Vintage Synth Repair

Hi Malke,

Sorry for the late reply, I hope you're still following this thread. The Chassis shield does indeed connect to the output of the opamp. I remember finding it strange also, and trying to disconnect it completely from the circuit, but doing that actually increased the voltage drift...

Of course the circuit has been modified, so I don't know what the original 'design function' of that shield was, but I know I can't just unplug it (unless of course I unplug the whole keyboard assembly, that makes the voltage hold very stable.)

Thanks for sharing your thoughts on this.

J.F.







--- In vintagesynthrepair@yahoogroups.com, Malte Rogacki <gacki@...> wrote:
>
> 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.
>