easy to use. The speed control consists of two knobs stacked on top of one another; the large one is for coarse control, the smaller one for fine adjustment. There’s a pair of selector switches, one for the flash and meter range (and calibration) the other is for selecting internal or external triggering.
This particular strobe dates from the mid to late 1960s; it was clearly one of the first models made by Dawes to use all transistor circuitry as this (then) novel feature is proudly emblazoned on the control panel. That date also ties in with the sort of transistors used, which are five ‘OCx’ germanium types (see Gizmo Guide below) made by Mullard. The circuit is very straightforward; the high voltage charge used to ‘fire’ the flash tube is stored on a bank of hefty capacitors, a two transistor oscillator supplies the variable pulses and these are used to trigger a chunky power transistor, which sends bursts of the high voltage charge to the flash tube. In theory there is comparatively little to go wrong, except that over time the kind of electrolytic capacitors used to store high voltages tend to degrade, germanium transistors are notoriously delicate, and flashtubes have a finite life, so it was a complete surprise to discover that this one still worked!
Like most of my best gadget finds this one came from a car boot sale. The muddy Kent field, overcast skies and threat of rain probably contributed to the very reasonable price, which was haggled down from £10 to £7.00. Even so, it was still a bit of a punt, given the previously mentioned problems with 60’s vintage components, and the fact that it couldn’t be tested, even if a mains supply had been available in the middle of that field. The problem was it didn’t come with a mains lead and it uses an old and obsolete 3-pin connector, known to its fans as a Bulgin PX0631. Fortunately this was something I had at home, but before I dared to power it up it was treated to a full strip down and spring clean, plus some basic circuit checks to make sure it wasn’t about to explode. It was in a fairly grubby state but it was mostly surface grime; inside it was just a bit dusty, suggesting that it had been quite well looked after for most of its working life, as befits what would have been an expensive precision instrument. It worked straight away, no fuss or troubling smells but given its age it’s only going to get used for special occasions as many of the parts are now well past their sell by date and a lot of them will be difficult to replace.
What Happened To It?
Dawe Instruments, later to become part of the Lucas group, appear to have been in business since at least the mid 1940s manufacturing a very wide range of industrial measuring equipment and professional photographic flash systems. There’s comparatively little about the company's history on the web and its later years are shrouded in mystery but it’s likely that, like a lot of small and specialist UK firms, it suffered badly from foreign competition. They seem to have shut up shop at some point in the 1980s. What was left of the company passed through a succession of owners but by the late nineties the name and brand had quietly faded away. As always corrections and clarifications are very welcome.
Stroboscopes are still as popular as ever and basic disco-type lighting strobes are cheap and plentiful – prices on ebay start at under £20. However, you can expect to pay upwards of £100 for a modern device with something approaching the spec of the Stroboflash, though properly serious instruments of equivalent quality are going to set you back several hundred pounds. There are usually two or three Vintage Dawe Stroboflashes on ebay at any one time and prices are all over the place. Two non-runners – both in apparently good physical condition -- recently sold for 99 pence and £30, whilst clean, working examples typically sell for between £50 and £100. If you’re handy with a soldering iron and have some basic knowledge of transistor circuitry there are some good opportunities, though you might have to scavenge a couple of basket cases if the repair requires any specialised or obsolete parts.
First seen 1967
Original Price £50?
Value Today £50 (0716)
Features Xenon flashtube, 130mm reflector, continuously variable flash rate 10 – 18000 flashes per minute, moving coil meter display, 50/60Hz mains frequency calibration, external trigger, pulse/syncro output, 5- transistor drive circuit (2 x OC71, OC139, OC202, OC28), carry handle
Power req. 110 - 240VAC
Dimensions: 180 x 210 x 185mm
Made (assembled) in: England
Hen's Teeth (10 rarest): 7
Micronta 22-195A Digital Multimeter, 1986
The ability to accurately measure the three basic units of electricity, namely volts, amps and ohms, has been one of the cornerstones of electrical and electronic engineering for almost 200 years. However, until comparatively recently it was more of an art than a science and relied, to a considerable extent, on the waggling of needles on meter scales, and the eyesight and judgement of whoever was doing the measuring.
Everything changed with the introduction of digital electronics and numerical displays in the late 1960s, which turned guesswork into certainty, and it has to be said, took some the soul out of the task. Most engineers wouldn’t dream of swapping their fancy pocket-size digital multimeters for a venerable old AVO 8, andno one can deny the benefits of digital technology when it comes to speed, accuracy reliability and cost, but it didn’t happen overnight. The first generation of affordably priced digital multimeters started to appear in the late 70s and the mid 80s professionals were starting to take them seriously, with their large and legible displays, improved accuracy and a comprehensive range of measurements that rivalled the best of the traditional analogue instruments.
Second generation digital multimeters, like this Micronta 22-195, dating from 1986, also introduced new features that were previously impossible, or uneconomical to include on analogue meters and the one that got the most attention was auto-ranging. In order to use a traditional multimeter you needed a pretty good idea of what you were about to measure, be it voltage, current or resistance, and more importantly, roughly how much of it there was. This meant you could set the appropriate units and range before you applied the probes. If you got it wrong with the probe leads the wrong way around or you tried to measure a hefty voltage with the meter on a low current or resistance range, for example, at best it was goodbye meter with a bent needle and a puff of smoke. At worst there could be a small explosion and the possibility of a nasty burn or shock for whoever was holding the probes.
