[R-390] capacitor analysis

[David Wise]

> From: Michael Melland [mailto:w9wis@charter.net]
> 
> > Unused filters are out of circuit, unless the bandwidth switch
> > is messed up.  See the Y2K manual, page 5-48.
> 
> Now I'm really confused.  My radio is being repaired/refurbed 

Well, you're not the only one.  My bad.  Unused filters are
not in the *signal* path, but they are still on the AGC line.
Your diagnosis has not been disproven.  If you aren't up
to paying for a replacement filter, you might be able to
rewire the bad one in a shunt-fed arrangement on the secondary,
where a blocking cap prevents it from grounding the AGC line,
and you add a resistor for grid return.  The resistor could be
oh I don't know maybe 100k.  You'd still get noises as the
filter went through its intermittent shorting regime, but
the AGC would stay ok.

To check your diagnosis, I'd disconnect the filter secondary
return bus from AGC and ground it.  This removes AGC from the
2nd IF, so you might want to back off the RF gain during the
test.  If the problem goes away, you've confirmed it's a
filter.  Then it's a simple process of elimination.

73,
dw

> at Miltronix.
> I had an almost exactly similar AGC glitch as John that was 
> traced to a bad
> 4 Kc filter that warmed up and would intermitantly short to 
> the filter case.
> It affected agc in all bandwidth selections even when 4 Kc 
> wasn't selected.
> Rick told me that one part of all the filters is always in 
> the agc line.....
> so did Wally Chambers.... and the IF filter information on 
> Chuck's site also
> states that a bad filter can affect agc and do other wierd 
> things even when
> not selected....