[sdiy] Jp-8 head scratcher

[Maarten Halmans]

Hello All.

> So you have *only* the KCV input driving the VCO pitch ?
> Even after it leaves auto-tune ?
> And it still has an error, even if you move the voice card to a different
> location ?

Yes I disconnected all modulation sources and the problem still persists! 
Moving the card moves the problem, so this rules out wiring, module 
controller board, interface board and cpu board. I moved the board serveral 
times because the problem I am having points more to these boards than to 
the voice board. I also replaced the tuning multiplexers, cv summing amp, 
transistor array.

> Could there be some kind of HF oscillation coming from somewhere that's
> affecting the tuning ?

I looked with my scope at the cv summing amp, tuning output of the 
oscillator but I don't see any oscillations nor on other places in the 
tuning circuit (sample&holds multiplexers etc).
When I look at the output of the comparator which goes to the counter I see 
no difference between working and bad channels. I also compared the output 
of the cv summing amp of the channels but the slew rate is the same. I stop 
the repair for now because I keep running in circles and keep testing the 
same things. But suggestions are still more than welcome.

Best,
Maarten