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" <[email protected]>
To: "Larry Haney" <[email protected]>; "Geoff Brozny" <[email protected]>
Cc: <[email protected]>
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: [email protected] <[email protected]> on
behalf of Geoff Brozny via R-390 <[email protected]>
Sent: Saturday, November 9, 2024 10:22 AM
To: Larry Haney <[email protected]>
Cc: [email protected] <[email protected]>
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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