Hello Phil,

Some years ago I had a TR-707 in my workshop with pretty much the same issue as yours.

Just like Florian Anwander suggests, it was easy with my scope to trace the source of the issue: It turned out to be a  burnt out TL064 in the Envelope Generator. A new TL064 and the TR-707 was like new again.

Just for a precaution I actually changed both TL064s. It seems like these TL064 JFET opamps are prone to fail in this manner.

A couple of TL064s won't cost many $$, so Phil, why not take the chance and hope that your TR-707 has exactly the same as the one I repaired

I wish you good luck. Keep us posted on your progress

Cheers

Bo