Cobra 142 GTL AM output low

[Hawkeye351]

Got a friend that's having an issue with very low modulation output on AM, but very loud on both SSB and PA modes.

SSB is around 20w peak, which sounds about right. And the modulation is loud.

PA mode modulation is very loud also.

But AM Modulation is very low, only bout 8w peak.

1. He swapped out the 1419 passthrough with an NTE-152.
2. Replaced all electrolytic caps in the audio chain with same UF values but increased voltage ratings to 25v.
3. Cleaned the mode control and modulation control, along with the AM Modulation adjustment inside.
4. R153 is 10 ohm, not 12 ohm.
5. R104 was clipped and still low AM Modulation. Replaced it after testing.
6. Checked voltages on the TA7222AP Audio IC.

Any suggestions I could pass his way? He's learning, and listens very well to instructions.

 

[nomadradio]

One thing nobody mentions when describing trouble in a Cobra 2000 is how old the radio is. Forty-five years old, or maybe "only" thirty years old? Can make a difference, but that date is not printed in any open format. Figuring it out from the serial number and codes seen on parts inside would be the subject of a whole separate thread.

Electrolytic caps deteriorated from age are a common cause of audio troubles, but the AM and SSB mike audio travel through the same capacitors.

But only up to a point. C174, C175 and C179 are in line with the AM transmit audio, but NOT sideband.

A 'scope probed at IC6 pin 9 and at the base terminal of TR42 should reveal mike audio at least 12 Volts peak-to-peak. Odds are that this is falling short. Question is why.

And searching for an audio fault without a 'scope is something I never had to learn how to do, thank goodness.

73

 

[Hawkeye351]

Thank you Nomad for popping in.

Is that C174, C175 and C179 for the 142 GTL?
If it's for the 2000 then I can compare the 2 schematics and figure out the difference.

 

[nomadradio]

Whups! My bad, I really was looking at the 2000 diagram, not the 142.

Those same three caps in the 142 would be C113, C118 and C119.

Musta been a long day.

73

 

[Hawkeye351]

Yea, I know how you feel, lol...
I looked over both schematics and found them. I told him to replace those, and also C114, not sure if it would cause this issue, but why not, lol...