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

[Jordan Arndt]

Ok....I hope this isn't too wordy...

No..I only replaced C551 with a 1.5uF cap in each of the 3 IF decks I 
have...

What actually happened was that I had bought a few 2uF film caps, to see how 
I could replace the original C551 with a 2uF cap under the chassis, but I 
didn't like the fit.

They wouldn't fit inside the C551 casing either so I installed the 1.5uF 
film cap in the casing. They fit diagonally inside the old case with some 
room in the corners because of the heat required to refit the base of the 
cap to the case and solder the perimeter.

I measured the DC voltage present at C551and was a little surprised to find 
~240Vdc there, but due to size constraints, I settled for 400V 
Mylar/polyester film caps.

The process is much the same as rebuilding bathtub caps and similar. I like 
to drill a few 'weeping' holes because I once didn't and ended up with hot 
cap juice blowing out of the seem due to the pressure built up from the heat 
of the torch used for desoldering.

As far as the Skirrow Carrier meter mod, the design is good, but it's made 
even better by adding an 18 ohm resistor across the input to the Skirrow 
circuit board so that the prior circuitry sees the same 18 ohm as the 
original meter presented.

I made two of my own boards using 741s (cuz I have tubes of them from back 
in the day) and didn't need the additional components for setting an offset 
for the op-amp used by Skirrow, so the circuit is even simpler and the meter 
is exceptionally linear in response.

I also added an 18 ohm resistor across the input on a Skirrow board 
installed in an actual R-390A I bought from Jan years ago. I was concerned 
that there might not be enough gain with the component values used by Jan, 
but there was enough room left in the variable feedback trimpot.

I also discovered that the meter itself does glow in the dark even though 
it's a replacement meter when Jan did the re-build...

There is also a version on QSL.net by KH6GRT which uses an LM308N...

73...Jordan

----- Original Message ----- 
From: "David Wise" <d44617665 at hotmail.com>
To: "Larry Haney" <larry41gm2 at gmail.com>; "Geoff Brozny" <geoff at glorb.net>
Cc: <r-390 at mailman.qth.net>
Sent: Saturday, November 09, 2024 1:49 PM
Subject: Re: [R-390] R-725 AGC quits working after 1 hr.


> Jordan,
>
> "film caps" plural?  Do you mean that you did the partial-mitigation for 
> moment of silence mod that uses one cap for LONG and another cap for MED?
>
> If I were to do it again, I would do Charles Steinmetz's moment of silence 
> mod, placing the caps behind the front panel, and abandon the original 
> C551 in place.  Once you've committed to removing the panel, you can also 
> illuminate the frequency readout with LEDs, and install Jan Skirrow's 
> universal carrier meter circuit if you don't feel like doing mine and Gary 
> Biasini's.
>
> Dave Wise
>
> ________________________________
> From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on 
> behalf of Geoff Brozny via R-390 <r-390 at mailman.qth.net>
> Sent: Saturday, November 9, 2024 10:22 AM
> To: Larry Haney <larry41gm2 at gmail.com>
> Cc: r-390 at mailman.qth.net <r-390 at mailman.qth.net>
> Subject: Re: [R-390] R-725 AGC quits working after 1 hr.
>
> On 11/9/2024 1:09 PM, Larry Haney wrote:
>> 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
>>
>
> Thanks Larry, I appreciate it. Hopefully I can get it back up and
> running this next week.
>
> Thanks,
> Geoff
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