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Cobra 148 GTL usb only

E

eldorado303

Member
Jun 17, 2014
15
1
13
45
Hello everyone. I am fixing a Cobra 148 gtl that had some problems.
Some issues are fixed, but some are still there.
The radio has reception on all freqs and modes, no problem.
The radio only transmits sometimes on USB, full 15 watts,
no audio problems, and when tx's it is not intermittent.
No tx on AM or LSB. I have checked the mode selector for broken
connections, and they all seem ok. Now, for the parts I have checked.
Driver and final test ok. Bias regulator also ok. Balanced modulator is ok.
tr41, and tr42 are also ok. Voltage regulator mb3756 is ok, carrier oscillator,
tr32, and am switch, tr31, also ok. Tr26, amx, also ok. TA7222ap, af amp
also ok. Right now, I dont have Bias current on TP7 and TP8, driver bias,
and final bias. There seems to be a short somewhere, or an open component
maybe, but I just dont know where. Also, driver and final collector voltages on tx,
seem a little low if I remember correctly, but ok on rx.
Any ideas are welcome.

Thanks in advance,

Joao G.
 

Robb said:
I would first check the AM/LSB diodes in the loop osc circuit to see if any had failed.
Then, I would check the mode selector switch/wires
Click to expand...

Thanks for the answer, Robb. The diodes, are ok. I then checked why
tr38, driver, wasnt getting collector voltage on tx, since tr41, had voltage on
the emitter ( AM mode ), and followed the connection to the mode selector
switch. The voltage came in, but did not came out through the other wire,
to TP8, and next to tr38 collector. It was a faulty mode selector switch.
I shunted the connection, and got AM power. Just need to check out
USB and LSB wiring, surely enough it will also be the faulty mode selector
switch. I have to open the switch and try to fix it, or try to get a new one.
Maybe I'll create a new post asking for a 148 gtl mode selector switch, just in
case I cant fix this one.

Best regards,

JG
 

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