[R-390] capacitor analysis
David Wise
[email protected]
Tue, 4 Jun 2002 14:45:23 -0700
> From: John Saeger [mailto:[email protected]]
>
> So far the symptoms are the same at 16kc, 8kc, and 4kc bandwidths. I
> haven't spent much time listening at 2kc bandwidth (yet).
The problem's not the filters, then.
> Michael Melland
> had suggested that it might be one of the filters themselves
> causing the
> dropout. That a short in a filter could cause a problem when
> listening at a
> different bandwidth.
Unused filters are out of circuit, unless the bandwidth switch
is messed up. See the Y2K manual, page 5-48.
By the way, there's a typo on that page that could confuse
your understanding of C501's role. There's a dot missing.
The bottom end of C501 (which goes into Z501) also goes to
V501 pin 1. In 2/4/8/16, C501 acts as a short around Y501,
completely overshadowing Z501 and C520. At 1KC bandwidth,
it is out of circuit. At 0.1KC, it effectively bridges R503
across R502, reducing the Q-spoiling resistance in series
with C503. (C503 in series with R502 or R502|||R503 is in
parallel with C524 and tunes L503.) This part of the IF deck
is shown correctly on page 3-13.
> I
> have noticed that the 1kc and .1kc bandwidth positions don't
> seem to do much
> in the way of narrowing the bandwidth, the signal level just
> decreases when
> I select one of these positions. Maybe this is another
> reason to worry
> about C501. I don't know. I don't really understand how this circuit
> works. The book says that C501 is a coupling cap when using
> 2, 4, 8, or
> 16kc bandwidths, but it's something different when using .1
> or 1kc. Maybe
> its part of some kind of resonant circuit. Maybe it's gone
> out of tune.
If you apply 1KC or 0.1KC bandwidth to an ordinary AM signal,
the sidebands will be almost completely eliminated.
The treble and midrange are gone, leaving only bass,
which the R-390A's internal audio amps are very short on,
so if you're listening through them, you won't hear much.
If you put a hi-fi audio amp on the Diode Load output,
you'll hear the bass part of the program at the same
volume as before. For 1KC and 0.1KC, use CW or an
unmodulated carrier and go by the carrier meter reading.
I seem to remember someone on this list finding that
his L503 was open. It seems to me that that would
degrade 1KC/0.1KC performance.
I can't remember when I last saw a bad ceramic cap.
73,
Dave Wise