[R-390] R-725 AGC quits working after 1 hr.

[Larry Haney]

Hi Geoff,  Welcome to the R390 group and reflector list forum.

The R725 has essentially the same IF deck as the R390, which means those
and the R390A have the same AGC speed circuit - bad.  Yes, you are right in
that those 2 caps are the problem, and it might be just 1 of them, but you
should replace both of them.  The caps you replace them with must have '0'
current leakage (usually electrolytics don't qualify).  Your symptoms are
that one of them is leaky.  There's been lots of discussion about this area
over the years.  There is also a major design flaw in this circuit, which
you will want to know about - see 'Ending the moment of silence' on our
website documents: R-390/R-390A AGC - Ending The Moment Of Silence
<https://www.r-390a.net/Wise-AGC-Mod-Long.pdf> and R390A AGC Moment of
Silence <https://www.r-390a.net/R390A%20AGC%20Mom%20of%20Silence.pdf>.

Regards, Larry

On Sat, Nov 9, 2024 at 6:36 AM Geoff Brozny via R-390 <r-390 at mailman.qth.net>
wrote:

> HI,
>
> First time posting to this list, have been subscribed for a couple
> months. I have not needed help with anything until now, and I am mostly
> new to this. I have a R-725 that works fine except for the AGC. After
> around 1 hr with the AGC in either Medium or Fast, it quits working.
> Audio distorted like I set it to MGC. I seem to be able to get it to
> work for what seems to be a random time by setting the AGC to slow, then
> after 10 seconds back to medium or fast.
>
> The only issue I seem to have found is the two 1uf oil filled caps, C546
> and C547 which are parallel to create a 2uf cap. Tested while cold I was
> getting right at 2uf, hot I was getting something close to 6, so that is
> my suspected issue. Should I just replace these caps with a pair of 1uf
> film caps? I do hope I can fit whatever the replacements are under the
> chassis so it at least looks original from the top.
>
> Thanks,
> Geoff, KD8GRN
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