[R-390] AVC voltages?

[Cecil Acuff]

It could easily be a cap problem.  Many don't show leakage at the voltages 
placed on them by ohm meters but will affect the operation of the circuitry 
at their normal operating voltages.  That's just a general statement about 
the caps in most old tube radio's.

With the antenna disconnected your carrier meter is setting 20db above the 
zero or is it only 20db high with signal input?  Does the AGC switch still 
do it's normal thing when switching around between the settings.....like the 
moment of silence etc....

Has the radio appeared to have lost sensitivity or has it maybe only 
developed a problem with the metering circuit?

I would still expect the culprit to be a leaky paper cap or a resistor going 
way out of spec.  Both of which we know happens...

Cecil...
----- Original Message ----- 
From: <DJED1 at aol.com>
To: <r-390 at mailman.qth.net>
Sent: Tuesday, May 16, 2006 10:37 AM
Subject: [R-390] AVC voltages?


> I've had a slight problem with my R-390A for awhile - the carrier meter 
> reads
> about 20 dB too high.   I   finally pulled the radio out of the cabinet 
> last
> night and did some diagnostics, but i need some sage advice from the 
> group.
> first, I set the IF gain according to the Rippel method (measure AF noise
> level), which also gave me      -7V at the diode load with 3 microvolts 
> in.   So
> that seems about right, but still the problem existed.   I measured the 
> AVC at
> the back terminal strip: -12 V versus a nominal 9 V for 10,000 microvolts.
> This implies the AVC on the controlled stages is not high enough. 
> Measured
> with a 10 Meg voltmeter and a URM-25.
> Then I measured the AVC voltage at all controlled tubes- With -12 V at the
> AVC terminals on the back, the IF stages had -12 to -10.5 V on their 
> grids, but
> the mixers had -10.8 to -8.1 V , and the RF stage had -7.8V.   I tried
> substituting NOS tubes in the RF and worst mixer to see if the tubes were 
> gassy, but
> no change.
> I then measured the resistance to ground of the AVC bus, and it was about
> 500K as expected.
> Sooo- does anyone have data on AVC voltages at the grids on a known good
> radio?   If I had leaky caps pulling the AVC line down, I would expect a 
> much
> lower resistance reading.   Any other suggestions?
> The radio has not been recapped, but I'm reluctant to pull the RF module
> unless I have some clear indication that that is the problem.
> Ed   WB2LHI
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