My Moog Liberation’s keyboard will not trigger the EGs. The EGs trigger fine from the LFO though. I have printout of the schematic, but the print is so small that I’m having trouble making sense of the circuit.
U14A is feeding a constant current through the 100-ohm resistors in the keyboard, and the scale trim is set for 1V/Octave. When I depress a key, I can see voltage corresponding to the depressed note appear at pin 5 of U24B.
After that, I’m not sure what to look for. Any help explaining this to me? It looks like some sort of sample-hold circuit but I’m not certain I understand what is going on.
Thanks in advance.
The Liberation gets its trigger from the “trigger extractor” circuit on the control board.
I first learned this stuff at ARP school where the signal that stays true as long as any key is depressed is called GATE, and TRIGGER is a pulse that occurs whenever any key is depressed regardless of any keys already gated or not. In this case U4 looks for the presence of audio at the Poly Out and sets V-Trig true (+8 volts) as long as audio is present. Check for a bad U4 on the control board, or a problem with the Poly Out signal. Keep in mind how easy it is to get fooled. It is always wise to achieve some understanding of the circuit purpose and function. Not likely, but there could be a shorted C1 or open R11 or a bad connection due to solder or pcb defect. Knowing what the circuit should do when operating correctly is frequently advantageous when looking for the problem.
Thanks! I’ll check U4.
The service manual copy I have is missing the control board schematic. I only have the keyboard board and the power supply schematics.
Do you have a PDF of the full service manual or know where I can find one?
Sorry, no, I do not. Kevin has most every other Moog doc at Synthfool, but I just looked and the Liberation is not included on his list. That “manual” is just two pages - each one folds out about 4 times, but not 8-1/2" OC. No easy way to do a good job of scanning it.
Problem solved. There was a hairline crack on a PCB trace.
The answer, when found, is obvious.
- Kent Rayman, The Oregon Bass Company
My old fishin’ buddy used to say that on such occasions. Anybody heard from Kent lately?
So, the problem was in the trigger extractor circuit then?