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 Post subject: 75A-4 K7CMS Audio mod?
PostPosted: Jan Fri 01, 2021 6:23 pm 
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Joined: Mar Fri 29, 2013 4:46 am
Posts: 233
Location: Livermore, CA
Going through a 75a-4 restoration trying to figure out what mods have been, and what hasn't been done. One mod appears to be half done the K7CMS audio mod. Reading the modification by K7CMS he states there needs to be a 470K resistor between C100, and pin 7 V13. There's already a 470K resistor between C100 and ground now is he saying to add a second 470K between the junction of C100, and R75 to pin 7 V13? Just want to verify what I'm reading is correct.

Dave


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 Post subject: Re: 75A-4 K7CMS Audio mod?
PostPosted: Jan Fri 01, 2021 6:30 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 22501
Location: Somers, CT
by William H. Beatty, K7CMS
4721 N. Bamboo Circle
Tucson, Az 85749

IMPROVED AUDIO
This modification removes inverse feedback between the plates of the 1st and
2nd audio stages and changes inverse feedback originating at the output
transformer secondary from the 1st audio to the 2nd audio cathode. Note: a
further improvement can be made by changing to a larger audio output
transformer and by trying different amounts of feedback by varying the
value of R71.
Remove R71 (33k) from pin 3 of V13 (12AT7) and move to pin 8.
Reverse primary leads of audio output transformer (T5).
Remove R109 (390k) and discard.
Add 470k between C100 (.01) and pin 7 of V13 (12AT7).

REDUCED SSB DISTORTION
This reduces the level of IF signal injection to the product detector.
Replace C87 (10pf) with 5 pf.
Add 50 pf in parallel with C99 (100pf).

IMPROVED SLOW AVC
This increased the slow AVC time constant. It also reduces the AVC attack time and allows the AVC voltage to drop instantaneously when switching to
standby.
Remove R90 (2200 ohm) and C112 (.1 mfd) and replace with 22k and .47
respectively, but with the bottom of the .47 mfd connected to junction of
R98
(22k) and R104 (270) instead of to ground.
Connect .001 mfd from junction of R89 (10k) and former R90 ( now 22k) to
ground.
Change C20 (.1 mfd) to .01 mfd.



... the above is a verbatim copy of what was in the article. No diagram
was included.

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Just because it can be done doesn't mean it should be done.


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 Post subject: Re: 75A-4 K7CMS Audio mod?
PostPosted: Jan Fri 01, 2021 9:09 pm 
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Joined: Jan Sat 12, 2008 12:25 am
Posts: 281
Location: Kingwood, Texas
Be careful with these mods. The AVC changes in particular are known to cause IF instability in many receivers.


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 Post subject: Re: 75A-4 K7CMS Audio mod?
PostPosted: Jan Sat 02, 2021 1:45 am 
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Joined: Mar Fri 29, 2013 4:46 am
Posts: 233
Location: Livermore, CA
hbr nut wrote:
Be careful with these mods. The AVC changes in particular are known to cause IF instability in many receivers.


The audio mod, and the SSB distortion mod work great, but the AVC YUCK! pure crap it made the AVC pump so hard that it sounded like someone was flicking ones finger against the speaker cone. I checked then rechecked my wiring, component values 5 or 6 times, reread the instructions 5 or 6 times no joy simply doesn't work. Talked with several A4 owners in the past they all boast about this AVC mod right now I don't get it. Maybe I'll see if one of them can draw a schematic I'm obviously doing something wrong, right now though I'm not seeing it.

Dave


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 Post subject: Re: 75A-4 K7CMS Audio mod?
PostPosted: Jan Sat 02, 2021 1:54 am 
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Joined: Mar Fri 29, 2013 4:46 am
Posts: 233
Location: Livermore, CA
Just a follow up on the K7CMS AVC mod found this on the Collins reflector this guy had the same experience I did with the K7CMS AVC mod. I'll have to try a locate this W1LSB mod and have a look to see what's different.