Auto-ranging removes a lot of the guesswork and perils involved in making measurements. Some models will do everything for you though this Micronta multimeter is a relatively inexpensive device and the user still has to make some basic choices about what they want to measure, i.e. voltage, current or resistance, but after that it mostly figures out the quantities on its own and where the decimal point goes without any need to twiddle dials or swap probe sockets. To those unfamiliar with the dark arts of multimetering this might not sound very important, or indeed interesting, but take it from me, when you’re trying to measuring a live circuit carrying several hundred volts, with both hands occupied holding the probes (usually in a dark corner, surrounded by lots of juice-carrying wires and components), the last thing you want to do is keep taking your hands out to mess around with range knobs.
The Micronta 22-195 has a number of other refinements and these include simple diode and transistor test functions, an audible continuity tester, a high-low memory function for gauging or comparing changes in a variable input, data hold, which freezes the display and it is well protected against overload and transient voltages and currents on all but the very high current range. Readings are displayed on large 4-digit LCD panel that also has mode, status and battery condition information. There’s a row of function selector buttons to the side of the LCD and the buttons below are used for secondary features, like the memory display, manual range control and the built-in buzzer. On the far right there are three sockets for the red (positive) and black (negative) colour-coded probes. The two lower ones are for everyday use; the one at the top is for high range AC/DC current measurements.
It’s housed in a slim, heavy-duty plastic case with a carry handle that doubles as a tilt stand. Power is supplied by four 1.5-volt C cells that live in a compartment on the underside. Build quality is very good indeed, it’s rugged too, which is just as well as test instruments that spend a lot of time in the field (quite literally in some cases) need to be able to withstand a lot of harsh treatment.
Micronta was one of Radio Shack’s house brands. The once mighty corporation had thousands of stores across the US, and several hundred in the UK under the Tandy name. The 22-195 made its first appearance in the US parent company’s 1987 catalogue, though it was almost certainly on sale from mid 1986 due to long product and catalogue lead times. It was priced at just under 100 dollars, which was a tidy sum for a multimeter (equivalent to around £200 in today’s money). I can’t be sure about it’s UK debut but typically products appeared here at or around the same time as the US.
This one was a chance find at a local car boot sale a couple of years ago and it has been sitting in the dustygizmos to-do box ever since. I can’t remember how much I paid for it but it would have been less than £5.00. Buying vintage battery powered gadgets is always a bit of a gamble, more so with early digital devices. They are virtually unrepairable due to the scarcity of critical parts, like integrated circuits and displays, which were often custom designed. The shabby condition is something else that would have limited how much I was prepared to pay for it but the battery compartment was clean and test instruments are designed to take a bit of rough and tumble, so it’s less of a concern. As it turned out all it needed was a strip down, a thorough spring clean, and with a fresh set of batteries installed it powered up worked straight away.
What Happened To It?
There are two stories here; the first was the sad collapse of Radio Shack, which finally filed for bankruptcy in 2015 but it had been going steadily downhill since the late 90s. Radio Shack, and overseas subsidiaries like Tandy were in almost every shopping mall and high street and they were a haven for gadget nuts. It pioneered many electronic technologies, and those of us of a certain age will be immediately familiar with their home computers (the legendary TRS-80), CB Radios, radio scanners, radio controlled toys, novelty radios, electronic kits and parts. For years Tandy was the go-to place for everything from batteries to cheap hi-fis and TVs, but they fell afoul of rival chains, their prices became uncompetitive, it was slow to respond to changes in home computing, they got left behind in the home entertainment, video and mobile phone booms and once the rot had set in, it was only a matter of time before they went into terminal decline.
The wider story of multimeters is less dramatic and for a long time Radio Shack/Tandy was the only place to go in the high street for decent quality test instruments like the 22-195. At the time, for the price, it was one of the most advanced models available to the general public, but this was a tiny market. By the early 90s Radio Shack had begun to reduce its involvement with the nuts and bolts side of electronics. In every store, usually at the back, there was an Aladdin’s Cave area, stacked to the ceiling with electronic components that drew in enthusiasts, constructors dabblers and DIYers. The shops changed, and the knowledgeable folk behind the counter were replaced by slick salespersons doing their best to shift shiny ready-made gadgets and black boxes. Interest in building and repairing electronics devices was also on the wane and with it went what was left of the market for multimeters. For those that wanted one there were plenty of cheap instruments online, though the vast majority of them were nowhere near as sophisticated as this one but for most casual users, who only wanted the basics, it didn’t matter.
The thing about a good multimeter is that one is all you will ever need, even if you only get it out a couple of times a year and if you are careful it should last a lifetime. Late vintage models, like this one, providing they are in good working order, are no less useful for being old – volts, amps and ohms do not change with the years – so they’re worth having in any case, just don’t expect them to go up much, if at all, in value. Really old instruments, and I’m talking pre WW II can be interesting objects in their own right, and have become very collectable, but they’re strictly for show, and definitely not for testing or any safety-related applications.
