Cobra 142 GTL C92 question

[Hawkeye351]

Got an old classic Cobra 142 GTL on the desk.

Issue:
Transmit low in all modes
Receive low in all modes
Controls/switches scratchy
Variable key not functioning correctly

Decided to do a complete recap first. Upon removing the old caps, I noticed the C92 location had a 330uf 16v cap instead of the typical 47uf. This cap does look as though it's newer than the others.

Is there any benefit to increasing C92 from the factory value of 47uf up to a value of 330uf?

Now, I know you can increase the value of the cap next to the AMC Timing capacitor in the 148/2000 for increased modulation, but never heard of increasing C92 in a 142.

While I'm on the subject and in the process of replacing all the caps in this 142, do you guys have any recommendations on certain electrolytic caps in a 142 that can be tinkered with for more modulation, other than C90 (AMC Timing capacitor)?

 

[Hawkeye351]

Well,
I went ahead and replaced that 330uf cap someone had placed in the C92 location, with the schematics value of 47uf. I noticed a slight change in the tone on receive, more natural sounding, and my locals said the tone of my transmit isn't as bassy as before, more natural sounding also.

After I replaced that cap and tested it out on the air, I decided to strap a 22k ohm resistor across the leads of the AMC Timing capacitor (sample and hold cap), C90 and then checked it on the air. Louder and clearer (crisper) audio, every syllable is maxing out the swing. In other words, every syllable, vowel, number that you speak will max out the swing and hold at that max swing the entire time you're speaking, no drop backs on certain words. That's with a stock cobra coffin style handmic.

All caps replaced,
All controls/switches cleaned,
New LED meter lights,
Variable key straightened out,
All previous work (nonsense) removed and put back factory,
No swing mods,
Complete alignment,

Receive now is amazing
Transmit now is great
All functions work perfectly
No heating issues whatsoever
No drifting issues
Excellent radio...

Deadkey varies from 1.5w up to 6w.
AM swing = 22w pep, bout 16w peak, bout 9w average. Regardless of carrier level.
Sidebands = 22w

You gotta love these old classics. Cleanest audio comes from classics.

 

[nomadradio]

C92 is a decoupling capacitor. A "filter" for audio noises that appear riding piggyback on the DC power that feeds the mike circuits. We found that turning down the carrier on this radio would cause a feedback squeal on AM modulation. Boosting the size of C92 stopped it.

Gotta figure other folks figured out the same thing, the same way we did.

73

 

[doffo]

Well,
I went ahead and replaced that 330uf cap someone had placed in the C92 location, with the schematics value of 47uf. I noticed a slight change in the tone on receive, more natural sounding, and my locals said the tone of my transmit isn't as bassy as before, more natural sounding also.

After I replaced that cap and tested it out on the air, I decided to strap a 22k ohm resistor across the leads of the AMC Timing capacitor (sample and hold cap), C90 and then checked it on the air. Louder and clearer (crisper) audio, every syllable is maxing out the swing. In other words, every syllable, vowel, number that you speak will max out the swing and hold at that max swing the entire time you're speaking, no drop backs on certain words. That's with a stock cobra coffin style handmic.

All caps replaced,
All controls/switches cleaned,
New LED meter lights,
Variable key straightened out,
All previous work (nonsense) removed and put back factory,
No swing mods,
Complete alignment,

Receive now is amazing
Transmit now is great
All functions work perfectly
No heating issues whatsoever
No drifting issues
Excellent radio...

Deadkey varies from 1.5w up to 6w.
AM swing = 22w pep, bout 16w peak, bout 9w average. Regardless of carrier level.
Sidebands = 22w

You gotta love these old classics. Cleanest audio comes from classics.

I did the 22k resistor that you did. I must say it made a great improvement! Thank you very much. So far did it for both of my 140 GTL radios, and will do it for my 3 Base 142 GTL radios whenever I get them from Nevada.

 

[Uncle Ronnie 336]

how does that look on scope with the resistor across it?

 

[doffo]

how does that look on scope with the resistor across it?

Unsure as I don't have an O-Scope (Some day hope to get one.) But for now I just limit my SSB watts to 10, and the AMC I also limit to 8 watts. Hoping limiting it that way won't be distorting my audio. Least it sounds fine on the other radio.

 

[Hawkeye351]

how does that look on scope with the resistor across it?

Without retuning all the RF stage afterwards, it may look a little nasty, but not that bad. After a retune of the entire RF stage, cleans the signal on the scope and spectrum analyzer right up.