[R-390] Local and Line pots plus AGC

[Charles Steinmetz]

Dave wrote:

>on the IF module that does work, here's what I get feeding the 
>service minotor into the BNC (balanced) on the back of the 390A, 
>measured at 16KHz BW.
>
>Oh, I forgot, on pin 3 TB102 I measure 470K after the DVM settles 
>down. on Pin 4 I measure around 1.8 Meg.
>
>How do these numbers look as far as AGC goes?

The voltages look decent, but the 1.8M ohms on Terminal 4 seems low 
to me.  Since the AGC detector already has to drive ~500k ohms on 
Terminal 3, the 1.8M in parallel is not a great additional 
burden.  But I think you may have more than nominal leakage on 
Terminal 4 that could indicate future AGC problems.  (However, I 
wouldn't dig into it at this point -- I'd wait for it to get worse, 
measuring the Terminal 4 resistance every 6-12 months.)

>How much AC hum should I be seeing on the local and line output?

1 or 2 dB above the broadband noise of the audio stages.  Enough so 
you think it should be lower, but not enough to seriously interfere 
with operating the radio.  The most prominent cause of R390A hum is 
the fact that the designers used the chassis for the heater returns 
(unless you have a tube installed that suffers from heater to cathode 
leakage, in which case the hum will be distinctly audible, 3dB or 
more above the broadband noise of the audio stages).

Best regards,

Charles