Communications headset tester
This is quite a ways out there for a PIC forum, but at least I'm in the Off-Topic section... so here goes....
I do some work on the side for a company fixing communications headsets. Usually, it's the standard stuff, broken wires at the connector, wiring pulled out, dead microphones, etc. And it's really old technology, but it works.
A couple of months ago, I started to get this different type of headset into the shop that I can't wrap my brain around. It's a military type headset used for ground communications and as far as I can tell, the earpieces are your standard 600ohm speakers, the microphone is a dynamic element (150ohm I think, I can't find the spec's on it). There is also a push-button inline with the cord which is a push-to-talk button. Even that's not difficult.
What's wierd about it is that there is a circuit board inline with this switch case. I can't make out what this board does, amplifier, filter, whatever. It's got 2 transistors (1 2N3906 and the other I think is another 3906 but the numbers are scratched up), 3 capacitors, 4 resistors, and a trimmer pot. I don't think it's an amplifier since that would require an external power source (no batteries inside this case).
About a month ago, I found this tester for these headsets laying in the back corner. Of course it didn't work. I was told by a guy that's been there since WAY back when that basically you plug the headset into it, press the push-to-talk button, blow into the mike and your hear yourself in the speakers. But he also told me that the guy that was there before him built it and he (nor anybody) has any schematic's, specifications, or anything for it. So I opened it up ('cause that what you should do right?).
I found 2 LM386's, a single diode, 2 9V batteries, and a pot. So, knowing what I know, I resoldered the connections, replaced the batteries and tried it out. No good. I drew out the tester schematic and found the LM386's were basically wired in parallel. Well, that doesn't work in my mind, so I switched it over to standard setup for using an LM386 (pin 1 & 8 open, gain=20, + input to the pot wiper, one side of the mike to the pot, 3rd pin of pot to ground, other side of mike to ground, - input to ground, output to headphones, other side of headphones to ground, and +18v to Vd, grnd to ground, etc).
I turned on the tester and got nothing but a squeal in the headphones unless I cranked the pot all the way to one side or the other. And of course, when I blow into the mike, I can't hear myself in the headphones.
At the moment, I'm rewiring it and trying various different configurations for the LM386's, including running them in series for more gain, putting them back in parallel (why I don't know). I'm going to try one side of the mike to one LM386 + input, and the other side of the mike into the the other LM386 - input, with each respective LM386 output drive a side of the earphones. I'll also try configuring a single LM386 for a 200 gain instead of 20 as the datasheets read, and so on and so on.
Question is....does anybody out there have any experience with any of these communications headsets at all, maybe seen something in the past, maybe work on them right now, maybe has a clue what this board inline with the switch might do?
JDG
