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

[Geoff Brozny]

Hi Larry,

I have seen that document before, but I just want to fix the R-725 as it 
was originally designed for the moment. I may chose to improve things in 
the future. I do have a R-390-a I need to eventually try to figure out 
what someone did to it. I think someone tried to improve the AGC, and 
add a product detector, and what they did in reality was create a very 
poor preforming radio. I just do not really have the time or the 
experience at the moment to try to figure it out.

Thanks,
Geoff

On 11/9/2024 10:03 AM, Larry Haney wrote:
> 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 <mailto: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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