First seen 1986
Original Price $99.95 (£65)
Value Today £20.00 (0616)
Features 4-digit LCD display, auto ranging, 300mV – 1000VDC, 3 – 750VAC, DC Current 300mA – 30A DC, AC Current 300mA – 10A, Resistance 3 Ohm – 30 Megohm, continuity, diode check, transistor check (hfe/gain), memory (max-min values), input impedance 10M/volt (AC/DC) 100M on 300mV DC scale, data hold, low battery indication, carry handle/tilt stand, fused protection (not high range AC/DC current)
Power req. 4 x 1.5v C cells
Dimensions: 200 x 125 x 68mm
Made (assembled) in: Taiwan
Hen's Teeth (10 rarest): 7
Advance PP5 Stabilised DC Supply, 1963
Here’s another unsung hero from the early-ish days of transistors. One way or another just about every electrical and electronic gadget you can name has at some point owed its existence to a device just like this.
It’s a stabilised DC power supply, a type PP5, made by Advance Electronics in the early 1960s who, at the time were based in Hainault in Essex (according to the ID plate on the back panel).
Now normally bench power supplies live and work far behind the scenes and to be perfectly honest, they don’t do a lot, other than converting high voltage AC mains into low voltage DC. They lack the glamour and glitz of other varieties of test and bench instruments, like oscilloscopes with their fancy dancing displays, and multimeters with slick digital readouts or giant meters (and you need to be an old-school electronics engineer to appreciate that...) but this one is a bit out of the ordinary.
It is full of character, and a real old-timer; and it is still going strong. The fact that it has lasted this long, in such good condition, says everything you need to know about the build quality, but the real surprise is that it’s a hybrid design, employing what is usually considered to be an unholy mixture of valves and transistors. Semiconductors and valves normally make uncomfortable bedfellows and as a rule they are kept well away from each other. To begin with they operate at vastly different voltages, valves get intentionally hot, transistors work best when cool or at room temperature. Valves do not stand up well to physical punishment and early transistors would blow if you so much as looked at them, so all in all it’s not a very promising sounding partnership. Except, that in this case it has clearly worked and continues to do so more than half a century after it was made.
The feature list is fairly brief, it has a variable DC output, controlled by the knob on the right, from 0 to 15 volts DC, and because it is stabilised that means that it shouldn’t deviate from the set voltage by more than a fraction of a volt, and there should be little or no evidence of AC mains, in the form of ‘ripple’ on the output. The supplied voltage is shown on the central meter, and at the flick of a switch (the one between the two knobs) it displays how much current whatever is it connected to, is drawing. This sort of thing is well worth knowing, particularly if the device or product is designed to run off batteries, and showing how many amps (or milliamps in this case) a device is actually drawing, compared to what it is supposed to be, is a useful fault finding aid. There’s another handy current-related feature and that’s a variable cutout, controlled by the knob on the left. Basically this lets the user set a current limit, between 0.05 to 0.5 amps, and if this is exceeded the supply is cut, hopefully protecting both the power supply and the circuit or device it is powering, from going up in smoke. Other items of interest on the front panel include the on/off toggle switch, a red neon power-on indicator, and two terminal posts, coloured red (for positive) and black (for negative) output cables. There’s not much to see around the back, just a round socket for the mains cable and on the underside there’s a folding tilt stand, so the meter can be read more easily when it is below eye-level, on the bench.
Construction quality is simply outstanding; the case feels like a solid chunk of metal, with the sides acting as cooling fins for the transformer and valves, which can get quite hot. The circuit board and all of the ancillary components are hand assembled and as you might just be able to see from the photos, all neatly arranged with connecting cables bundled together in miniature looms. All of the seriously hefty metal parts are bolted together and it looks as feels like it could go on for another 50 years. Topping it all off is a tough leather folding carrying handle. To say they don’t make them like this anymore would be a serious understatement!
The date of manufacture is a bit of a guesstimate but there are a few clues. The germanium OC30 and OC44 transistors it uses date from the early 1960s and according to the company website (after several changes of ownership they're now known as APS or Advance Product Services), Advance outgrew their small Hainault factory in late 1960s, by which time the reliability and performance of transistors had improved to the point where it would be highly unlikely that they would still be using valves.
I have owned and used this PSU for the best part of 25 years; a retired engineer friend gave it to me. I sensed at the time that it was a real wrench for him to be parted from it, so much so that I had to promise never to sell or scrap it, otherwise he would demand it back so he could be buried with it when he popped his clogs. He's gone and now it’s now going to have to be prised from my cold dead hands...
What Happened To It?
Bench power supplies have never gone away and some modern ones are smothered in knobs, buttons and displays but essentially they do the same job as the PP5. To be fair the output voltage of recent top-end bench supplies are even more stable, and generally ‘cleaner’ than this one (or at least they should be), but they are, for the most part, fairly dull grey boxes lacking the character and classic 60s styling of the PP5. What's more few, if any of them will still be earning their keep in 50 years time.
It has to be said that bench power supplies do not stand a snowball’s chance in hell of ever becoming collectable, or achieving more than their scrap value, especially if they no longer work (I reckon this one is worth about least £10, but only because it still works). They are quite rare though, because not many were made, but no-one in their right minds could ever be excited by them, however, it would be a real pity if quirky old ones like this vanished from sight or memory and they do deserve at least a little recognition for the important part they’ve played in the development of electronic technology.
First seen 1963
Original Price £?
