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Old 6th March 2012
Here for the gear

Thread Starter
Received tools today and got to work on the Jupiter 6. Swapping parts between the 2 voice and 4 voice boards to try and see which part causes the problem to move. First I swapped the 4051 multiplexer on IC20. Then swapped the comparator. Next swapped the 4051 on IC15. All these swaps made no change in the tuning problem.

Next I swapped the 4051 multiplexer on IC16. The tuning issue did change. It did not move to the other board as expected. It is still on the 2 voice board...but instead of vco 1 voice b being 1/4 tone sharp all the time it is now about 2 octaves flat all the time. Also vco2 is now being affected.
On the 2 voice board:
Vco2 voice รค" is always about 2 octaves flat.
Vco1 voice "b"is always about 2 octaves flat.

Not sure if I damaged the 4051 chip or if is related to the original problem. I did have to resolder a couple pins several times... which I did not have to do on the first couple of swaps. Ordered some replacement 4051's today.
Just trying to get a bit of experience before I swap the larger DAC chips.

If anyone has a minute to look at this and share a clue... please let me know.
The service manual for the jupiter 6 can be found here.