I drew out the schematic for the circuit board inside the push-to-talk switch, and while I probably had a few connections wrong, it looked a lot like a few of those example 'dynamic to electret' converter schematics I saw on .epanorama.

AND...somehow I found an old schematic for a headset tester. The paper is all beat up and barely readable, so I could be wrong on a number of points.

It looks to me like power (18v, 2 9v back-to-back) comes in, goes thru a 1K resistor (and a 1K pot for volume control I assume) and feeds one side of the electret mike (which is actually the output from the dynamic mike run thru the converter circuit mentioned above), then across a small cap, connected to the other side of the mike, which is also grounded.

The 1st output of the mike (nearest to power thru the 1K resistor) is fed directly into the + input of the LM386 thru a coupling cap. The output of the '386 back to the headsets thru a coupling cap. Nothing on the gain pins, '386 negative input to ground, bypass open.

Now that's the way I figured it would run. However the actual tester unit that I have, has 2 LM386's in it (and again, it was ruined by another shop, not me). If I had to guess, I would say that the output from the 1st '386 fed the input of the 2nd '386 and back out to the headsets from there.

Shouldn't the negative input of the '386s be connected to the output or is that just when using op-amps, since the '386 has a gain of 20 with nothing connected to 1 & 8. Or am I wrong again....

JDG