[sdiy] sh09 cv out drops again

[rburnett at richieburnett.co.uk]

> It has no guard ring on the pcb around LF13741, but datasheet talks
> about that. It has a cardboard with metal foil connected to ground
> above this area.

Maybe the foil-backed cardboard screening sheet has absorbed moisture 
and become slightly conductive.  You could try putting some polythene 
sheet between the cardboard and the sensitive sample-and-hold area of 
the PCB.  Or temporarily removing the foil/cardboard sheet, and see if 
the leakage problem goes away.  (As far as I know this screening sheet 
in the SH-09 is to stop 50/60Hz electric fields from the mains wiring 
underneath from introducing pitch warble into the expo-converter on the 
PCB.)

I guess that ideally the PCB would have a guard ring and be coated with 
conformal coating to keep leakage currents in the S&H under control in 
high humidity environments.

-Richie,