New 148gtl : No AM Audio

[sdmahr]

First of all, I know all about the new 148gtl and the reputation it has. I got two N.I.B. units for nothing......so I'm experimenting to make them "work" as they should.

One radio actually works quite well, but the other works well only on SSB (Upper and Lower) but on AM it puts out very little power, no audio, and a "Hum" on my listener radio. The S-Meter doesn't swing when keyed either.

Any clues where to start ??

 

[Robb]

AM Regulator would be my first blush guess.

 

[sdmahr]

Did that already...........

Keep the suggestions coming ! !

 

[sdmahr]

No more recommendations?? The radio performs surprisingly well on SSB.....but will not transmit in AM.

I did replace the AM power transistor..............same condition. Microphone checks out o.k.

No power output, no S/RF movement when keyed.

Any other ideas GREATLY appreciated !

 

[Robb]

SSB uses a different power flow to the finals than AM does.
That should be a useful clue.

 

[redlght]

Check TR41 and 42 and do you have receive audio if so the audio amp is good then check TR36 and 38 should see 12 volts on tx in SSB mode and 6 volts on tx in AM mode

 

[sdmahr]

Radio DOES HAVE AM receive so I'll check TR36 and TR38 and report back.

Thanks ! !

 

[sdmahr]

So I checked out the voltage at the finals........


TR36

It has 0V on the pin it should have 7.2V on during X-mit

TR38

It has 0V on the pin it should have 7.2V on during X-mit

This thing swings almost 20 watts on SSB so the finals are working properly in the SSB mode.

The power transistor for AM is functioning properly. Seems like it isn't being "switched" to supply the finals in AM Transmit. Can anyone point me in the direction of how this 7.2V gets switched to the finals and what component I should check next ??

 

[sonoma]

here is the schematic for the later version of the 148. should be the one you need and can trace the schematic for what parts you need to check.

https://downloads.cobra.com/CB/Models/148-SERIES/148_GTL_Late_Model/148GTLschematic.pdf

 

[sonoma]

check IC4 a 3 legged am regulayor for TX a 7809 used for am TX.

 

[RatsoW8]

I hope you get them straightened out. I'd venture to say that these radio succumbed to the golden screwdriver of some bone head self proclaimed "CB Tech" before you got them for free. Free is good and the 148 GTL is a good radio that's been around for a long time.

 

[unit_399]

SDMAHR-

You never mentioned if you checked TR41 & TR42 as Redlight suggested. My best guess too.

- 399

 

[9C1Driver]

So I checked out the voltage at the finals........


TR36

It has 0V on the pin it should have 7.2V on during X-mit

TR38

It has 0V on the pin it should have 7.2V on during X-mit

This thing swings almost 20 watts on SSB so the finals are working properly in the SSB mode.

The power transistor for AM is functioning properly. Seems like it isn't being "switched" to supply the finals in AM Transmit. Can anyone point me in the direction of how this 7.2V gets switched to the finals and what component I should check next ??

That radio has been fiddled with for sure if you are seeing 20 watts pep on SSB. The new 148's cannot keep up with that much power, I would back the SSB peak down to 10-14 watts or it will sound like crap.

Once you figure out what is broke fix that part, then give it a factory alignment and a very conservative tune. After that you might luck out and sound average at best with it on the air. Free is free though so what the heck I guess.

 

[sonoma]

SDMAHR-

You never mentioned if you checked TR41 & TR42 as Redlight suggested. My best guess too.

- 399

I thought by now he had checked tr41 and 42. OH well

 

[sdmahr]

Voltages on TR41 and TR42 are not correct. The schematic / service manual and my radio don't match completely.........trying to figure out what is what.

Thanks for the inputs...........I gotta step away for a while and take a break.