[ARC5] More Vodka-Scented Ether (long)

[David's Mail]

Vodka Scented Ether
Or
The Continuing Saga of
The Low B+ Russian Radio
"УC-П" (pronounced "oo-ess-'pay")

http://home.netcom.com/~arc5/usp/2.jpg

I'm really enjoying the challenge of getting
this old "Red" radio perking again.
Here's a photo of it paired with Soviet transmitter
"RSB-F" in radio set "RSB."
I probably wouldn't trade one of my daughters
for an RSB-F..... probably... :

http://home.wxs.nl/~meuls003/gallery/usp.html

In my early collecting years, before I knew better,
I "recapped" an authenticaly rare set, thereby
ruining the fine workmanship under the chassis.
I have since resolved never again to be guilty of that,
but I still wish to operate my radios once in awhile.
I have, therefore, been working on ways to operate my radios
without resorting to mass cap and resistor changes.
While I'm still learning every time I heat up my iron,
I'm guided by these principles:

  + Make no change that is not 100%
    restorable to the original configuration.
 + Do as little as possible.
 + Do not expect a "Model "T""
    to be a Porsche; enjoy your Model "T"
    or give it to someone who will.
  + Preservation trumps performance.

 I've found that the large majority of old caps
which fail at their rated voltage
will still perform their function in an acceptable manner at
a much reduced voltage, and that when a stage is running
at a fraction of the intended B+, resistors far out of spec
don't seem to matter much.  There are no "free lunches,"
of course;  I am willing to accept some trade-offs
in performance to both keep the receiver as near "stock"
as possible and still enjoy operating it.
In most cases, the trade-offs have been, IMHO, minor.
The underchassis of the УC-П is pretty cool,
so I determined I was not going to "gunch it up."

http://home.netcom.com/~arc5/usp/4a.JPG

WF2U kindly sent me the manual for the Russian УC-П receiver-
Thanks a million, Meir!
He also pointed-out the reason for the radio being “deaf “ on AM:
the detector is biased heavily off, which provides
a crude sort of  “squelch,”
giving the Russian pilot’s ears a rest until someone close by transmits.
 Moving the pin-8 ( cathode two ) connection of the dual-diode detector
to ground gives normal sensitivity.
Although, to an English speaker, the detector tube appears
to be marked "6X6," the "X" is actually the Russian letter "Khah"
(pronounced "ha"), and is the closest thing in their alphabet
to an English "H."   The tube is a equivalent to a 6H6G.

The tube filaments had been wired for 6 volts when I got the set,
but the manual calls for 12 or 24 volts. I decided to restore the wiring
to the 12-volt state.
I purchased some monolithic block DC-to-DC converters
at a local surplus outlet for $4.95 each,
which included the circuit board mounts and
a couple of filter caps.

http://home.netcom.com/~arc5/usp/3a.JPG

The converter inputs were speced at 9-18 VDC with
plus-and-minus 15 VDC at 1 Amp outputs. The boards included
a little trimmer resistor used to adjust the output voltages.
At zero resistance, the outputs were 13 volts.
At infinity, they were 15.2 volts.
By connecting to the + and – outputs and ignoring the center common point,
I got a floating 26 to 30 volts. I connected two of these outputs in series
for 60 volts- plenty to run a low B+ receiver. I tried three for 90 volts,
but it was too stressing on the aged components and didn't enhance
performance much.  The only problem with going down to 60 volts was that,
over a small portion of the dial scale at the most "full mesh" of the tuning 
cap,
the local oscillator quit.  Everything else worked fine.
This aren't important spectrum to me, being neither ham nor SWL band,
and it isn't worth the extra strain to get that little bit, so I let it go.

Digging around in my junk produced an old “Archer” Radio Shack
transistorized audio amp that runs on 6-9 volts.
You probably remember these little hi-Z input project amps,
which came in several power levels. They are very handy and I always grab
any I find in a hamfest box.   A little 5-volt regulator dropped the 12 
volts
to 5 for the audio amp.  I tinkered this all together with utterly
inadequate RFI suppression.  Here's my messy "prototype:"

http://home.netcom.com/~arc5/usp/pwr.JPG

Even so, some .01 and .1 caps on
the input and output leads, plus some ferrite beads killed 99% of
the switching hash and most of the "birdies,"  leaving a few
attenuated ones here and there.   Grounding the power chassis
direct to the receiver attenuated them more.

Once I got it perking, a proper alignment really brought it to life.
With a 112 KC IF, you can't play "good enough" with alignment.
A good sig gen, freq standard and scope and the proper
procedure are required if you want to keep the images down.
It's a "touchy" set to align.  The dial calibration does not track
especially well and there are no trimmer caps on the LO side to
make it track; only the LO coils.  I got it to track reasonably well
over 160, 80 and 40 meters and decided to accept that, rather
than risk damage in an attempt to "make a Porsche from a Model T."

I've been running the set on my bedstand for several days,
letting it run overnight to see if the supply is going to hold up.
So far, so good.  The set sounds great with all the audio I can stand.
In fact, the input to the little Archer amp is swamped
with a 4.7 K resistor to ground and a 2.2 meg in
series to keep the level from the radio down.
The morning AM crowd sound as good as they do on
any  of my receivers (but then, I'm half deaf ;-).
One of the "trade-offs" of low B+ is that the AGC
doesn't really work much.  I'll accept that;
"preservation trumps performance."

http://home.netcom.com/~arc5/usp/5a.JPG

The only problem that's developed is some background "crackle"
noise when I turn on the BFO, which is probably either a
cap in the BFO circuit that is too far gone even for low B+
or the Russian BFO 6K7.  I'll put a scope on it this weekend
and find it.  Oddly, the longer the converters run,
the less pesky the birdies.
The peak 12-volt current draw with the set playing wide-open
is about 1.8 amps, close as I can measure.  Averages about 1.2 amps.
Total 60 volt B+ current at full output is about 14 milliAmps,
averaging 10 mA.  Not bad for an eight-tube radio.
You could run this thing on a car battery for a long time.
Who knows?  Maybe Boatanchor Field Day, Battery Class
one of these days.

I just found some surplus AT&T converters which are less than
half the size.  I'm going to build the "neat" version with them
and let you know how it comes out.

There is one other problem:
 "David!  Must you listen to that noise all night long??"
Women; they'll never understand radio....

73 Dave AB5S.