TOA ER-58 Megaphone Microphone

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I've been trying to get an old ex-army megaphone going. As is,
you press the talk button on the mic and hear a pop from the
speaker, but can scream for all your life and not get anything
else out of it.

The microphone (which is suspiciously easy to replace in the
handset) looks to be an old carbon type, but I thought it was
worth trying some dynamic microphones and an electret one
hooked up instead. None produced any sound at the speaker.

Today I hooked it up via a 2K2 resistor to my laptop's Line-Out
port (in place of the microphone) and played music through it
without any severe distortion. It wasn't very loud with the
volume at max., but it scaled well according to the laptop's
volume setting, so I expect it just wants more voltage/current
from the "mic".

Up to this point I assumed that it was from the 60s or 70s, but
then I found this page:
which reveals that the model is actually from 1957, so a carbon
microphone is quite likely (and it's quite impressive that the
rest of the electronics are still OK).

###The Questions###

What options are there for fixing/replacing/substituting-for
a carbon microphone in something like this?

There's no part number that I can find on the mic. itself.
is it a common type?

It runs on 6VDC (4x D cells) and output power is 8W. Also,
it was made only four years after TOA released the world's
first electric megaphone!

Pictures (I took the opportunity to compare some free image
hosting services, so you can take your pick of server - the
images are the same for each batch):
[other links didn't work]

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Re: TOA ER-58 Megaphone Microphone
On 21/08/2018 6:01 pm, Computer Nerd Kev wrote:
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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  

Re: TOA ER-58 Megaphone Microphone
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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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Re: TOA ER-58 Megaphone Microphone
On 22/08/2018 9:24 am, Computer Nerd Kev wrote:
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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.

Re: TOA ER-58 Megaphone Microphone
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Carbon microphones have significant power gain, in old style
telephones the only amplifier in the whole device was the carbon

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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...


Re: TOA ER-58 Megaphone Microphone
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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:
     ?                |~|
      \  |    ||+     | |
        >|----||----->| | 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.

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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?

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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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Re: TOA ER-58 Megaphone Microphone
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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.


Re: TOA ER-58 Megaphone Microphone
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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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Re: TOA ER-58 Megaphone Microphone
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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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