Vintage Synth Repair

When you part it out, can I have your battery cover? Just kidding.

> On Oct 12, 2013, at 9:28, <aaronkotilainen@...> wrote:
> 
> Very happy to join this group! 
> 
> I picked up a mint looking SH-101 for a song earlier this week because it didn't output any audio. It powered up fine and the LFO indicator was functioning as it should, but didn't have any audio output. The guy I picked it up from said he bought it new and it sat in his parents house for the past 20 years unused. When you move the volume knob back and forth while playing it you can hear it trying to output sound, albeit very very low. I think this is just ground noise. He just recently brought it to another tech who was no help. The tech said he replaced a shorted diode in the power supply and traced the output back to the VCA output which had a very low output high screech to it. He didn't know how to proceed further. 
> 
> I popped her open and traced the signal back to the VCF and Voila! The oscillator noise, and filter are functioning fine. Sounds great. The arpeggiator and sequencer are working as well so I assume the CPU is still ok. I also measured the ENV fader and that seems to be working correctly. All of the voltages seem to be correct as well. There seems to be an issue right before the signal reaches the VCA. Measuring TR28 produces the same low output high screech before it hits the VCA. 
> 
> I replaced the VCA chip with another BA662 I sourced locally and added a socket to easily swap in the original. That didn't fix it. I also replaced C53 which is before pin 1 of the VCA. Replaced C64 on the +15 rail, as well as IC18 the IR9022 right before the VCA. Nothing has worked. 
> 
> I'm getting stumped. 
> 
> I'm beginning to think this is a another dead op-amp or transistor in the envelope or on the control board. Another person has suggested a bad trace somewhere, which I haven't found yet if it exists. If anyone has some experience with these synths, I want your brain power. 
> 
> Any thoughts or ideas would be much appreciated! 
> 
> Thanks!
> 
>