Value Today £10 (0715)
Features Mains powered stabilised bench power supply (0 – 15 volts DC), presettable current cut-out (0.05 – 0.5 amps), meter display switchable volts/amps, neon indicator, screw/banana terminal output, leather carry handle, valve-transistor hybrid circuitry (1 x GD83M, 1 x 15OC4, 5 x OC44, 1 x OC30, 1 x OC35), folding tilt stand.
Power req. 240V AC
Dimensions: 160 x 143 x 160mm
Made (assembled) in: England
Hen's Teeth (10 rarest): 8
Simpson Model 389 Ohmmeter, 1955
Apart from a handful of arguably mad collectors of vintage test instruments – and I am pleased to be one of them – most normal people probably wouldn’t give this rather uninteresting black box more than a second glance. Those who do may spot a somewhat bizarre warning message embossed in red capital letters on the front panel: ‘NOT TO BE USED FOR TESTING EXPLOSIVES’. That’s clearly good advice, in fact testing explosives by any means has to be a fairly risky business, but the fact that this instrument makes the point so prominently suggests that it might be the sort of thing that it could, but definitely shouldn’t, be used for.
It is, in fact, an ohmmeter, Model 389 to be precise, manufactured by the Simpson Electric Company of Chicago, possibly some time in the 50s or early 60s. It does just one thing and that is measure electrical resistance, and in that respect it is not especially rare or unusual and one of hundreds of similar test instruments on the market at the time; but that warning message makes it a bit different. This turns out to be a surprisingly obscure model and confusingly Simpson uses the same 389 model number on a range of temperature measuring meters. I have been unable to find any reference to it in the company’s extensive archives, and never seen one before, or on ebay, which suggests that it may have been a special design, and the odd reference to explosives might indicate it was made for the military, rather than the general population of electrical engineers.
In case you are wondering about that warning message, it has to do with the fact that many types of explosives are set off using electric detonators. Basically these are small devices that go bang when connected by a (long) cable to a battery or hand-operated generator, and this initiates the main explosive charge. The implication therefore, is that following a failed detonation it would be tempting to test the electrical circuit using an instrument like this. However, somewhere down the line this was found, or judged to be inadvisable, as it could accidentally fire the detonator.
And so to the Model 389’s more conventional applications. It has two measuring ranges, 0 – 2,000 ohms and 0 – 200,000 ohms and this is determined by connecting a test lead to one of the screw terminals ether side of the central ‘Common’ terminal. A small voltage passes between the two test leads, which are used to complete a circuit, with the resistance of said circuit – i.e. whatever the leads are connected to – displayed on the meter. There are no controls as such but there are a couple of adjustments. The first one is a set-screw built into the meter, for setting what would normally be the zero point, though in this case the meter reads backwards, as it were, with maximum resistance, shown on a logarithmically calibrated scale, on the left. The other adjustment, on the lower end of the case, is the actual ‘zero set’. The procedure is to short circuit the Common and one of the two range terminals, effectively creating a zero resistance between the two. The screw, connected to a small potentiometer, it turned until the meter reads zero on the right side of the scale. This adjustment has to be made periodically, to compensate for a failing battery, or when a new battery is installed. For the record it is powered by a single 1.5 volt D cell and with normal use this lasts many months. This also means that there is a danger that the battery will be forgotten and eventually leak; fortunately this one had been well looked after.
The case is made from a tough Bakelite-type material and the back panel, which covers the battery compartment, is held in place by six small screws. There’s not a lot to see inside, just a few wires and a couple of resistors, all well protected by waxy insulation. In common with most good quality test instruments it is designed to take the knocks and bumps and this one is especially rugged, quite possibly to a military-spec. This assumes – quite rightly -- that it will be subject to some pretty challenging conditions and rough treatment. Suffice it to say that after what looks like a fairly arduous life, this one still works perfectly.
What Happened To It?
Simpson Electric, now based in Florida, are still going strong and continue to be a leading light in the test instrument business. They still make ohmmeters, though these days the majority of theirs, and those of most other manufacturers, now use digital technology for measurement and display. This little meter came my way via a box of electrical and electronic bits and bobs given to me by a friend. It had been gathering dust in his father’s garage for some years, and they were having a clearout. How and where it came into his possession no one can recall, so its exact origins are unknown.
Meters like this turn up fairly regularly in bric-a-brac shops, antique markets and car boot sales and generally attract little attention. Needless to say the collector’s market is quite small and this is reflected in the prices, which are usually quite modest, especially considering the build quality, workmanship, comparative rarity and what they would have originally cost. The £10 value I’ve put on this one is probably optimistic, which is okay, and it means that anyone discerning (daft...) enough to want to start a collection isn’t going to upset the bank manager -- or get rich....
First seen 1955?
Original Price £?
Value Today £10
Features Moving coil meter, dual range 0 – 2000 & 0 – 200,000 ohms, zero set adjustment, screw terminals
Power req. 1 x 1.5v D cell
Dimensions: 125 x 72 x 48mm
Made (assembled) in: USA
Hen's Teeth (10 rarest): 7
GPO 12B/1 Multirange Test Meter, 1965?
For many of us the General Post Office, or GPO conjures up images of chunky black Bakelite telephones and red phone boxes. It was the highly visible and public face of the communications division of the government organisation that became BT in the early 80s. Behind the scenes, though, there was a large army of engineers who tended the ageing and often temperamental equipment and a fair number of them were issued with instruments like this. It’s the 12B Mk 1 multi-range meter, dating from the 1960s and one of a vast array of test meters used by the GPO over the course of its 70-year history. This particular one is a lightly customised version of a model 127A multi meter, made by Taylor Electrical Instruments Limited of Slough. .
It’s the sort of thing that would be used to diagnose routine faults in exchanges, phone lines and customer installations; nothing too exotic, it measures AC and DC voltages up to 1000V, currents of up to 100 milliamps and resistances of between 0 and 20 meg ohms with a fair degree of accuracy. It can also measure decibels, though I have yet to meet anyone who actually did such things with an analogue multimeter... Above all, it was easy to use and built to withstand the rough and tumble of fault-finding in the field. Ruggedness was taken very seriously back then and it came with a really tough leather carry case that looks capable of protecting the meter from a fall from the top of telephone pole, which probably happened quite a lot…
It is housed in a Bakelite case, with the large meter movement taking up around half of the front panel. Below that is a single knob for selecting the AC, DC voltage current and resistance ranges, a switch for AC or DC/Ohms measurement, 5 sockets for probes and optional plug-in shunts (to extend the current measurement range) and a knob for zeroing the meter prior to making resistance measurements. It’s all very straightforward but it did have one minor flaw. One of the sockets, marked S, is used for the aforementioned shunts and is connected directly to the meter coil, which has a rating of just 40 microamps. If a test probe was plugged into this socket by mistake there was a very good chance of wrapping the needle around the end stop and blowing the meter; wise engineers blocked the socket or fitted a blanking plug, to avoid unfortunate accidents.
Apart from that there is little to go wrong, it’s mostly filled with precision resistors and a couple of rectifiers. Resistance measurement requires a source of power and this comes from two batteries that fit into a compartment on the rear panel. They were then and still are fairly specialist types. The BT121 has an output of 15 volts and the other one is a U10, which is a 1.5 volt cell, but close enough in size and shape to a modern AA cell. They are both still available, at a price, and the BT121 can be cobbled together using contemporary button cells, so one way or another it is still very useable.
What Happened To It?
Doubtless one day someone will document the history of GPO test instruments (maybe they already have – if so please let me know) but for the moment the precise dates at which this one came into and went out of service is pure guesswork. The Taylor 127A, on which it is based is almost certainly 1960’s vintage, and like a lot of multi-meters of the period, it would have lasted well into the 70’s and early 80s, though by then increasingly sophisticated instruments were required to maintain the network, which was well into the process of being upgraded from analogue to digital technology.
I came across this one recently at a local car boot sale, priced at a fiver and haggled down to £2.50 as it didn't come with any probes. Sadly that is about all that it is worth; for those that are interested they turn up quite regularly on ebay. According to ballpoint markings on the inside of the case it once belonged to P. Townsend, and if he's still around he may be pleased to know that it still works, and will probably continue to do so for many decades to come, which is more than can be said for many modern multimeters. This sort of thing doesn’t rate very highly as a collectable so its value isn’t likely to rise by very much, but the 12B Mk 1 and its ilk are well worth preserving and would be a great shame if this rarely visited backwater of communications technology were to be forgotten.
First seen 1960
Original Price £?
Value Today £5
Features 20k opv (ohms per volt) DC, 1k opv AC, +/-3/4% fsd (full scale deflection), AC/DC 0 – 1000v 6-ranges, DC current 0-50uA, 0-100mA 5-ranges, resistance 0 – 2k, 0 – 20k & 0 – 20M 3-ranges, decibels (-10 - +55 5 ranges), zero ohms adjust, external shunt socket
Power req. 1 x BT121 (15v) & 1 x U10 (1.5v)
Dimensions: 145 x 95 x 48mm
Made in: England
Hen's Teeth (10 rarest): 5
Mention the name Clive Sinclair and those who know of him probably think of cheap and cheerful home computers or the ill-fated C5 electric vehicle. Maybe if they are of an age, or a gadget nut like me, then miniature televisions, tiny radios, calculators and watches might also come to mind, but I doubt that very many people associate him with test instruments. As it turns out Sir Clive had relatively little to do with this side of his business but it seems that for a while it was a profitable enterprise and almost certainly funded some of his more wacky products and projects.
Throughout the seventies Sinclair Radionics produced a succession of digital instruments including multimeters and frequency meters, starting with the DM1 in 1972. This was quite revolutionary and one of the very first low cost portable digital test meters to come onto the consumer market. This model ran for three years, when it was replaced by the DM2 in 1973. Things really took off in 1977 with the PDM 35, which we’ll be looking at in a moment, as well as the PFM 200 frequency meter and in the following two years there were ever more sophisticated multimeters and even a portable oscilloscope.
The PDM 35 resembles several other products in the Sinclair range and shares the same case as the Oxford calculator, which was also used to house the PFM 200. The specification is unremarkable by current standards but back then digital multimeters were more likely to be found in a lab, than on a home experimenter’s work bench, nor were they as small or as cheap (it sold for £33.00 by mail order) as the PDM 35. It covers a useful, rather than extensive range of measurements, including AC and DC volts, DC current and resistance. The mode is selected by a pair of slide switches on the top panel and the probes plug into a row of sockets along the bottom edge. Readings are shown on a tiny three and a half digit LED display and power comes from a standard 9-volt PP3 type battery.
