[Collins] 75A-4 Excessive Audio Level

[GBabin73@aol.com]

I have the same problem with My 75A-4. I pulled My hair out looking for a 
cause and never really got it under control. I had problems with the pot and 
finally got it somewhat under control. The taper of the pot is VERY critical 
and must be audio taper. The slightest resistance (even 50 ohms) is enough to 
drive the audio section to a very high level. It sounds like You have a cold 
solder joint somewhere, or a bad pot. The pots are a little difficult to 
find, due to the thickness of the panel and MUST be audio taper.

Good Luck and 73!
DE N5MCJ (Jerry)

In a message dated 1/1/2003 2:19:01 PM Pacific Standard Time, 
byorslf@earthlink.net writes:


> The 75A-4 that I have fought my way through restoring electrically and
> physically is nearly done but there is one last problem I am in need of
> experience or opinions about.
> 
> This problem is that my audio output to the 270G3 speaker is just too loud
> to be comfortable at anything much above a 1 to 1.5 on the AF gain control.
> Even with the knob at 0 there is still some audio leak through.
> 
> I have measured all voltages on the first, second and audio output
> amplifiers and all voltages are nearly dead on. The one thing I have found
> is that R62, the 500K AF gain pot, while it is 500K across the pot, doesn't
> reach zero ohms at the minimum position, rather it still has about 50 ohms
> of resistance to ground. The actual terminal on the pot is at ground so 
> it's
> an internal condition.
> 
> I suspect this high resistance to ground would be enough to cause the no
> cutoff condition but what have others done to move the range of the AF pot
> operational level higher in the scale?
> 
> Does anyone have any measurements for AC level that I can use to make sure
> I'm not feeding the first audio an excessive amount of gain?
> 
> Thanks, Roger NJ2R
> 



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