50 kHz filter (late model)
Seems to be applicable to serial nos 21000 and higher. Make a a 50 kHz network
of a 10 mH choke and 1000 pf capacitor in parallel. Add a .01 uF 250 volt cap
in series with one end. Install this network from pin 7 of V6 to ground. This
should be the plate pin. The receiver in question should have a T7C (not a T7
IF can).
Lead dress for this mod can be critical and is noted by an increase in audio
hash and hum.
This change is applicable ONLY to R-4C receivers that feed B+ to the plate of
the 3rd mixer through the notch filter. My own experience is that this change
seems to do little, but it does no harm either. Its a very popular 'secret mod'
that you may want to try, just for the hell of it.
Its *supposed* to act like a tuned IF transformer for the plate of the 3rd
mixer to minimize mixing products.
Found this on http://www.dproducts.be/DRAKE_MUSEUM/nouvelle55.htm
I also remember doing this mod on the R-4C that I had (s/n was, I think,
24xxx):
50 kHz IF Coax (late model, early series)
Late model R-4C (tapped IF transformer T7C) could have some additional CW
crystal filter loss due to use of high capacitance shielded cable running from
T7C. Replace this audio cable with RG/174 and change the value of C49 to 430 pf.
You don't need this change if there is no CW filter installed in the radio.
Your receiver is eligible for this change IF you have a T7C (NOT a T7) IF
transformer and if C49 is currently 390 pf in your radio. In this case, the
coax in question will have a slightly larger diameter than the replacement
RG/174 and will have a white colored center conductor.
Happy Drake-ing...
Steve Wedge, W1ES/4
To be is to do - Socrates
To do is to be - Plato
Do be do be do. - Sinatra
All my computers have my signature with various pearls of wisdom appended
thereto.
From: Kevin Nathan
Sent: Sunday, January 16, 2011 2:20 PM
To: 'Steve Wedge'
Subject: RE: [Drakelist] Late R-4C's and noisy mixers
Thanks much Steve. I plan on looking into that mod, it just depends on the
price tag to go with it. <GRIN>
Thanks much and very 73.
Kevin :)
Amateur Radio: K7RX
--------------------------------------------------------------------------------
From: [email protected] [mailto:[email protected]] On
Behalf Of Steve Wedge
Sent: Sunday, January 16, 2011 10:36
To: [email protected]
Subject: [Drakelist] Late R-4C's and noisy mixers
I seem to recall installing a mod to the 3rd mixer on my R-4C years ago that
was available from Sherwood. I believe it was an inductor (20 mH comes to
mind) and I do recall that it quelled the frying pan that was inside my
receiver at the time :-)
I haven't looked it up but figured I'd pass along what I remembered about it.
Steve Wedge, W1ES/4
"I can't complain, but sometimes I still do."
- Joe Walsh
If the above message appears, it came from Steve's Son of Laptop!
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