I’m in the middle of fixing a really fun one here. This System 35 thinks it’s a Theremin! As you can see in the video below, 921B oscillator frequency changes based on proximity to the module and cabling. Some 921B’s are more prominent than others. Ironically, the 921 a few spaces over is dead on and maintains rock solid pitch. My supply rails are spot on, and I’ve replaced all of the electrolytics in this thing.
I’ve got a gut feeling that there is some sort of impedance or resistance in the current return path of the oscillators or oscillator controllers, but that’s about it at this point. It’s located in an old building and there could be some issues with ground integrity.
I’ve got a few different theories of why it is doing this and I’m going to work on some solutions this week. What do you guys think? Has anyone here ever seen this kind of a problem before? I’ve never seen anything like it in 20 years I’ve been doing this.
Do the symptoms go away when the CV1 buss is disconnected? The lighted switch for the CV1 buss is lit on both 921A controllers and there is no connection from this buss to the large 921. Might be a bad connection to the rear from the multi pin connector that connects CV1 from a keyboard (951) or ribbon (956) depending on what is connected. Would disconnect the multi pin connector from any controller if this is the problem and diagnose the controllers.
Sorry, but no help if flipping those switches does not make the wobbly CV go away.
The symptoms are still there when the CV1 buss is disconnected. I’m going to try and disconnect the sync pins on the edge cards and see what happens next. This is a really strange problem. Everything appears correct and ground appears to be correct. I must have missed something I guess.
Hello,
I would try to isolate the issue first by removing the 921A and 921B (the most troublesome one) from the cabinet and putting it on your bench with another power supply and connectors.
Based on the schematics, it only requires +12 -6 Gnd and then the summing node, reference ground and the PW input from the 921A.
That should allow you to eliminate the rest of the system and help pinpoint the issue.
