Cobra 2010 GTL WX No AM

[DXer]

Hi Everyone,

I have a Cobra 2010 GTL (Same radio as Cobra 2000 but with a face lift) that works perfectly on USB and LSB but on AM mode, it recognizes the mic is keyed but has no carrier and no modulation. It receives on AM no problem just no modulation and no carrier. The SWR/CAL/MOD switch was just replaced when this started. I suspect something on the MOD side of the switch maybe wired wrong as only the MOD portion works on AM mode by design. Any ideas what I should check first?

[nosaj]

Well if you suspect the mod switch is wired wrong I would start there and verify via schematic that is indeed correct.

nosaj
N1EBC

[MDYoungblood]

If it happened when the switch was replaced then my guess is a bad switch, fried a diode or 2, or some solder slag hit the board and is shorting. 3 areas to check.

3's

Greg

[DXer]

I checked and found no solder slag and swapped out with a new switch and have the same issue. Do you know what diodes I should check?

[MDYoungblood]

From what I can see from the schematic off of CB Tricks the modulation components are on a small board marked EPT201040Z, it has VR1 for adjusting the meter, check the components on that board, if any are shorted the modulation won't work and this might be causing the problem.

3's

Greg

[DXer]

I checked everything on EPT201040Z and they were everything checked out fine. I also eliminated the board all together and still no AM TX. I thought it possibly could be IC4. I had a spare new one so I replaced it and I still have no AM TX. USB and LSB works fine. Any other suggestions?

[gunner121]

I believe in those radios if they are indeed like the old 2000/148 models there is a pass(power) regulator for am tx. Ssb is controlled from the band switch on front of radio. Which when in ssb mode the regulator is bypassed. I'll have to dig up the tr location for you.

[gunner121]

I believe in those radios if they are indeed like the old 2000/148 models there is a pass(power) regulator for am tx. Ssb is controlled from the band switch on front of radio. Which when in ssb mode the regulator is bypassed. I'll have to dig up the tr location for you.

[MDYoungblood]

The voltage regulator is bypassed on SSB so that would be a place to look. Sorry don't know the location in that radio but it is a rather large transistor mounted on the side using the chassis as a heat sink, I believe it it a TA-72222.

3's

Greg

[DXer]

Is there an easy way to test it to see if its bad? Do you know what legs should have what voltages?

[gunner121]

Check tr41 in am tx. Should see
E-5.8v
C-13-13.8 volts
b-6.8 volts or so.

[DXer]

Thanks for the info. I have fixed AM and it is now TXing fine. Weird thing is, now USB does not TX. It is locked on frequency 28.310. LSB & AM work just fine. Ugh. Think I have one issue fixed and then another one arises. Any suggestions?

[DXer]

Disregard last post. I found the issue with USB and it works now. I thought I had AM fixed using a dummy load but its not quite fixed. I am able to TX now but it TX's at full power and there is no audio present while TXing. I wonder if I have a wire crossed somewhere but cant seem to figure out where. Any ideas?

[gunner121]

Did you end up replacing the regulator or any other parts?

[DXer]

Yes I replaced the regulator and IC4.

[DXer]

Any other ideas as to what to check? I am stumped.

[DXer]

Nothing?

[gunner121]

Are you able to adjust the carrier with the internal vr?? You are getting zero modulation?

[DXer]

No I cannot. It basically goes full power TX and the audio is not existent. No issues on SSB.

[gunner121]

Check tr42. Also when turning vr10 (am power) see if voltage changes on emitter of tr41.