Operationally there’s very little to say. According to contemporary reports it worked well, though the small display was said to be hard to read at a distance or in bright light, but on the plus side it was small, light and reasonably accurate. It also proved to be fairly robust, unless you dropped it or did something stupid, like trying to measure very high voltages on the resistance or current ranges.
What Happened to it?
Unfortunately the instruments division went down the pan with the rest of Sinclair’s operations in 1979 but it was saved and re-emerged as Thandar Electronics in 1980, later to become Thurlby Thandar Instruments, which has grown into a successful international business.
Although it was advanced for its time, developments in digital test instrument design came thick and fast in the late 70s and 80s. LEDs gave way to larger and easier to read LCDs, and manufacturers in the Far East came out with cheaper and better products, leaving the PDM 35 and its successors looking a little old fashioned. For a few short years, though, this and the other Sinclair test meters sold well and they were produced in quite respectable numbers. A fair few of them seem to have survived as they regularly turn up on ebay, often in good condition and usually still in working order.
I never actually owned a PDM 35, or felt inclined to buy one as I always preferred analogue multimeters but that didn’t put me off seeking one for my collection. This example came from ebay and cost £25.00. That’s about right for one in such good condition and as an added bonus it came with its original box, leads and even a set of instructions. Apart from a loose connection on the battery clip it has performed faultlessly. Test instruments like these are unlikely to ever become mainstream collectibles or worth very much but it would be shame if they drifted into obscurity as they definitely deserve a place in the history of digital technology.
GIZMO GUIDE (manual)
First seen: 1977
Original Price £33.00
Value Today? £20
Features: 3.5 digit resolution, 10M ohm input impedance DC Volts 1mV to 1000V, AC volts 1V to 500V, DC Current 1nA to 200mA, resistance 1 ohm to 20M ohm,
Power req. 1 x 9v PP3
Dimensions: 155 x 74 x 33mm
Made in: England
Hen's Teeth (10 rarest): 4
White Electrical Advance Lecture Ammeter , 1965?
Anyone who has been through the UK’s secondary school system will have sat in front of one of these, or something very much like it in physics lessons. It’s an ammeter, a device for measuring electrical current, but on a grand scale, so even the kids at the back of the class could see it. Not that I can remember many of my contemporaries paying much attention to amps, volts and ohms. I on the other hand was fascinated, thanks to 'Gaffer' Groom, my inspirational physics teacher, who undoubtedly steered me along the path that led me to a lifelong interest in electronics.
But back to my giant meter, which was made by the White Electrical Instrument Co. Ltd. and is known as The Advance Lecture Model. White Electrical used to be based in Amwell Sreet in London and moved to Malvern in Worcestershire in 1966. (see below). I haven't been able to put a precise date on it but this model was in production from the early 1950s and possibly before that, until the mid 1980s, at which point I suspect that many school labs switched to digital equipment, and interest waned in the workings of analogue meter movements.
For me and generations of kids learning about electricity it works on two levels. Firstly as a measuring instrument for electrical experiments, and secondly as very clear demonstration of how a moving coil and moving iron meter movements work. The innards are clear to see, from in front and behind through glass panels and there’s no mistaking a reading given on the huge scale.
It’s a real piece of craftsmanship too, beautifully made of wood and glass, and I’m guessing it’s pretty sturdy, designed to withstand the rigours of the school lab. This one has three large terminals, a red one marked c (probably for common) and two black ones marked 3 and 30, indicating that it can read currents in the range 0-3 and 0 – 30 amps (AC and DC). The only other feature is a small wire arm that twists when you turn a knob on the side of the case. The tip of the arm is bent at right angles and when in position it limits the movement of the needle to just a few degrees, probably to prevent damage to the movement when it is being moved.
What Happened To It?
The digital demon put paid to wonderful display instruments like this one. These days everyone expects clear unambiguous numbers and of course, you can’t argue with a row of bright LEDs or an LCD, but we’ve lot something. Analogue meters can tell you much more about the volts, amps and ohms being measured. The movement of the needle shows changes and trends far more clearly than winking digits and you can see exactly what makes it tick – try explaining the workings of a digital multimeter to a class of 15 year olds.
I have no idea what it is worth, I have seen similar large scale display instruments from time to time on ebay but there’s no consistency in prices or condition and they sell for anything from £10 to £100. This one I found at a car boot sale in mid Sussex. It was in a fairly grubby state and the stallholder had no idea what it was or what it was worth. He said ten pounds, I said how about eight, and the deal was done. It looked a lot worse that actually was and half an hour spent cleaning off the caked grime, and a liberal application of good quality furniture wax and it looks great, oh yes, and it still works.
Update. My grateful thanks to Alex Worswick for filling in the many missing details about this instrument. After leaving the RAF Alex started working for Whites as an apprentice in the 1950s and retired as the Technical MD in 2000, shortly before the company closed in late 2000.
First seen: 1965?
Original Price £30?