I put the K7CMS AGC mod into my 75A4. I thought I had
> performed the mod incorrectly until W1LSB published an
> article in Electric Radio with a different mod to
> accomplish the same thing. He made an indirect reference
> to the K7CMS mod. He mentioned that a"popular" mod caused
> AGC bus oscillation which is exactly what I was
> experiencing. That is why I thought I had done something
> wrong. The oscillation is not that noticeable on SSB but
> is terrible on CW which is the mode I use most. I
> reversed the K7CMS mod and put in the W1LSB mod. The
> results were spectacular. The W1LSB mod did everything
> that the K7CMS mod promised without the negative effects.
> So, I recommend the W1LSB mod and I do NOT recommend the
> K7CMS mod. I'm sorry I don't remember the issue that the
> W1LSB mod was in, but it was about 4 years ago.
>
> Now if I could just solve the overloading problems in the


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 Post subject: Re: 75A-4 K7CMS Audio mod?
PostPosted: Jan Sat 02, 2021 6:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 22501
Location: Somers, CT
I installed the W1LSB mod based on reports I read on various reflectors concerning the AGC instability issues with earlier modifications... how much of an improvement did you observe with the SSB distortion and audio feedback changes? I'm debating how far to go with my 75A4 before calling it a day and good enuff.

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 Post subject: Re: 75A-4 K7CMS Audio mod?
PostPosted: Jan Mon 11, 2021 5:13 am 
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Location: Somers, CT
BTW, I am still hearing some attack time artifacts with the AGC mod installed. Most noticeable on Slow AGC; just irritating enough to let me it is there on strong signals.

Pete

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 Post subject: Re: 75A-4 K7CMS Audio mod?
PostPosted: Sep Sat 18, 2021 5:12 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 449
Location: Missoula, MT
Was the question in the original post ever resolved? - - Reading the modification by K7CMS he states there needs to be a 470K resistor between C100, and pin 7 V13. There's already a 470K resistor between C100 and ground now is he saying to add a second 470K between the junction of C100, and R75 to pin 7 V13?

Is the instruction to add a new 470k in series between C100 and the R75/pin 7 connection?


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 Post subject: Re: 75A-4 K7CMS Audio mod?
PostPosted: Sep Mon 20, 2021 11:48 am 
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Joined: Oct Thu 01, 2020 10:48 am
Posts: 91
I have a 75A-3 which I don't think has the audio NFB back to the driver. I think that was a 75A-4 improvement. I'll never forget how disappointed I was with the audio quality of the A-3, the first time I connected a speaker and antenna, fired it up and tuned in a station. Even with a barn door wide RC bridge across the mechanical filter slot, it was pretty bad. Keep in mind this set was not made with SSB in mind. My eventual solution (on the advice of others, in particular W3JN) was to tap the AF gain pot wiper with a 0.1 cap (or maybe .01) and bring the detector audio out in a shielded cable to an outboard p.p. audio amp driving a speaker. I used a Heathkit p.p. 6BQ5 amp. That gave me the big vintage hi-fi sound I was looking for. I pulled the audio output tube to protect the now unloaded output transformer and laid it on its side on the chassis and left the set's audio stages stock. Easy reversible mod. but you have to have a preamp/amp preferably tube (in my opinion).


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 Post subject: Re: 75A-4 K7CMS Audio mod?
PostPosted: Sep Mon 20, 2021 12:35 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 449
Location: Missoula, MT
As it turns out, the instruction is to insert a new 470k in series between C100 and the pin 7/R75. Found it in issue 50 of ER magazine.

I use outboard audio often here too, but thought I'd explore this simple mod anyhow while I have the rig on the bench.


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 Post subject: Re: 75A-4 K7CMS Audio mod?
PostPosted: Sep Tue 21, 2021 5:11 am 
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Joined: Mar Fri 29, 2013 4:46 am
Posts: 233
Location: Livermore, CA
stevef wrote:
As it turns out, the instruction is to insert a new 470k in series between C100 and the pin 7/R75. Found it in issue 50 of ER magazine.

I use outboard audio often here too, but thought I'd explore this simple mod anyhow while I have the rig on the bench.


Glad you figured things out I ended up just using the stock audio configuration to my ears K7CMS mod didn't sound well to me at all. Also didn't care for the AGC changes he made ended up using another mod found in ER's 75A-4 book. In the end I did very little in the way of mods to my A4 I think the biggest improvement was sweeping the IF and getting the IF tweaked visually instead of just peaking via a VTVM.

Dave


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