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One of the things I need to do to put my Heathkit SB-102 into service is connect a microphone. I've got a suitable crystal mic, with four conductors in the cable -- two for the mic element, and two for the Push To Talk (PTT) switch. The PTT switch is to be connected to short one of the pins in the connector to the shell (chassis ground), which closes the transmit relay(s).

The problem is identifying the unmarked pins in the radio's mic connector.

By examination of the circuit schematic, it appears one pin connects to the first stage voice amplifier; the other connects to the low B+ (300V DC). When I test the pins with a voltmeter to the shell, however, I find around 30V on one, and no potential on the other. I'm not savvy enough on electronics to be confident shorting either one to the chassis to check if it keys the transmitter; further, I don't see how that will do anything on the pin with no voltage.

I also don't have the original assembly manual, only the testing and operation section and schematic -- the manual would have told me which wire was which (I do have an SB-101 manual, this is likely to be the same on both, but I hadn't thought of that possibility until just now).

I'm pretty confident the low B+ isn't completely dead, as the set receives well. How can I determine reliably which pin should get the PTT and which the mic element?

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    $\begingroup$ Have you tried examining the internal wiring and matching it to the schematic (if the connector is visible without too much disassembly)? $\endgroup$ – Kevin Reid AG6YO Aug 26 '19 at 18:47
  • $\begingroup$ Haven't yet had the bottom off the rig. Limited work space and time, and it wasn't clear I needed to do so. So I don't know if I'll be able to see the connector (it's below the PCBs in the front panel, so may not be easily visible from the bottom, either). $\endgroup$ – Zeiss Ikon Aug 26 '19 at 19:09

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