Value Today? £50
Features: Large scale display or classroom ammeter, 0 – 3 & 0 – 30 Amps, see through movement, needle lock
Power req. n/a
Dimensions: 420 x 150 x 420mm
Made in: England
Hen's Teeth (10 rarest): 8
At least, I’m fairly sure it’s made by Taylor Instruments, and the date is a bit of a guess as well. There are no manufacturer’s marks or dates anywhere but it looks a lot like barographs made by the company, of that era. In case you’re wondering what a barograph is, it’s an instrument for recording changes in air pressure, and if you’re familiar with aneroid barometers -- the sort you hang on the wall, with a round dial, marked ‘stormy’ and ‘change’ – then you may recognise part of the mechanism inside the case. The stack of round cylinders on the right is a type of bellows, all the air has been sucked out so when the atmospheric air pressure changes they expand or contract.
The top of the bellows is connected by a lever mechanism to a simple pen nib and ink reservoir, on the end of the arm, and this draws a continuous line on a strip of chart paper, attached to a drum that rotates once a week. The drum, in this case, is driven by a battery powered clock movement in the base, which makes it a bit unusual. The drums on most ‘classic’ barographs are usually driven by clockwork movements, which is probably why ones like this are eschewed by serious collectors, and can still be found relatively cheaply.
It’s small size and the fairly plain, functional design and Perspex dust cover makes me think this particular model was made for schools and libraries and so on, rather than for ornamental, scientific or domestic use, even so it’s a really interesting object to have around, and functional too. The pen trace gives you a real-time indication of the weather, as it is now, and as it has been, and with a little practice you can spot trends and take a fairly good guess at what the weather will be.
What Happened to it?
Barographs of this type are still being made and good ones cost hundreds, if not thousands of pounds, and antique ones – especially fancy models from top name makers – cost a small fortune. Nowadays, though, anyone seriously interested in recording air pressure will use an electronic instrument or one kind or another.
Barographs can be quite expensive to run. Blank recording charts are quite difficult to find, and when you do they can be silly prices, so I make my own. I found one the right size and scanned it, erased the ink trace with PaintShop Pro and print them out as needed. To prevent the ink soaking in I spray them with a fixative spray. Special barograph ink is also very expensive, so I make my own. Ordinary ‘Quink’ type pen ink dries out in a few days, so to stop that happening just mix it with glycerine, a 50/50 mix works just fine.
I bought this one from good old ebay a few years ago for £30, the only trouble was the seller was Canadian, so it cost me another £30 to have it shipped over (and careful packaging is essential) but it was money well spent and I have seen them selling for two or three times as much, through the occasional bargain still slips through, especially when the seller doesn’t know what it is and it ends up in the wrong category, or can’t spell the word barograph….
First seen: 1930
Original Price £?
Value Today? £100
7-day movement, continuous barometric
pressure recording, Perspex dust cover
Dimensions: 198 x 111 x 135mm
Made in: USA
Hen’s Teeth (10 rarest): 4
AVO Model 8 Multimeter 1965
In my opinion you are now looking at one of the finest electrical test and measuring instruments ever built and until a few years ago, if you ever needed to have a piece of electronic equipment repaired there’s a very fair chance an AVO meter, and quite probably a Model 8, had something to do with it.
By current standards the AVO 8 is fairly basic; all it does is measure AC and DC voltage and current and electrical resistance. You can buy a pocket test meter in Maplin for under a tenner that does all that, and quite a bit more besides, and probably more accurately -- but I absolutely guarantee it will not be still working in 40 or 50 years time. AVOs even older than that are still in daily use. What an AVO 8 and analogue meters lack in fancy features they more than make up for with the extra information they provide about the circuits they are being used to test. It takes a while to learn and understand the behaviour or a wiggling moving coil meter but it’ll tell you more than a bunch of digits ever will. However, what really sets the AVO 8 apart from almost every other test meter is its rugged construction. In short it’s built like a brick outhouse and can take a ridiculous amount of physical punishment, and if you do abuse it electrically the fast mechanical cut-out usually saves the day.
There’s not really much to say about the technology, it’s simple and it works, the only points of interest to those unfamiliar with mechanical test meters are things like the curved mirror on the meter scale. This is used to improve accuracy; it’s elegantly simple, when reading the dial you position your eyes so that you can’t see the reflection of the needle, at which point you know you are looking directly down on the scale. One less welcome feature is the really unusual 15 volt battery it uses to power the resistance measurement circuit. Fortunately they last for ages, and they are still available, though it’s a constant source of worry that one day they’ll stop making them
What Happened to it?
The Model 8 was introduced in the early 1950s and this one, one of two that I own, is a fairly early example because it has a (notoriously inaccurate) decibel range. Legend has it that it was designed to meet military specifications but the manufactures decided it was so good it was developed for the civilian market. As a matter of interest the Model 8 is still being made and costs around £600, though AVO has long since moved on to more hi-tech products and sadly most of it’s model range is now manufactured overseas. I can’t honestly remember what I paid for my two AVO 8s, one I’ve had for at least 25 years, the other I found at a car boot sale ten years ago so it probably only cost £5 or so. They are not especially collectible so you can expect to find bargains but for a generation of old hands that grew up with them they are still very useable test instruments and their worth goes way beyond mere monetary consideration.
