Problems with Cobra 148GTL CB radio
 

wrote...
Ok--I solved the problem with the radio!

Yes, it had been modified. Someone grounded pin 10 of the PPL chip and
fiddled with the VCO for above-band operation. I removed the jumper on
pin 10 and then used a frequency counter to realign the VCO.

Realign the VCO using a DMM, rather than a frequency counter.
Use the frequency counter when aligning the crystal oscillators.

As it had clearly been got at, I would take the time to check the frequency of each crystal
oscillator and the carrier balance. Depending on which version of Cobra 148GTL you have, there
could be a few crystals.
Even more important, check the output and drive bias settings. If these are not set correctly,
the transistors may not last long.


Regards,

Peter.
http://www.citizensband.radiouk.com/

Problems with Cobra 148GTL CB radio
 

Peter wrote:

Realign the VCO using a DMM, rather than a frequency counter.
Use the frequency counter when aligning the crystal oscillators.

As it had clearly been got at, I would take the time to check the frequency of each
crystal oscillator and the carrier balance. Depending on which version of Cobra
148GTL you have, there could be a few crystals. Even more important, check the
output and drive bias settings. If these are not set correctly, the transistors
may not last long.

Peter,

Thanks for the suggestions. Your advice is more constructive than
some of the previous posts, some of which are little more than ad
hominem techno-babble.

Actually, I used my oscilloscope and volt meter, as well as a
frequency counter, to realign the VCO and other components. (In the
interest of brevity, I didn't mention this or other details of the
repair in my previous posts.) Also, I poked around the final output
amp and modulation circuit, and found nothing notable, although I did
elect to reduce the carrier level a bit.

-Dave, K3WQ