[R-390] 60 Hz Hum Output from R-390A IF Module

[Flowertime01 at wmconnect.com]

Jon,

A thought has come to me that you may want to change the B+ caps for the 
audio and B+ in the audio deck. If you still have the original plug in cans or 
some other old caps, it could get you enough leakage to ripple through the 
IF and audio decks.

How much hum are you getting? Most receivers will give you some hum when 
the RF and Audio are set to max. Run 150 UV of 455 into the IF deck and look 
for -7 volts on the diode load buy setting the Gain pot on the IF deck.

With modulation on you should get 1/2 watt out on the local audio pins of 
the rear terminal board. And 100 Mili watts on the line audio as read with 
the line level meter.


Turn the modulation off and you should get a 30 dB drop in the output on 
the local audio pins. Use an AC volt meter with a dB scale and a 600 ohm load 
resistor to measure the half watt of power and 30 dB down from that point 
with the modulation off.

If the receiver will do this for you, what ever level of 60 cycle hum you 
have is within bounds for the receiver. If your hum hold the noise level up 
so you cannot get the 30DB difference, then you do have a problem and do need 
to do some work to find the problem.

Use the procedure in the Y2K manual or a PDF copy of the TM for setting the 
IF gain pot. Start reading there to set up the signal generator and the 
receiver for the test and adjustments.

Roger AI4NI