cobra 148GTL receives on AM/LSB but does not transmit on LSB

[alexandre9099]

Hi,

I have an old cobra 148GTL which seems to be working fine on RX side, but seems broken on TX for sideband (IIRC AM transmits fine).

What can i do to troubleshot this? I have checked PLL voltages and all those seem OK according to this schematic [Please login or register to view this link]

EDIT: oh, and one of those ferrite things on the input power is broken in half and the other has the wire all blackened. The light dims a little bit when i PTT but the current on the input never exceeds 1A at 12V (the PSU i'm using at home is 12V 2A)

[MDYoungblood]

Is it both USB and LSB not working or just one. Old radio most times means dirty contacts, possible the mode switch. It could be a cold solder joint or broken wire at the switch or follow the wires down to the board and check there. Last is the adjustment points are dirty or broken, the loading coils on the transmit circuit in old radios do get brittle with time.

3's

Greg

[alexandre9099]

LSB at least works fine for RX so i guess that the rotary switch is good. I have no idea about USB cause i don't know anyone with a cb radio that supports USB and i find no one transmitting there.

Maybe i can try to transmit AM on a known channel where people are on LSB, but that is not a good thing to do and would be the last thing i would do.

The meter seems to go "low" when i key PTT and then jump when i release it. On AM with SWR or CAL selected the meter shows values between 1 and 2

[51]

ok so to get this straight. on AM the radio transmits and receives properly, on USB/LSB the radio receives properly, and it does transmit but it has no rf output. is this correct?

[alexandre9099]

I am not sure on AM RX cause i haven't had anyone talk to me yet, on TX i know it works cause the meter moves (a lot more than on ssb) and a friend of mine had a radio with RX only and said that he got sound on his end.

About LSB, i can hear (maybe only stations that are close to me, not sure about distances, sometimes someone near me is talking to other guy but i can only hear the one that is closer) but it seems like i can't transmit cause no one hears me.

Sorry for being so vague but i have no other means of testing it

[51]

its hard to give you a specific diagnosis and repair when your symptoms are not clear. assuming the AM transmit and receive is fine, and the SSB receive is fine just no transmit rf output, the first thing i would check is the alc circuit control. whoever went in there messing around (assuming) probably turned the alc pot all the way down. find the alc pot(vr11) and turn it up, then check output. if that is the cause re align transmitter to specs, done. let us know what you find

[MDYoungblood]

I would agree with 51, go ahead and do a quick realignment. you can find some help on that on the CB Tricks website. I might be just a dirty contact, going through a realign would break the bad spot. I would suggest getting a second radio for your bench if your plans are to work on radios, it will shorten the work time on repairing, you can hear and see what is going on.

3's

Greg

[alexandre9099]

So i got a RTL-SDR and it seems to transmit ok, though on AM there is LOTS of harmonics on adjacent frequencies (i'm not really willing to use AM so... yeah, not that bothered with it), on SSB i can hear my voice just fine, but i'm still not sure to why others can't hear me. I think this is due to a little offset of 192kHz (not really sure how to test this the right way or even if the RTL-SDR is acurate enough for this, i just went up and down on gqrx and the final offset to 27.205, channel 20, was of 192kHz).

Hopefully next week i can get together with a ham radio guy which might be able to help me tune the radio, meanwhile is there some pot i can try to adjust to change the offset? The vr11 that was sugested did no actual difference (i still got no answer, so i left it how it was)

[51]

you are seeing all those harmonics and spurious emissions because the amc is cranked wide open, i almost guarantee it. without the proper test equipment there is nothing you can do that will resolve your radio issues. just randomly turning things, not a good idea. the radio needs a full and proper alignment as well as an operational checkup, bottom line. hopefully your ham radio friend can help you with that.

[jjhhff9]

check the regulator q41 and see if it is good