[Collins] 75S-3B hum (yes, again)

[Gerald]

On Thu, 2005-06-16 at 10:27 -0400, Craig Roberts wrote:
> My "new" 75S-3B is humming -- and it's not a merry tune.
> 
> I know this topic's been covered before, but -- well ---
> 
> My hum is low level and independent of volume control setting. It's 
> there with the gain all the way down.  My receiver has a new 40/40/40 uf 
> electrolytic can and IF filter shields top and bottom. 
> 
> The hum seems to mimic the vibration of the power transformer.
> 
> The hum MAY have appeared or increased after I performed a mod, i.e., 
> replacing the inductor L-16 in 75S3-B with an 820 ohm resistor to reduce 
> audio distortion (as suggested in a Reflector post some time ago).    
> I've noted on the schematic that the original RF choke was magnetically 
> shielded.  Could this have done it?
> 
> Many thanks and 73,
> 
> Craig
> W3CRR
With the volume control full down, hum ordinarily can't come from the IF
filters. The noise coming from them was more a hum modulated noise
anyway.

First question is whether the hum is 60 or 120 Hz. Makes a difference in
the troubleshooting.

60 Hz hum often comes from heater to cathode leakage in a tube. A common
failure mode. That hum will be sinewave. Would only be the two audio
tubes. One or both.

In the 75S receiver, 60 Hz sawtooth hum can come from the bias supply.
The selenium rectifier wasn't all that great when it was new and now its
positively leaky and can work the filter capacitor hard. Not in a can.
Late production ought to have a silicon rectifier instead of the little
rectangular block with terminals. But that rectifier and filter can be a
source of 60 Hz sawtooh ripple.

120 Hz sawtooth hum comes from the B+ supply. Sometimes new
electrolytics aren't quite new.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer