Octave Cat SRM VCO scaling
2018-04-18 by concken@...
I need some help repairing an Octave Cat SRM. I have one remaining problem but I think I've got the rest figured out..
First, the keyboard would trigger the ENVs but would not change pitch. Traced this to the "From Master" jack.. normalled jack was open. Then I found that VCO2 had a very narrow audible range. Replaced the tempco R150, although the tempcos for both VCO1 and VCO2 are labeled R150 in the schematic so who knows.. replaced the VCO1 tempco too just for good measure. Suboctave didn't work on VCO1 but replaced the 4013 and now both VCOs have suboctave.
After both VCOs were oscillating I tried to calibrate things (following the procedure) and I'm not able to get the VCOs to scale properly. In the middle of their range they're both somewhere around 2 volts/octave, you have to span about 2 octaves on the keys to get the pitch to go up one octave. This gets worse as you go up in pitch and better (closer to 1v/oct) as you go down.
The CV out from the keyboard circuit measures good and the trim pots seem to be ok. Since the one tempco was bad I eyed the rest of them carefully and desoldered one of the legs of the one in the poly circuit to check it. It measured pretty close, but I found that with it out of the circuit I could get VCO2 to track perfectly. With it back in I get the same behavior, neither VCO will track. Because of this I suspect something in VCO1 or the poly circuit for VCO1, but I'm stuck here...
I've swapped around the ICs that have an equivalent on the board, like the CA3140s, CD4093s, CD4066s, LM307s, NE532s (these were all replaced with LM358s before the synth got to me), and replaced the TL082 and CA3046 with ones I had on hand. None of this had any effect on anything.
Any hints or pointers would be appreciated.
Cheers!
Christian