Tangerine
Thanks for all the fish!
I've already posted this under my blog that covers the re-build of the A-9:
Having fitted the new transistors:
2SA1295
2SC3264
to the (GWX-593/4) Driver Assemblies (that's a post on its own) and recapping the (GWH-148) Power Amp Assembly we now have parasitic oscillation (PO) present running RF between the rail voltages. So it's BIG TIME positive feeback. Notwithstanding -ve feedback paths for the servos there's the protection path that may be the cause (since writing this proven not to be the cause). These three elements are the only boards in play at this stage.
With the protection ccts disconnected hopefully this will cure it (no it didn't), otherwise we're going to chase the RF to gnd and inject an audio tone to trace it to a point where the oscillation is having an effect (didn't work).
The Driver Assemblies heat sinks are grounded, so aren't acting as antenna's.
My obvious question to the better technically adept (than I, have forgotten more than I know) here is have you come across Parasitic Oscillation in a NS Amp before, even better if it's an A-9??
I'm so disappointed that we've not been able to get the A-9 back into service, and am looking for help.
Cheers,
Les
Having fitted the new transistors:
2SA1295
2SC3264
to the (GWX-593/4) Driver Assemblies (that's a post on its own) and recapping the (GWH-148) Power Amp Assembly we now have parasitic oscillation (PO) present running RF between the rail voltages. So it's BIG TIME positive feeback. Notwithstanding -ve feedback paths for the servos there's the protection path that may be the cause (since writing this proven not to be the cause). These three elements are the only boards in play at this stage.
With the protection ccts disconnected hopefully this will cure it (no it didn't), otherwise we're going to chase the RF to gnd and inject an audio tone to trace it to a point where the oscillation is having an effect (didn't work).
The Driver Assemblies heat sinks are grounded, so aren't acting as antenna's.
My obvious question to the better technically adept (than I, have forgotten more than I know) here is have you come across Parasitic Oscillation in a NS Amp before, even better if it's an A-9??
I'm so disappointed that we've not been able to get the A-9 back into service, and am looking for help.
Cheers,
Les