[R-390] re: Call for measurements - 100dB carrier reading

[David Wise]

There were several responses advocating a 10-turn
or low-ohm 1-turn pot.  These can relieve
the symptom (but see the quote below from the
Cost Reduction Report), and as such will be
fine for most of those who find the stock
pot annoying.

***********
A carbon potentiometer is used in the carrier level meter
adjustment circuit. It offers less erratic operation and smoother control than
the wire wound control.

- Cost Reduction Report, section 4.3.3
***********

This quote suggests that the wirewound pot used in
the R-390 had visible jumps as the slider contacted
successive turns of wire.  I have never seen an R-390
pot, so I can't comment, but I suspect this was a
rationalization; cost was the primary motive.

One person recommended a 20-ohm pot from
Surplus Sales of Nebraska.  He feels that 18
ohms is the ideal value.  I disagree, 8 is ideal.
The nominal reference voltage, dictated by V506A's
saturated plate current, is about 60mV.  If
the tube had infinite perveance, the current
would be (205/82) or 2.5mA, which would develop
67.5mV across R548.  The reference voltage is
V504's cathode current through R523.  The
manual obliquely says it is 13mA.  I measured
10 to 12.  Therefore, the worst-case maximum
value of R523 is 67.5/10 or about 7 ohms.
I found a 10-ohm pot at SSN, part number 140-6233.
It's $1.50 .  (You'll probably want to convert it
to screwdriver-adjust by grinding a slot in the end
of the shaft.)  With this pot the adjustment range
will be 45-65%.  Assuming it uses fine wire, it's
the best cheap palliative around.

I know I'm in the minority: I find the pot-replacement
tactic profoundly unsatisfying.  It's so... brute-force.
There is a smart way to fix it too.  Never mind that
it's 55 years late, it's how they should have done it.
Now that satisfies!

* * *

Graham had a lightbulb moment about the suppressor
grids.  Their use as diode clamps is mentioned briefly
in the TM11-856A manual and maybe others.  It seems
like a weak argument to me.  They aren't very good clamps.
Turn down the RF gain (so the gain-controlled tubes don't
become clamps themselves), pull out V506, and see what
the AGC line does.  On my set, it goes a couple of volts
positive.  That's all the suppressor grids can manage.
V506's control grid is the real clamp.

A big thanks to Jim Miller N4BE for the reminder about
DeOxit conductivity and his recommendation for a
remediative treatment.  I presume that Graham is already
satisfied that his AGC line isn't leaking to ground.
Mine isn't, I have more than usual.

Regards,
Dave Wise