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In Reply to: RE: Completely re-cap the AM/FM front end while you're at it! posted by gusser on July 25, 2017 at 11:54:15
guesser - in your experience is it likely an alignment would be needed if the caps are changed in the AM/FM sections?
what is the likely failure mode if these caps are bad? is it possible the AM/FM sections could take out the output transformers or create a short?
what is the likelihood the alignment for the AM/FM section isn't wrong with the factory caps that are potentially bad?
would you replace all caps regardless of type or just the paper caps?
in the case of this HK224 here I have found that my FM section is a bit flaky, volume levels of received signal fluctuate. there seems to be minimal frequency drift however... I'll probably change those caps out soon. for now I did the coupling caps from/to the concertina phase splitter and put the receiver in service.
there is always the ongoing battle of "recap everything" vs "recap what is bad". I've found the old timers generally prefer to only fix what is broken... and everyone that is roughly 50 years old and younger tend to lean toward "change everything."
Follow Ups:
You never want to replace caps in RF circuits unless they are truly defective. Mica and Ceramic caps, which are commonly used in RF circuits, rarely go bad.Especially in point to point wiring, just moving caps can alter the RF alignment.
Edits: 07/25/17
I miss-communicated there but appreciate what you said.
I meant the paper/wax capacitors in the FM/AM sections. I know the mylar and ceramic are fine.
Does anyone happen to have a spare fisher 19kc oscillator coil for the MPX unit on this fisher receiver?
Thanks
That's such a difficult job, rebuilding a tube-type tuner. Even 40 years ago, when these units didn't yet need such extensive maintenance and replacement of parts, most owners couldn't afford to have a front end refurbished. I've seen things fail in tuners (both tube and solid-state) that you just wouldn't think would ever go bad. I still remember a KLH that wouldn't tune properly... after many, many hours of testing, it turned out to have a shorted turn in the discriminator transformer. The customer was happy... me, not so much.
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