First seen: 1951
Original Price £30
Value Today? £10
28 ranges: DC Current 50uA - 10A, DC Voltage 2.5 –2500 volts, AC
Current 100mA - 10A, 0 – 20M Ohms, insulation resistant ace up to 200M (with
external 150 volt supply) sensitivity 20,000 Ohms/volt, 1% accuracy
Dimensions: 195 x 170 x 115mm
Made in: UK
Hen’s Teeth (10 rarest): 6
Anyone who has worked in the electrical or electronics industry will be familiar with the name AVO. British made AVO (short for Amps, Volts and Ohms) test meters and instruments are justly famous for their accuracy, reliability and above all, the kind of rugged construction that means they can take a great deal of punishment. The design of AVO meters changes little over the years and the classic Model 8 analogue multimeter, which I was using back in the 1970s is still being made, and nowadays costs the thick end of £600!
The Multiminor was designed for portability and use in the field or up ladders so it’s relatively small and light, and very easy to use. There’s only two controls, the large range/mode switch and the small ohms ‘zero’ preset, which you twiddle to compensate for the aging effect of the single AA battery, used to measure resistance. There’s also a meter zero adjustment, though this would normally only be set if the meter had suffered a severe shock, or set to the wrong range, and the needle had wrapped it self against the end-stop…
This model range has also been around for a long time and I have found references to Multiminors dating back to the 1930s. This particular one is almost certainly from the mid to late 1960s, judging by the materials and the design of the leather carry case. The top panel and switch are all made from black Bakelite and the lower part is a hammer-finished steel pressing; earlier models were all Bakelite.
The leads are not original, and like most well-used AVOs they are probably the third or fourth set, earlier ones being lost, stolen, destroyed or the insulation burned by a carelessly placed soldering iron.
What Happened to It?
Analogue test meters are now very rare, having been largely replaced by digital instruments, nevertheless, AVOs and their ilk will continue to find favour with engineers, especially those from the old school, who appreciate the extra information they can give, and their inherent reliability. Analogue AVO meters were produced in fairly large numbers, so they’re not especially rare, and they’re virtually indestructible, so you’ll regularly find good examples selling on ebay, often for a fraction of their real worth (or original cost). A good example of a practical and genuinely useful collectible, but probably not much of an investment.
First seen: c1966
Original Price £50 - 150
Value Today? £10
Measuring ranges DC volts: 2.5, 10, 25, 100, 250, 1000; AC volts: 10, 25, 100, 250, 1000 V; Current: 0.1, 1, 10, 100, 1000 mA; Resistance:
x1, x100k ohms
Dimensions: 143 x 92 x 35 mm
Made in: Archcliffe Road, Dover, Kent, England
Hen’s Teeth (10 rarest): 4
TTC C1001 Multimeter, 1971
Back in the 60’s and 70s there was huge amount of dabbling going on. Boys of all ages (and it was very much a boy thing), especially nerdy ones, were into electronics in a big way. For a short while electronic construction kits were all the rage but for the hardcore enthusiast it had to be DIY all the way, which meant learning which end of a soldering iron to hold, and buying your transistors, resistors and capacitors from the dozens of companies that sprung up all over the place.
There was a healthy magazine market too, with titles like Practical Wireless, Radio Constructor, Practical Electronics. Elektor and Electronics Today International (ETI, who gave me my first job in journalism). Each month these magazines published detailed plans for impossibly complicated electronic gadgets, most of which never worked, and the must-have accessory was a Multimeter, so you could find out what went wrong with it.
Incidentally, after working for various electronic constructor magazines over the years I can tell you that at least half the things we published never worked and one of my first jobs was to put together the corrections page each month. Also, my sincere apologies for anyone who received shocks from the many dodgy mains-powered projects we occasionally and most unwisely published…
Anyway, this particular multimeter dates back to the early 70’s and was ideal for simple projects, being able to measure AC and DC voltages, small currents and resistance. It was reasonably accurate and a pocket-money alternative to serious multimeters like the magnificent AVO models used by serious teccies. This one is based around a large angled moving coil meter, housed in a sturdy bakelite case and it came with a leather carry case and pair of tests leads.
What happened to it?
Most test meters had gone digital by the late 70’s and very accurate they were too, giving precise readings of volts, ohms, amps and much more besides to several decimal places. However, call me an old stick in the mud but I still prefer to watch a flickering needle. I genuinely believe it tells you more about what’s happening in an electrical or electronic circuit than a set of digits. Changes in current or voltage, for example, are much easier to see when represented by a moving needle. It’s also easier to judge the performance and condition of a capacitor by measuring its resistance, and watching the charge quickly rise and slowly fall. Most moving coil multimeters of this era were built like brick outhouses and they didn’t reven need a battery for measuring volts and amps (the battery was used for checking resistance).
Old test meters pop up now and again in junk markets and car boot sales. However, it is unlikely that cheap little ones like this will ever become seriously collectable but big old AVO meters are definitely worth having; they are superbly well built and to anyone who has used one, a thing of beauty and precision.
First seen: 1971
Original Price £8.95
Value Today? £2 - £5
Features: Moving coil meter, DC volts 5 – 500/2.5k, AC volts 10 – 1000, DC current 0-5uA/0-250mA, DC Resistance 0-infinity 2 x ranges
Power req. 1 x AA
Dimensions: 115 x 85 x 28 (very approx)
Made in: Japan
Rarity: 6 (1 = common, 10 = Hen's teeth)
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