[Collins] 75A-4 Excessive Audio Level

[Jim M.]

Looking back at copies of some 75A4 "modificatio" articles from various
old magazines, a couple caught my eye.  Could a previous owner have made
one or more modifications suggested by various authors in the past? 
Suggest you carefully compare the underchassis wiring with the original
schematics in the area of the AF stages.  For example, here's a
modification to "improve" audio response from April 1970 Har Radio
Magazine.  Quoting from the article written by W6ZO R.F. Rinaudo:  "...I
became annoyed by an emphasis in the bass of the audio response ....  A
surprisingly high peak appears at 100 Hz...  P. Rockwell W3AFM November
1966 QST suggests that feedback resistor R71 be removed...I tried this
but I didn't like the increase in audio, which required riding the AF
gain control..."  So here is at least one old modification that could be
causing your audio overload if a previous oner did it.  There may have
been others.  Check your component values and wiring to see if any
hacking occurred in the past life of your 75A4.

Roger Shultz wrote:
> 
> 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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