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?

  • 1
    $\begingroup$ 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, 2019 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, 2019 at 19:09

1 Answer 1


Referring to the circuit diagram for this set

The mic connector has 2 pins and the grounded body as the 3 connections.

As you have guessed the 2 pins are mic input (measured at 0 volts) and PTT switch (measured 30 volts). Both return to the body of the connector.

The PTT pin is connected to the cathode of the Rx/Tx relay driver (V12B) which is pulled up by the 470k resistor R219 (using the 300V low B+) and should be limited to +15V max by the 15V zener D202.

Shorting this pin to ground will operate the Rx/Tx relays so make sure you have a dummy load connected to the antenna and/or disable the finals to avoid doing any damage. There should only be power at the output if there is some audio input (SSB) or if it's key down on CW, but you never know.

If you have 30V at this point it's probably worth replacing the the zener.
It only has 0.6mA flowing through it (300/470000) when V12B is biased off but might have enough current through it when using the VOX to have changed its value. Measure the current through the relays (30v pin to earth PTT current) to know what the zener current max is, use something capable of much more to avoid it getting hot.

  • $\begingroup$ @rclocher3 thanks for the edit, I couldn't see how to do that so left it as text. Cheers. $\endgroup$
    – PeterH
    Mar 10, 2020 at 1:26
  • $\begingroup$ Not just an answer to the question, but better information on how to (probably) fix the underlying problem. Nice! $\endgroup$
    – Zeiss Ikon
    Mar 10, 2020 at 11:31

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