I received a LP from a customer awhile back that said his friend replaced the filter board.I believe it was because of a broken control or erratic control. Afterward, the board did not work properly. Upon receiving it from him I have connected it and after about 20 seconds it begins to generate a tone much like a signal generator. Fairly loud through headphones and volume control has no effect.The sound cannot be turned off. I notice an odor {heat} from R3 and R4 on this board. They are surface mount resistors marked 100, and are fairly large for surface mount so they are designed for some bit of heat. But they are blistering hot. My question is, has anyone here done any work on this unit and had this symptom? I have repaired some MOOG synths before, but this LP has become sort of infamous for control knob issues and debugging. No keys play, but all panel lights work and the display is perfect. I can change settings, and get into the utilities.
I see where MOOG has discontinued making, servicing and supplying parts and info on the LP. These compnents are R3, R4 and sm transistors Q3 and Q4. Before trying to track down a short and some tiny parts, could this be normal heat and a debugging problem? Anyone care to venture an opinion? I have a schematic I downloaded but so far R3 and R4 are nowhere to be found in it’s 50+ pages.
Thank you.
The fact that R3 and R4 don’t show up easily in your schematic could be because they are part of a sub-circuit or a secondary power rail or signal path that is referenced in a different section of the schematic. It’s possible these components are located in a part of the circuit that deals with power regulation or filtering 