[R-390] Saturday's Line & Local Report

[Craig Heaton]

Hello Jim,

There is continuity between the pot pins at the front panel, per the
schematic. Pots at terminal three (3) are grounded at chassis, checked many
times over. With today's little test, I'd say the AGC is working, had my
doubts. Injecting the sig-gen into the antenna input and watching the
results at the IF output, tells all. With the function switch set on AGC,
Motorola & Amelco are the same at that stage. If the signal from the last
tube in the IF of the Amelco had a greater output than the Motorola, it
would be time to back up to either the IF or RF. I think?

Craig, 

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of N4BE_Jim
Sent: Sunday, January 05, 2014 6:58 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] Saturday's Line & Local Report

I would also suspect the wire harness.  Check continuity between the pot
pins at the front panel and their ends in the various modules.  Are the pots
grounded to front panel or through the harness?  

One end of gain controls are typically grounded.  If the grounds are lifted,
the pot has little or no affect.

I once had an old SW 390a that would produce an awful popping noise on
strong signals.  After much hair pulling, it turned out to be the little
shielded cables, particularly the one carrying diode load signal to the rear
apron. Who would have thought?  The dielectric was breaking down under
"high" diode load voltage (if you consider 10 volts to be high).  Took a
good part of a day to thread new miniax through the harness.

Another thing to look at is AGC not being able to control some stages.  It
is conceivable that AGC is controlling early stages ok, but due to a bad
bypass or agc cap in later stages, the IF is being over driven.  That can
make for high audio.

Another fun anomaly is oil or contact cleaner on the phenolic antenna tune
shaft.  Looks like a low impedance and will kill your AGC.  Same for contact
spray or lubricant used carelessly on band switches.  High impedance tube
circuits are such fun!

Jim N4BE

Sent from my iPad
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