Oberheim 4 voice

XpanderXT · Joined: Oct 22, 2007 Posts: 137 Location: the flat universe

HI,

I am trying to rebuild a FVS and have one issue I cannot figure out. I have a VCO that is oscillating at about 50kHz. If you touch a scope probe to a number of areas, the freq. comes down to normal range. The pots and CV's are all good and match VCO 1, which works fine. I'd rather not do the shotgun approach and start replacing stuff if someone can think of why it might be doing this.

blue hell · Posted: Fri May 27, 2016 5:20 pm Post subject:

Could it be the probe applying pressure? as in that that there is some bad connection that goes into conduction with applied pressure on the PCB, like a solder joint or a broken trace.

XpanderXT · Joined: Oct 22, 2007 Posts: 137 Location: the flat universe

I have gone over it all with a magnifying headset and pushed and pulled on most things. I have touched up a lot of the solder joints and they look pretty good. This board had a voltage regulation issue so I am wondering if one of the transistors or FETs could be bad. The highest voltage it could have gotten was about 22v but it might have been enough to mess things up.

I think maybe it has to do with the resistance or capacitance of the probe in the circuit that pulls it bad into the audio range but I don't know enough about these types of oscillator circuits to know for sure.

richardc64 · Posted: Fri May 27, 2016 6:42 pm Post subject:

Sounds like the probe & 'scope are supplying a termination that is missing somewhere. What is at the points you are touching that cause the change?

XpanderXT · Joined: Oct 22, 2007 Posts: 137 Location: the flat universe

Turns out that module 3 has the same exact symptoms for both oscillators. Both of these modules had a bad voltage regulator (the main power supply was outputting something in the 22V range which probably took these out).

There is a 1000pF cap that if you probe it, the freq. comes back into range. There is also a pin on the exponential convertor that will do it too.