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

[Larry Haney]

Geoff,  the minimum voltage for that cap is a good quality 250 vdc or more
(200 vdc on the plate of the 'time constant' tube).  You might find one
2mfd cap that will work.

Regards, Larry

On Sat, Nov 9, 2024 at 8:16 AM Geoff Brozny <geoff at glorb.net> wrote:

> Hi Jordan,
>
> It looks like I may have enough space under the original caps to mount a
> terminal strip on one of the mounts for the original cap, and I may be
> able to fit a pair of 1uf 400V film caps. I do not have any on hand,
> only 600v ones, and they are too fat to fit in there. I have never tried
> to open one of the caps before to re stuff them, so I am a bit hesitant
> to make this my first try..
>
> thanks,
> Geoff
>
> On 11/9/2024 10:13 AM, Jordan Arndt wrote:
> > I've replaced C551 with 1.5uF film caps in the 3 390A IF decks in my
> > radios...
> > It's not an elaborate solution to the 'moment of silence' issue, but the
> > recovery time is much much faster than the original 2uF caps produced...
> >
> > I took C551 apart and inserted the film caps inside the original
> > assembly, although that might not be possible in the R-390/R725 IF decks.
> >
> > my .02 cents worth....73...Jordan VE6ZT
> >
> >
> > ----- Original Message ----- From: "Larry Haney" <larry41gm2 at gmail.com>
> > To: "Geoff Brozny" <geoff at glorb.net>
> > Cc: <r-390 at mailman.qth.net>
> > Sent: Saturday, November 09, 2024 8:03 AM
> > Subject: Re: [R-390] R-725 AGC quits working after 1 hr.
> >
> >
> >> 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
> >>> ______________________________________________________________
> >>> R-390 mailing list
> >>> Home: http://mailman.qth.net/mailman/listinfo/r-390
> >>> Help: http://mailman.qth.net/mmfaq.htm
> >>> Post: mailto:R-390 at mailman.qth.net
> >>>
> >>> This list hosted by: http://www.qsl.net
> >>> Please help support this email list: http://www.qsl.net/donate.html
> >>>
> >> ______________________________________________________________
> >> R-390 mailing list
> >> Home: http://mailman.qth.net/mailman/listinfo/r-390
> >> Help: http://mailman.qth.net/mmfaq.htm
> >> Post: mailto:R-390 at mailman.qth.net
> >>
> >> This list hosted by: http://www.qsl.net
> >> Please help support this email list: http://www.qsl.net/donate.html
>
>