Archive of the former Yahoo!Groups mailing list: Discussion about the Korg PolySix synthesizer

Ok, here we go.

I'm currently debugging KLM367 that modulates (from the MG section) both
the VCO and VCF but not the VCA. This seems to be a somewhat similar
problem; and I'm trying to debug it here in theory first before I test this
theory.

Since in both your and my case the VCO gets modulated we have established
the the actual LFO circuitry is working. The LFO signal simply doesn't
"find" its way to the destinations.

Swapping anything directly INSIDE the LFO circuitry isn't going to help; so
swapping IC13 and IC15 probably wouldn't have helped anyway. I'm suspecting
a problem in the switching matrix but we will work backwards here.

I hope you have an oscilloscope or can borrow one?

First, we'll put the LFO into a "basic" configuration: Freq at 5 to 10,
Delay at 0, Level at 10. This will give us a strong signal. Put the
modulation switch to "VCA" and measure CN03-05 (that's the third pin from
the bottom for the bottom connector on the LEFT side of the KLM367). If
there is no signal we go backwards through the signal chain. This leads us
to IC1, pin4. If there is an LFO signal on this pin it means that the trace
is somehow cut. If there is still no signal (which is what I suspect)
measure IC1, pin3. In theory pin3 should carry such a signal. If it is it
means that the (electronic!) switching at this point doesn't work.
Now we must establish if the switch (IC1) itself is broken or if it doesn't
get a proper switching signal. The switching signal for the VCA is on pin5
so I will measure this one. If it is high (and goes to low when moving the
modulation switch away from VCA) this means IC1 is probably broken; if it
is low it probably means that the problem is further up the chain.

And so I'm going to test this now. Stay tuned.