[Boatanchors] SX-122 hum and noise

[WA5CAB at cs.com]

I don't know anything about the SX-122 but assuming it is tube based, a 
technique I've found useful in military sets with hum problems is to start at 
one end of the audio chain and ground the grid.  Then move to the next or 
previous stage and repeat.  If shorting the grid kills all of the hum, then the 
problem is upstream of that point.  If it only reduces is, then there is 
more than one source and you should start at the output amp and work back 
locating and fixing the problems.  Fortunately, civilian sets seldom have tight 
wiring harnesses with decades old MFP slathered over them to contend with.

In a message dated 3/25/2010 9:45:33 PM Central Daylight Time, 
bjtatum1 at att.net writes: 
> Hello-
>     I am trying to root out two types of objectionale noise coming out of 
> audio channel on mySX-122. The receiver has a  new filter cap, that I have 
> ruled out, as there is no ripple on the B+. I recently installed shielded 
> leads on the AC power run from rear panel to front panel on/off switch. It 
> did not seem to help.
>         1. A constant hum is present in the audio, it is not too terribly 
> objectionable but quite a bit more than any of my other tube receivers.
>         2. A lower pitched hum is added in when the BFO is turned on, and 
> this hum is noticably louder. The hum does not change in any way as the BFO 
> is tuned.
>     I believe hum in the audio has been a common problem with these sets, 
> does anyone have any advice or info?
>                                                         Thanks, Byron.  
> 

Robert & Susan Downs - Houston
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