Vintage Synth Repair

I had another look at your diagram for the s/h circuit. I'm not completely
sure if it's correct.

If I'm not completely mistaken it looks as if the "keyboard chassis shield"
is connected to the output of the opamp and the input of the VCO? Doesn't
sound plausible to me. Could it be that this is just another output for the
keyboard CV (perhaps for the filter) and not the chassis shield?

Other than that the part is practically identical to the circuitry in the
Odyssey, except two minor changes:

1. The Odyssey doesn't use the trimmer.
2. The Odyssey doesn't use the diode.

The remaining stuff is completely identical, including the "loop" around
the input end of the hold capacitor.

Odyssey and Axxe (and perhaps other ARP synths as well) used an IMF3958
(2N3958). It looks like those are mostly available from specialised vendors
(like vintageplanet.nl) and cost around 7 to 9 EUR.