Widget Of The Week
Mehanotehnika Intercom Telephone 1973
This may not be the first vintage intercom
featured in dustygizmos, far from it, but it is most definitely the
first, and probably the last one to be made in the former Yugoslavia. It’s the Mehanotehnika
Intercom Telephone, dating from 1973. Okay, so it’s a toy, but we
won’t hold that against it. The design goes right back to the first principles of telephony, established in the late 1870s and
-- lest we forget -- ably demonstrates that there was once a time when you
could do quite clever things without bucket-loads of microchips.
The two stations are roughly three-quarter size
replicas of domestic telephones, complete with rotary dials that are used to
‘call’ the other station. Although this appears to be a very basic function,
routing call and two-way audio traffic through a two-core cable is a
surprisingly complicated business involving a fair amount of what we now call
logic, yet it achieves this using just a couple of simple switches and some
ingenious wiring. These days it would all be handled by a microcontroller, at
the very least.
The phones are powered by four 1.5-volt D cells
(two in each phone) and connected together by the previously mentioned two-core
cable, which is around 10 metres long. Each phone also has a red call light and
an internal ringer, or rather a buzzer. To call the other station you have to
lift the receiver and turn the dial at which point the lamp on the other phone
lights up and the buzzer sounds, It’s also possible to set off the lights and
buzzers on both phones by leaving the receiver in its cradle and turn the dial.
To answer the call the person at the other end just has to lift the receiver
and chat away.
Internally the two phones are identical. Two
leaf switches are mounted on top of the battery holder; one is linked to a
plunger or button on the handset cradle and the other is actuated by rotating
the dial. The buzzer – a simple electromagnet with a spring metal interrupter
switch – is on the right side of the base panel and the call lamp is mounted on
the left side (incidentally, this looks a lot like a Christmas tree bulb). On
the rear of the panel there’s a polarised two-pin socket for connector cable.
Inside each handset there’s a magnetic earpiece and a carbon microphone.
According to the stallholder at the Kent car
boot sale where I found them, they started the day with a £10 asking price.
That would have been fair, had it not rained stair rods in the half hour before
I arrived. What started out as a nice little outfit, complete with its original
cardboard box, now looked a very sorry sight indeed. The box had almost
disintegrated in the downpour and in the stallholder’s haste to pack his more
precious wares away, the cable and instructions it allegedly started out with
had vanished. Clearly he’d had enough and the price of everything left floating
in the mud had been reduced to £1.00. Not wishing to add to his misery by
haggling I paid my pound and scooped the soggy mess into a carrier bag. As it
turned out the casualty list was fairly short, with the box being the only part
of the package beyond help.
The two phones appeared to be in good shape and
just needed a wipe over to remove the mud and muck. After fitting a set of
batteries and connecting the two phones together I wasn’t too surprised when
nothing happened. There were several minor faults. The easiest ones to fix were
the contacts on the switches on both phones, which were badly tarnished, but it
was nothing a quick scrub with a glass fibre brush couldn’t remove. The cable
sockets were also a bit manky. Wires going to both earpieces had come adrift,
and were swiftly reattached with my trusty soldering iron. This time the two
phones came to life with the signalling lamps and buzzers all working, but
there was still nothing in the way of voice communications. A few dabs with the
multimeter suggested that the problem lay with the two microphones, and the
likely cause was moisture getting into the capsules, which contain a small
quantity of fine carbon granules.
Carbon mikes are one of the oldest microphone
technologies. The carbon granules, about the size of grains of salt, are
contained within a small metal pot, which acts a contact with the electrically
conductive granules. A second smaller conical-shaped electrical contact, attached
to a thin metal diaphragm, sits in the pot of granules, insulated from the
metal pot and sealed from the atmosphere by a soft rubber gasket. Sound waves
make the diaphragm vibrate, agitating the conical contact, which you’ll
recall is also in contact with the granules. The agitation of the granules
changes the resistance between the two contacts, so a current passing between
them varies in direct proportion to the frequency and intensity of the
vibrations. What could possibly go wrong? Very little, as it happens, except
that damp granules stick together and do not agitate.
Finding a pair of direct replacement
microphones for this vintage toy would be difficult, and probably quite
expensive. Working carbon mikes scrounged from old phones might be persuaded to
fit but I thought I would have a go at fixing them myself. As it turned out it
wasn’t as difficult as I feared. The front and rear parts of the mike capsule
are held together by fold-over metal tabs and once they are separated it is
possible to gently lift the diaphragm assemblies out of the rubber cupped metal
pots containing the fine granules. I emptied the contents of both capsules onto
a clean sheet of paper and placed in the oven (150 degrees C) for around 15
minutes. I would like to say all this had been carefully researched or worked
out but the truth is the time and temperature were just guesswork. Fortunately
it seemed to work and the sticky granules came out of the oven as a fine loose
dust. I carefully poured them back into the containers and the capsules were
It was a partial success. One of the mikes
worked quite well, the other was barely audible. It’s possible that the
division of granules wasn’t quite equal, or the contact surfaces of the duff
mike might be tarnished, but taking them apart again risks breaking the metal
tabs that hold the parts together, so it’s on the to-do list and at least I
know that the phones can be made to work.
What Happened To It?
The manufacturers, Mehanotehnika, were founded
in the early 1950s, and according to the company’s archived history, the name
came from the first toy they made, which happened to be a puzzle. Throughout
the late 50s and sixties numerous other toys and games followed; details are a
tad skimpy but apparently they were produced with ‘superior psychological and
pedagogical content’… Stamps on the bases of both phone show that they were
made in 1973 and it seems to be the part of a move to more sophisticated
mechanical and electrical toys. In 1990 the company changed its name to Mehano
and by this time it had become a highly regarded manufacturer of model trains,
which it continues to make to this day. Bizarrely this is in spite of the
company being declared bankrupt in 2008, following a decade of declining
profits and mounting debts.
Toy telephone style intercoms used to be fairly
common though they tended to fizzle out in the 80s as most kids, and parents,
would have regarded them as rather old fashioned. In any case by that time
there were much more exciting ways for children, and big kids, to communicate.
Walkie talkies had been around since the early 60s and the market was awash
with cheap, mostly character or TV show themed models, aimed at pre-teens. These
were technically illegal but tolerated as they operated at very low power levels
but at around this time the brief craze for Citizen’s Band radio had begun to
take hold. More sophisticate and powerful walkie talkies started filling the
shelves, and eventually a sanitised system was legalised in the UK but the
novelty quickly wore off as rival attractions, like video games and electronic
toys began their apparently unstoppable takeover.
Mehanotehnika/Mehano Intercom Telephones from
the 70s turn up on ebay fairly regularly, though all of the ones I have seen
so far were clean and boxed examples. Most of them were in good working
order and priced at between £30 to £50. This one, in its current partially
working state is probably only going to fetch between £5 and £10, for spares or
repair but this is an area where bargains can still be found and there is clear potential for bargain hunters and collectors.
£5.00 – £50.00 (0418)
intercom/telephone, buzzer and signal lamp, rotary dial, handset plunger,
magnetic earphones & carbon microphones, 10 metre connecting cable.
4 x 1.5 volt D cells
128 x 153 x 95mm
Made (assembled) in: Yugoslavia
Hen's Teeth (10 rarest)