Old microphones

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I have a bunch of old microphones, some which may date back to the 1950s. These were used with old reel to reel tape recorders.

While the were cheap, They look sort of neat. I tried connecting one to the mic input of a modern computer. I got no input.

I figured I could google how to test an "unknown microphone with a scope."

Hah! all I got were ebay listings. Tried looking under the phrases "How to" "Troubleshoot" "identify"

All I get are websites that want to sell me something. The "-" exclusions were getting so long. That I have all but given up.

Problem is that most of these are sealed in bakelite cases whit an RCA plug. I can not open it to see if it is carbon, condenser, magnetic or one of the other types.

I am really surprised there is not a tutorial that explains how to use something like a battery, an a scope to measure and determine the parameters of the mic.

Especially these old cheap tape recorder ones.

Any ideas of where to start?

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Hook the scope to the rca and go 'Wooo!' real loud. Should see several 10s of mv. Use the x1 probe setting.

Imagecraft compiler user

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Pretty much any microphone you find from the fifties or sixties will be a moving coil type. Carbon granule mics were noisy, required a large DC component to work (they change their resistance when the sound pressure wave hits them) and were as far as I know used only in telephone handsets - because they were cheap.

The mic input of a computer soundcard should have sufficient gain to deal with most moving coil mics; they've essentially been unchanged in level for decades.

A moving coil mic will have a DC resistance of around (usually) one to two k, and will produce an output in the range microvolts to millivolts; usually easily visible on the 1mV range on a scope as Bob suggests.

An alternate possibility is that they are piezoelectric. These have a much higher DC impedance - hundreds of k or sometimes megohms - and an output of perhaps half a volt but only into a high impedance load.

The third possibility - unlikely until the late seventies or so - is that they are electret mics which will usually have an internal batter compartment or a three-terminal connection. Some use a two terminal connection and require a DC fed through a few hundred k to charge the diaphragm capacitance but to be honest you're unlikely to see those except as either the mic module itself, or as professional mics with phantom power requirements and a balanced output.

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The first thing to check is the DC resistance. It's rather low for a moving coil (dynamic) Microphone and high for a compensator- or piezo- microphone.

The output level of a dynamic microphone can be lower than that of more modern electret microphones, especially if the DC resistance is low (e.g. 600 Ohm). But even then the difference is not that much. So one should still get some response.