TOA ER-58 Megaphone Microphone

How about this?

When I started with Telecom back in 1980 the standard rotary dial phone came in many colours ranging from corpse-ivory to puke-green. They had a carbon mic in the handset. We replaced the mic inserts on occasion when they became noisy or insensitive. A few years later the phones came new out of the box with a not-carbon mic of the same form factor and with no apparent mods to the rest of the phone circuit. Some sort of electret mic in the same shaped metal insert with a tiny PCB inside as well. Maybe on the same principal as the schematic in the PDF. Worth a try perhaps, considering that the schematic shows three very common transistors and twenty or so resistors, caps and diodes that could be mocked up on a piece of strip-board to see if the idea works. If it does, that's when you gut the handset and fabricate a final PCB with a small electret capsule that you can squeeze into the available space.

Thanks, somehow I didn't think to search "carbon microphone replacement" and all my other searches kept bringing back pages about carbon fibre and carbon monoxide alarms.

I've got all the parts already, so I'll be sure to give it a go.

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I got a million hits for carbon water filters on Ebay. The circuit isn't very mysterious and even I can understand it. If it works, be sure to get back to the group and tell us how it went. With boat-anchor WWII transceivers still in use by a few historically minded hams thirty forty and fifty years after the war, I'd guess there have been many schemes to substitute for carbon mics.

Carbon microphones have significant power gain, in old style telephones the only amplifier in the whole device was the carbon microphone.

rest?

A: another carbon mic.

B: an electret mic module and lots of electronic gain.

you've got 4 wires to the handset, you could perhaps install a class D amplifier module and an electret in there...

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Yes, I know that carbon mics work as amplifiers, but this thing proclaims itself to be a "transistor" megaphone.

I decided to have a look inside for curiosity, and to consider what changes might be needed to the suggested substitute circuit using an electret mic.

More electronics than I was expecting actually. Three transformers and at least five transistors (two power types appear to be mounted on the under side of the "chassis"). I haven't tried to take it fully out of the case because I suspect this will cause some of the wires to break. As it is one was loose, but I think I connected it back to where it's supposed to go and it's now working the same as before, so it probably broke because the speaker wires caught it when I opened the case up.

The circuit around the mic (as far as I can see it) seems to be:

6VDC______[MIC]________ | ? |~| \ | ||+ | | >|----||----->| | 1KA ? / | | || | | VOLUME Transistor| |_| | |_____6VDC |~| | | | | |_| | ?

That doesn't make much sense actually... The 6VDC connection is the wire that was loose and looked to connect to the volume pot. . However without it the circuit would be: ____________ | | ? |~| | \ | ||+ | | ~~~ >|----||----->| | 1KA MIC ? / | | || | | VOLUME ___ Transistor| |_| | | |____________| |~| | | | | |_| | ?

That doesn't make sense either. At least the push-to-talk button connects GND to the circuit, so there are still 6V and GND connections in the handset for an electret amp. to use.

I couldn't find them on Ebay. Also, are the specs for all carbon mics the same? This is quite a different circuit to an old telephone, so would a telephone mic still suit?

I was planning on using the electret amp circuit linked to before. However now I don't know what to do with the emitter of the output transistor. How did a resistive carbon mic. produce a signal when there wasn't a voltage difference across it? Now I'm wondering if it just looks like a carbon mic...

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I think you've mis-traced the circuit. if headphone level signals only produce feeble output after amplification, you very probably have a carbon mic input.

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I've since checked that wiring a number of times and I'm sure that I'm right with the first circuit that I drew out. I tested it again with a headphone output, this time from a portable music player, and at full volume (on both the player and the megaphone) it's actually pretty deafening, so maybe there was just a bit of resistance at one of the clip lead connections during my last test.

So I've decided to go with what I know works and use a simple electret line-level amplifier circuit in place of the old mic.

But before building that amp, I decided that I could no longer resist pulling apart the old mic to see what it really was.

The result:

The (fixed) centre part of the round mass that protrudes through the card ring, and sits under the blue cone which is sandwiched beween the two havles of the case, is magnetic. Two small wires go from this assembly to the electrical contacts. The lower part is presumably a step-up transformer, I guess the interaction of the magnet and the thin blue cone bending(?) with the air pressure causes some variance in the magnetic field which is picked up by a surrounding coil.

I'm guessing it's not a carbon microphone based on the magnet, and the fairly narrow inner assembly. But I don't know much about any microphones, carbon included. It's hard to tell whether the blue cone touches the magnet or not when it's assembled, it's close enough to be attracted by it (I wonder if there was originally a gap and as the metal weakened, it started touching and so the mic stopped working). I can't see a match with any of the microphone types on the Wikipedia "microphone" page or in a book I've got that describes common types. There's no electrical connection to the blue cone.

Being an electromagnetic device (of some sort) makes more sense of the circuit, the signal voltage apparantly varies above and below Vcc. Though does that mean I'll need a step-up converter for my replacement amplifier? Otherwise the volume pot. which connects to Vcc instead of 1/2 Vcc will distort the waveform. I guess putting a capacitor on mic. amp. output would solve that. Sorry, just thinking aloud at this point...

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I finally got around to this and it worked very well. Through trial and error I settled on a gain of 11 for the electret amp., which results in a maximum peak output voltage of about +/- 1V into the megaphone's amplifier. This is connected via a 10uF bipolar electrolytic cap.

The megaphone is very effective over a good distance, now I just need some people to boss about with it...

My microphone amplifier, should be easy to remove if I ever do find out what the original mic. was and get a replacement:

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