Julian Bunn wrote:
Pin 26 measures +12V, so looks like it is an output signal, probably
the "SYNC" as you say.

Good catch. I didn't think of trying to measure the voltages when
nothing is hooked up.

The other two, pins 24 and 25, look to be inputs.

I tried grounding pin 24, putting it at -12V and at +12V, with no luck
- nothing changes.

I would advise against connecting it to -12 V unless you have pretty
good documentation that negative voltages are OK on that input. The
digital logic levels in most consumer devices are 0 volts and some
positive voltage; +3.3, +5, and +12 are popular but it can be almost
anything.

Pin 25 - same story. (In the DIN plug this is pin 3, and is shielded
by the braid ground. The wire gauge is also thinner.)

From the turntable schematic, I would expect pin 25 to be at about +9
volts, if the turntable is in the same configuration as it was when the
schematic voltage readings were taken. There should be a note somewhere
that describes the conditions: for the turntable, record loaded/not
loaded, playing/not playing, etc; for the receiver, tuner/turntable/
tape/aux selected, volume muted/low/high, etc. This note might be in
the part of the manual that you don't have, though.

It does seem that pin 24, the "STP" pin, should be the one that
signals the turntable to play or not. But connecting it to any rail
of the PSU doesn't do the trick.

It might be a sequencing issue; maybe STP has to be in a particular
state when the power comes up in order for the turntable to recognize
a change in state of STP later. Or, worse, maybe it expects the
receiver to flick STP off and on once at powerup... something like
that.

It's possible that STP is some kind of serial data output from the
receiver, but the fact that it participates in that diode-OR seems to
argue against that, for me.

If you can get at pin 13 of IC151 in the turntable, or R152, you might
measure that point with a voltmeter to see if it tracks changes in the
state of pin 24 on J106. With pin 24 at 0 V, the IC pin should be
about 7.3 V; with pin 24 at some higher voltage (probably approaching
+12 V), the IC pin should probably go to around 0 to 0.3 V. If this
doesn't happen, IC151 may be screwing up.

Matt Roberds