[R-390] Below 8mhz problem

[Dan Martin]

I took my 390A down for "below-8' sensitivity problems after I confirmed
suspicions that it was considerably less sensitive than my 51J4 on 40
meters. I'd recently noticed that fact when doing A/B comparisons on 7290
and had always assumed it was due to different antennas on each rig. Then
one day I used the same antenna and found the same thing. Also noticed the
calibrator signal was 40-plus dB on 8 and above but barely 10 dB below that.
On 40 meters the calibrator barely kicked the needle! First thing I did was
confirm the 1st oscillator signal. Whether looked at p-p on a scope or by
checking the rectified equivalent in standby at E209, I had lots of good 1st
oscillator signal. Then I found that pin 6 on V202, the first mixer grid,
was lazily 0.0 to about 0.5V positive instead of -0.4V. The resistance to
ground for this pin, which should have been a couple of series resistors, a
small choke, and the AGC line when in AGC, or just the series discrete
components when in MGC, was infinite, or open. Hmmm...  I pulled the RF deck
and was very surprised to find the three series resistors, a 27 ohm
parasitic suppressor right at the tube pin and a 1 meg and 22K resistor in
series to a choke and the AGC line all checked fine when tested
individually. However, the entire series check from the grid pin back to the
J208E was again infinite! To shorten the story, after repetitive checks I
found a stone-cold solder joint on a stand-off post where R231 (1 meg) and
R232 (22K) join! I reheated the joint and got a perfect -0.4V and the proper
resistance. Because this cold joint effectively opened the series line the
grid was biased-off slightly positive and had no resting state -0.4V. This
was my personal 390A I've had for years, a nice '67 EAC, and this particular
solder junction just went south for no particular reason! Anyone else had
this happen? I've often heard of a couple of guys that "reheat all the
joints in the RF deck using silver solder" during their refurbishment
efforts. I've always thought that to be snake oil. While I still don't
personally recommend doing that preemptively I AM more thoughtful now in
checking for such conditions during my trouble shooting efforts! Finally, to
get the set back to 100% I had to do a thorough alignment. I was so
frustrated in my early evaluation of the problem that I fiddled around with
some of the transformers to see what effect it would have. Of all of the
stages I aligned, I must say the 1st variable IF took the most time. I did
repetitive alignments of this stage, coils at the low end and caps at the
high, over and over and over. I must say the adjustments were very
interactive. Each time clear gains were realized however, and  I could hear
the rig coming more and more alive. Finally, after several passes I could
make no further improvements. I reset the IF gain using Rippel's method. The
rig has spectacular ears now and specs out fine. I also reset the PTO
endpoints while I had it out. It had gone about 6 kHz long since Chuck
showed me how it was done in his shop in the summer of '98. I learned a few
things doing the PTO endpoint here, but that is another posting sometime ...

Dan
WB4GRA

----- Original Message -----
From: "David Wise" <David_Wise@Phoenix.com>
To: <r-390@mailman.qth.net>
Sent: Wednesday, January 23, 2002 7:25 PM
Subject: RE: [R-390] Below 8mhz problem


> > As an update, I have found that E209, before the 1st mixer
> > has low voltage.
> > It should be approx -6.5 volts, and it actually is -.85.
> > Measured with a
> > 1meg resistor it is -.76.