148 GTL RX but no TX

[jimbowilly]

Hi,
I have a 1993 Cobra 148 GTL I can receive on just fine. But I cannot TX at all.
The finals have this chalky color around them both on the heat sinks.
The previous owner told me the radio just stopped TXing one day while talking to another radio.
Any idea what I should be testing for on the circuit board?
Thanks
Jimbo

[420Snowman]

That chalk color around the finals is probably the dielectric grease that is factory installed, that'd be normal I think. Does the radio key at all? I mean does the needle move when you key it?


Snowman
Sent from my HTC mobile.

[jimbowilly]

Yes the needle does move when I key up.
I just don't hear anything when TXing.
Now I'm told the previous owner may have connected the power cord backwards.
I would think there would be more wrong if the power cord had been run backwards. "?"
All and all, I think I have a mic problem. It is a battery operated five pin power mic.

[TheCBDoctor]

Yes the needle does move when I key up.
I just don't hear anything when TXing.
Now I'm told the previous owner may have connected the power cord backwards.
I would think there would be more wrong if the power cord had been run backwards. "?"
All and all, I think I have a mic problem. It is a battery operated five pin power mic.

Hi,

Does the radio transmit on sideband? If so, do you hear the audio receive? If you do not then the audio chip TA-72222 is popped. That is what happens when a radio is hook up backwards and the polarity protector is opened.

The radio throws a carrier but does not modulate because the audio chip modulates the carrier on AM. On sideband the audio chip is not used for modulation. The modulation is produced at the balance modulator on SSB.

If these are your symptoms, then the audio chip is N/G, and the polarity protector is probably open. You will need to replace both of these components.

Respectfully,

[jimbowilly]

Rick,
The radio has no TX on LSB or USB, or AM settings.
Dead key produces 2 watts on AM setting. No indication of carrier on LSB, USB.

Balance modulator and polarity protector are not components I have had to work with up until now.
And have no idea of their location, or what component(s) is used for their intended purpose?
I'll post a good clear photo of this radio's pcb this evening after work.

Thank you,
Jimbo

[jimbowilly]

Took my wife out for her birthday last night.
Didn't get a chance to post the photo .
I'll get on it after work.
Jimbo

[jimbowilly]

Ok.

I found the External Speaker Jack was the problem. The solder point between the external speaker jack and the pcb had worked loose.
I noticed a short while I was listening to the radio using head phones an a dummy load.
I re-soldered this joint and wha-lah! Instant TX once again.
I looked over the pcb on and off for 20 hours looking for a short.
This Cobra 148 GTL with extra channels is a really great radio.
Wouldn't you know it was something simple.

Jimbo

[TheCBDoctor]

Ok.

I found the External Speaker Jack was the problem. The solder point between the external speaker jack and the pcb had worked loose.
I noticed a short while I was listening to the radio using head phones an a dummy load.
I re-soldered this joint and wha-lah! Instant TX once again.
I looked over the pcb on and off for 20 hours looking for a short.
This Cobra 148 GTL with extra channels is a really great radio.
Wouldn't you know it was something simple.

Jimbo

Hi jimbowilly,

It is always something simple after you spend 10 hours fixing a radio.

The next time you will know what to look for and have it done in no time. I used always tell my customers that I don't charge by the hour I charge by the knowledge. The 5 minutes it took me to fix that radio took me 2 hours the first time I saw the problem in another radio.

I am glad it worked out for you. Remember, every repair is simple; you just have to figure out what is wrong with it.

Respectfully,