Hey there,
so I have this Model D and its volume envelope starts cycling by itself when I turn down loudness sustain below 9 o’clock.
This is a really weird issue and I can’t find the error.
I replaced all the faulty/old capacitors on the contour generator PCB and most of the semiconductors (every 2N3392, 1 TIS93 (Q19) and 1N4004) also have been replaced (not the germanium diodes though). I also checked the voltages noted on the schematic (also the waveforms/pulses/voltages noted on a later revised schematic).
Holding a key or connecting a shortened plug to the keyboard trigger jack and dropping the loudness sustain below 9 o’clock on the front panel starts the cycling of the volume envelope (not the filter envelope though).
Once the voltage on pin 16 of the PCBs connector drops below 1.2V the envelope starts cycling.
I attached the schematic for the board and I appreciate any ideas.
I ran out of ideas to be honest.
Thanks!
Since the loudness contour envelope’s flip-flop seems to be constantly setting and resetting when the sustain level is set below a certain voltage, I would suggest replacing CR6 first. If that doesn’t fix the issue, then I would replace both Q15 and Q25.
Also, CR8 might be faulty and not maintaining a high enough voltage (to stop the flip-flop from cycling) when a key is held down.
Thanks for your reply.
Since I already replaced the flip-flop pair (Q15/Q25) I’m going for CR8 and/or CR6 next.
Can you recommend a replacement for the IN34A?
Oops, I had missed that you had replaced most of the semiconductors… Sorry.
NTE109 comes to mind as an alternative replacement for those germanium diodes. Not sure it has exactly the same characteristics though…
I think there might still be some parts stores online that carry the 1N34a, as this was a very common diode back in the days.
CR8 was indeed faulty.
Since I was already in there, I replaced all germanium diodes on that board with SD103A (Shottky) diodes, in case they would fail in the near future as well.
(side note: Q12 also was faulty, which I discovered during my troubleshooting)
Everything is working fine now.
Thank you for your help! *thumbsup
