So i have a nos cobra 148gtl, when i powered it up receive is fine, on transmit there the red and green tx/rx light blinks back and forth super fast so it looks amber. Final and driver transistors are fine, mic is fine, alignment is fine. Tried it again today, same symptom but now the transmit died. When i tx no red light. Is there a switching transistor or a cap that is responsible?
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Cobra 148gtl transmit died
- Thread starter groundwire
- Start date
I would start with TR35 and make sure D43 (I think that's right) is good.
Having not seen it said, I am just going to throw this out there.......
Check the mic connector wiring.
If the switching wires in the pins had any problems...and maybe just a whisker or two making at all.... that could also cause an unstable "switching" situation....and perhaps the wire just finally broke loose.... so no more transmit.
Like I said... I just hadn't seen any mention of the mic......
Check the mic connector wiring.
If the switching wires in the pins had any problems...and maybe just a whisker or two making at all.... that could also cause an unstable "switching" situation....and perhaps the wire just finally broke loose.... so no more transmit.
Like I said... I just hadn't seen any mention of the mic......
Ok. with that....check the CB/PA switch too.
That has to be good and solid in CB because the ground from pin 5 of the mic goes to the emitter of TR35 that 999 mentioned which serves to turn TR35 on....
if that ground doesn't get to the emitter..... that radio won't transmit.
JOining up with 999's assessment here........
TR35.... the emitter has to be pulled to ground.....
AND the base has to be "on"...... so that the collector of TR35 is pulled LOW to enable transmit. If you key up.....TR35 collector should be pulled low.....
If it isn't.... you found the area you should be looking in.....
That has to be good and solid in CB because the ground from pin 5 of the mic goes to the emitter of TR35 that 999 mentioned which serves to turn TR35 on....
if that ground doesn't get to the emitter..... that radio won't transmit.
JOining up with 999's assessment here........
TR35.... the emitter has to be pulled to ground.....
AND the base has to be "on"...... so that the collector of TR35 is pulled LOW to enable transmit. If you key up.....TR35 collector should be pulled low.....
If it isn't.... you found the area you should be looking in.....
A little more info........
Key the mic..... TR35 emitter should go to ground.
If it DOESN"T go "dance" on the CB/PA switch and see if it is dirty/broken...because the ground from mic pin 5 isn't making it to the emitter.
If it DOES....then look at TR35 collector.... it should be low.
If it WASN'T LOW....then either TR35 is open OR the base is low/grounded which says.... The PLL is unlocked OR the channel switch thinks you have an illegal setting selected.
If it WAS Low... you should be transmitting.
Key the mic..... TR35 emitter should go to ground.
If it DOESN"T go "dance" on the CB/PA switch and see if it is dirty/broken...because the ground from mic pin 5 isn't making it to the emitter.
If it DOES....then look at TR35 collector.... it should be low.
If it WASN'T LOW....then either TR35 is open OR the base is low/grounded which says.... The PLL is unlocked OR the channel switch thinks you have an illegal setting selected.
If it WAS Low... you should be transmitting.
mine ended up being a bad switch, the volume power on. Had to take it to Larry's CB shop to find it.
When TX mode, but RX then comes up - TR35 or MB3756 is at fault.
TX is not holding up - so what could drop the 8 volts regulation - except for either a TX issue, which now has modified to NO TX - or you have a failed cap (or mod) that works TX side - now could have damaged TR35 and or MB3756 regulator.
Either way, whatever caused this failure - has now tripped the MB3756 into fail. NO TX.
TX is not holding up - so what could drop the 8 volts regulation - except for either a TX issue, which now has modified to NO TX - or you have a failed cap (or mod) that works TX side - now could have damaged TR35 and or MB3756 regulator.
Either way, whatever caused this failure - has now tripped the MB3756 into fail. NO TX.
I thought the same but my issue has come closer to the surface of my remembering. 
It started out as no xmit and became no power on with a sticky volume switch. Then Larry fixed it by replacing the volume/power switch. IIRC. I do KNOW there was nothing else done because Larry lets us watch him fix stuff.
It started out as no xmit and became no power on with a sticky volume switch. Then Larry fixed it by replacing the volume/power switch. IIRC. I do KNOW there was nothing else done because Larry lets us watch him fix stuff.
Found out decades ago the people wanted to see what's being done. Took some getting used to the first year or so, but it got me customers the other shops didn't get.
73
Hmm, well, Mic KEY for TX - between Pins #'s 3, 4 and 5 - then you have RX light returns.
You won't hear RX, unless you unkey - to physically restore the speaker ground.
So the TR35 "quitting on you" reference, look at what your Mic PTT has to use, Key thru - to work...
Do you have the 5-pin or 4- pin?
5-Pin...
And if you use a 4 pin, the Mic wiring is similar to any Galaxy Cobra 4-pin
So, I'll keep this focused on the Jack (Radios' Mount) side.
Ok, Now ...
No, it's not Fine - anymore - possibly you damaged something - look at your TX side - any mods, wiring mistakes MOSFET conversions not followed correctly?
Relay for Amp not wired right? How? Bad wiring? Possibly, even may have a Blown (Relay spike) diode now - so now has spiked MB3756???
Look above, that TX Pin 5 that shorts to Pin 4 "common" heads to CB/PA switch that then hooks up to TR35 - or pulls everything else to ground so all you have is Receive.
The only reason you still have Receive is due to TR35 and the MB3756.
They kept the rest of the radio from shutting down.
But, if (and since) you had it switching that fast - you may have damaged not just TR35 - but took out the TX control line for the MB3756 rendering it unable to toggle.
4-pin Cobra is similar - so you just go Pin 3 to CB/PA switch and Pin 1 becomes (The Radios') everyone's "common" Pin 4 now is speaker Return. Pin 2 no longer is SHIELD - that is now AUDIO - Pin 1 is the Shield and Common.
It's solid state sir, not indestructible.
You won't hear RX, unless you unkey - to physically restore the speaker ground.
So the TR35 "quitting on you" reference, look at what your Mic PTT has to use, Key thru - to work...
Do you have the 5-pin or 4- pin?
5-Pin...
And if you use a 4 pin, the Mic wiring is similar to any Galaxy Cobra 4-pin
So, I'll keep this focused on the Jack (Radios' Mount) side.
Ok, Now ...
No, it's not Fine - anymore - possibly you damaged something - look at your TX side - any mods, wiring mistakes MOSFET conversions not followed correctly?
Relay for Amp not wired right? How? Bad wiring? Possibly, even may have a Blown (Relay spike) diode now - so now has spiked MB3756???
Look above, that TX Pin 5 that shorts to Pin 4 "common" heads to CB/PA switch that then hooks up to TR35 - or pulls everything else to ground so all you have is Receive.
The only reason you still have Receive is due to TR35 and the MB3756.
They kept the rest of the radio from shutting down.
But, if (and since) you had it switching that fast - you may have damaged not just TR35 - but took out the TX control line for the MB3756 rendering it unable to toggle.
4-pin Cobra is similar - so you just go Pin 3 to CB/PA switch and Pin 1 becomes (The Radios') everyone's "common" Pin 4 now is speaker Return. Pin 2 no longer is SHIELD - that is now AUDIO - Pin 1 is the Shield and Common.
It's solid state sir, not indestructible.