I'm working on a PA-76A, doing the obvious stuff - replacing the filter
caps, the tantulum timing cap, the low-voltage filter etc. All well and
good.
What raised my question is the bleeder/equalizer resistors across the HV
filter caps. Apparently stock, each cap has two 120K 2-watt carbon comp
resistors across it in parallel. With 2400V B+, that looks like 60K ohms
across 400V, or ~ 2.7 watts per 4-watt pair. All the resistors are
either open or close to it, interestingly enough.
First, I looked at a few posted images on the internet. Yep, it looks
like that's what Alpha used, although the schematic I have says each
"unit" should have 220K @ 2 watts across it...
So: Inquiring minds would like to know what to use to replace them?
I'm thinking maybe 2 220K Ohmite OY (in parallel of course) across
each? That would lower the bleeder current, but I'm not seeing a huge
problem with that at first glance. ?
For whatever its worth, the original problem with the amp was that there
was substantial 120hz hum modulation on the output signal.
Since this is an original two-holer, I'm hoping the problem is strictly
in the B+ supply since a cathode-filament short in one of the 3CX400's
would mean either a retrofit or something equally undesirable...
Tnx es 73,
Jim N7CXI
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|