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

[Geoff Brozny]

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
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