[R-390] A Bunch of Questons About an R390A

[flderoos@mmmpcc.org]

I've been working on a R390A that I got through a trade and have several
questions.  It is basically a Stewart Warner receiver (RF deck and
chassis), but the IF module is an EAC, the audio and power modules are
Collins and the PTO is Cosmos.  So far I've recapped it and checked the
values of most of the resistors.  I've replaced the power cord with a
3-wire cord and cleaned and the gears, bearings and slides.  As soon as
the Deoxit arrives and I can clean switch contacts and assorted sockets,
I'll put it together and see if it works, then align it.  Now to the
questions.

I decided to measure the resistance from the AGC lines to ground.  The
IF module showed infinity, as it should.  I then measured the AGC line
on the RF deck and found approximately 80 K ohms (way too low).  I don't
know why I did this, but while troubleshooting I removed V203 and V204
and found that the resistance went up to 3-4 megs (better).  Well that
suggested some pretty bad tubes if they could show low resistance from
grid to ground without the filaments on.  After checking them and
finding them to be ok (no shorts and not gassy) and subbing them with
known good tubes and finding the same decrease in resistance, I started
looking at how they were wired.  The first thing I noticed is that the
27 ohm resistors that go to the grids were connected to pin 1 on each
tube.  A quick check of the base drawing shows that pins 1 and 5 are
internally connected, so that explained why with the tubes inserted I
got the low resistance.  I was measuring the resistance of the B+ line
to ground!  Well, with further checking, I found that the cathode
resistor/capacitor was connected to pin 2 of V203 and to pin 7 of V204
and there was a jumper to pin 2 on V204.  So, it appears that the jumper
was left off of V203.  Now the real strange observation, pin 6 (grid) of
V203 and V204 were both connected to ground!!!  I've never heard of a
grounded grid convertor.  So, the first question is does this mod look
at all familiar to anyone?  Was there a mod that just didn't get
completed correctly or was there a tube change?

Now the second question.  The AGC line to V201 goes through R201, which
is listed as a 270k resistor.  My deck has a 3.3 meg in parallel with a
diode for R201 with the cathode toward the transformers.  It also has
R233 replaced with a 3.3 meg resistor and another 3.3 meg resistor from
E208 to ground.  I seem to remember a change in the AGC for the RF amp,
but can't remember it.

And the third question is what is the purpose (if there is one) of an
added diode between terminals 3 and 5 on TB102? This is the AGC DIV/NOR
connection on the back.  I forget the number, but it is a 3-digit number
and looks like it might be a germanium diode.

And the final question is what should the DC resistance of the
mechanical filters be?  Three of mine measure 50 ohms for the input and
output and the other one (8 khz) measures 40 ohms for the input and the
output.  I think I've seem 40 ohms listed several times, but am
surprised that I would have three bad filters that measure so closely.
And, if there were shorted turns, the resistance should be lower.

These are quite the receivers.  I can't wait to get it working and
actually use it.

Thanks in advance for any help/suggestions.

73,
Fred
WA0GMH