Microwaving carbon granule transmitters

[Mister C]

This may seem like a strange question, but bear with me....

Over the past few months I have accumulated a bag of type 16 transmitters from 700 series phones which I've swapped out due to 'frying'.

I heared recently that someone recommended cooking these in a microwave oven for a short time, and that this seemingly cured the problem.

Although I'm slighty wary of blowing up my microwave by experimenting with this, I may give it a try, but I thought I'd ask on here first to see if anyone has already tried this and has any cooking instructions....?

(BTW - at one time putting anything metal in a microwave was a definite no-no, but the instructions for my Bosch which was bought just over a year ago specifically instruct you to put a metal spoon into a drink before warming it up...)

[richwk1]

Hi there,
I have read a similar thread on this on line sometime ago ( can't remember which site ) but have never tried it myself. As to whether it works i'm sure there will be an explanation as to how it excites the carbon molecules thus rejuvenating them but as to whether i would try in in my microwave, doubtful !!
Someone on here may be able to give you a more definite answer as to if it works and is safe, Anybody !!!
I'm not sure it would do any harm to a dead granular mic but as to your microwave and kitchen Hmmmm!!!!

[Peter.N.]

I would think it just dries them out thus preventing random changes in resistance, I wouldn't think it would do any harm providing you only cook it for a few seconds at a time.

[gpogal]

Has anyone been brave enough to try this yet ? I have a few carbon granule transmitters that would be good to save and recycle

[dseymo1]

Something like a spoon is normally fine in a microwave oven.

Interestingly, accounts of possible hazards vary. My own understanding (based mainly on observation) is as follows, but I'm entirely open to correction:

Any metallic object can potentially act as an antenna, hence a P.D. is developed across the object when placed in the microwave field.

Problems arise when a metallic object forms a closed (or near-closed) loop. A current is induced in the loop, which then heats up rapidly. If there's a gap in the loop, an arc can be drawn across the gap.

A fairly vivid example of this can be seen if an item of crockery with a gilded rim is used - the gilding will often vapourise in places, and arcing will occur in the gaps.

I'm not sure, but would guess that such a short circuited antenna might overload the magnetron and cause damage.

As a carbon mic casing effectively forms just such a closed loop, I'd personally be wary of attempting to microwave one!

[beamcurrent]

If I were going to do this, I would include a glass of water as a dummy load. If there is little or nothing to absorb the power, the microwave oven will be unhappy and what small amount of absorbent material will get very hot very quickly.

Brian

[Dave757]

Originally Posted by beamcurrent

If I were going to do this, I would include a glass of water as a dummy load. If there is little or nothing to absorb the power, the microwave oven will be unhappy and what small amount of absorbent material will get very hot very quickly.

Brian

Hi Brian,

You beat me to it! This is the method I learnt many years ago
for testing the suitability of plastic coil formers for carrying RF.
The glass of water kept the microwave oven happy, and if the
plastic was unsuitable, it would warm up!

Kind regards

Dave

[JHGibson]

I heard that you could revive an old carbon mike by giving it a couple of hard bangs on the bench top.
John.

[Millay_2010]

I got a carbon granular microphone that was a tad fried, and noticed it had a sticker on it saying it had been refilled in 1977. This led me to the idea that these are more sevicable than was thought. so with a pair of pliars i peeled off the metal top and dismantled the mic, I found a lot of the granuales had escpaped into the area between the plastic diagprham and the base, I removed the plastic diagphram and washed this scopped all the carbon back into the base and then reasembled, the microphone was almost as good as new. Although the problem is putting the metal top back onto the plastic case it doesnt look pretty but it holds tight.

[Billy T]

"Back when I were a lad" we used to give them a good band around the circumference to break up any clumping and it seemed to work. Thses were Mics used on marine and land mobile radio telephones so moisure could have been a problem on the marine ones perhaps.

Why not try slow drying out in a low-heat oven? Fast evaporation of moisture by microwave is just as likely to be followed by condensation internally and the problem will come back sooner rather than later.

Cheers

Billy

[russell_w_b]

Might not the problem with carbon microphones be that the little granules, over years of passing (sometimes excessive) current, develop burns, spikey bits and pitting, resulting in a more irregular contact that no amount of drying-out will cure?

[dagskarlsen]

Originally Posted by russell_w_b

Might not the problem with carbon microphones be that the little granules, over years of passing (sometimes excessive) current, develop burns, spikey bits and pitting, resulting in a more irregular contact that no amount of drying-out will cure?

