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

[Jim M.]

What about a filament to cathode short in a tube? I have seen that cause hum before. 
------Original Message------
From: Bill Hawkins
Sender: r-390-bounces at mailman.qth.net
To: 'Barry'
To: r-390 at mailman.qth.net
Subject: Re: [R-390] 60 Hz Hum Output from R-390A IF Module
Sent: May 8, 2009 11:40 PM

Barry - N4BUQ,

Only if they were radiating hum within the cable harness. Which they do,
but the high impedance (>>>50 ohms) audio lines are shielded from that
effect.

Heater current through the ground extension wires causes the problem.
The current is about 2 amps. Say the ground resistance is 0.010 ohms.
If the detector/limiter generates a very quiet zero volts, the audio
module will see 20 mV of 60 HZ at its input. That's audible hum, which
should go away if the extension harness is not used, or the heater
current is removed from the ground extension wires.

Bill Hawkins
 

-----Original Message-----
From: Barry
Sent: Friday, May 08, 2009 7:25 PM

Could the heater lines be run through shielded cable in the extender?
Just a thought.

Barry - N4BUQ

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Jim