Yes, of course. You may never get it back to "as new", but you may make them better. In the really old times they changed the carbon granulate and maybe the diaphragm during service. Anyway, the real carbon transmitter will always have a little noice, and never give the same sound as a modern electronic replacement capsule.

I use the carbon transmitters on most of my telephones, but electronic on those used daily, people complaines on the carbon sound.

dsk

[Mister C]

Haven't been on the site for a while, apologies for not providing an update...

I eventually tried it, 800w microwave, first 30 seconds on full power (no difference), then 60 seconds on full power (again, no difference). :-(

I didn't fancy trying it for any longer than that...and maybe, as the last couple of posts suggest, the problem is incurable anyway.

Interestingly there are some electronic transmitters doing the rounds now which are built up on old type 16 backs with a modern electronic mic inside and a red (tyep 21-esque) front plate on, the only ones I've seen available are around the £6 mark, which probably reflects the time and materials involved in the refurbishment of each unit.

[Millay_2010]

Originally Posted by Mister C

Interestingly there are some electronic transmitters doing the rounds now which are built up on old type 16 backs with a modern electronic mic inside and a red (tyep 21-esque) front plate on, the only ones I've seen available are around the £6 mark, which probably reflects the time and materials involved in the refurbishment of each unit.

Interesting, i have two different types of 21/A transmitter, The red cased one might fit in a 16 transmitter, the back cased one almost defiantly would. But the effor involved wouldnt justify doing it.

Do you have a photo of one? or a link to where I can buy one to have a look?

[Mister C]

The red cased one might fit in a 16 transmitter, the back cased one almost defiantly would. But the effor involved wouldnt justify doing it.

I think it's more a case of a new electronic microphone being put into the back of a type 16, with a new red plastic front on it.

Where to get them - my research so far has found the following sources, but obvoiusly verify this for yourself.

http://www.antiquetelephones.co.uk/ (£6)

Alternatively, Ray at www.gpospares.co.uk has made some red fronts (I think around £1 but check with him), but you'd need a microphone and an old type 16 to make up the unit yourself.

Telephone lines in Cheltenham quoted me around a fiver for the bare microphone on its own, apparently a mic on a little circuit board. So you'd have to take apart an old type 16 transmitter, solder fly leads from the terminals to the circuit board, secure it somehow (foam maybe), and stick the red front on.

I haven't tried either of the above, but interested to hear from anyone who has (and if anyone has a picture of the 'innards' and an opinion on how easy it is to secure them in a convincing manner).

(BTW I hope it's not against any forum rules to provide details of suppliers - some forums are very touchy about this - just sharing what I know for the greater good rather than advocating any of the above suppliers!)

Alternatively, any other suggestions welcome...presumably the electronic microphone innards of other phones could be salvaged if they are of the same resistance...?

[russell_w_b]

Originally Posted by Mister C

presumably the electronic microphone innards of other phones could be salvaged if they are of the same resistance...?

Not so, I'm afraid... The type 21A - both electret (red) and dynamic (blue, manufactured by AKG) - were designed as a 'drop-in' replacement for type 16 carbon mics. The mic in the sort of telephone in question forms part of the 'loop', and carries the loop current. A self-contained bridge-rectifier and IC contained within the T21 allow direct replacement. More modern instruments use VOGAD ICs and dynamic mics (can't speak for them all, mind) and have different circuitry incorporated into the telephone, so that some of the telephone speech-element circuitry PCB would need to be replicated in the telephone under conversion.

Apart from the pragmatic replacement of a dicky mic by a T21 insert (and the improvements guaranteed by doing so), I don't think it's worth the hassle of stuffing modern electronics into a carbon-mic generation telephone; might as well go out and buy a replica!

[gpogal]

Originally Posted by Mister C

Haven't been on the site for a while, apologies for not providing an update...

I eventually tried it, 800w microwave, first 30 seconds on full power (no difference), then 60 seconds on full power (again, no difference). :-(

I didn't fancy trying it for any longer than that...and maybe, as the last couple of posts suggest, the problem is incurable anyway.

Interestingly there are some electronic transmitters doing the rounds now which are built up on old type 16 backs with a modern electronic mic inside and a red (tyep 21-esque) front plate on, the only ones I've seen available are around the £6 mark, which probably reflects the time and materials involved in the refurbishment of each unit.

Many thanks for the update. Its a shame it did not work as I hate to throw these old transmitters out :-( I have tried every other method I could think of with no joy.