Cobra 29LTD no RX and TX

[dss56]

Seems the radio just quit RX and TX

I get volume
every thing works squelch rf gain but no tx and rx.

I tried a radio keyed next to it and nothing.
The 29 meter is seeing static noise but when I key the other radio the 29 meter goes back to zero.
I have green light and red light when i transmit but no output.
Radio was working perfect si I know all the cans should be ok it has to be a transistor, Fet, or cap.

Tried many test and nothing.

Any suggestions on where to look or what part it might be. The pll has all its voltages.

So this one has me puzzled.

thanks

 

[kaos513]

Test your final and driver

 

[dss56]

I did both are good and would that stop the receive from working, I can see if shorted but both show good. I feel its in a mixing stage where receive and transmit come together but just cant seem to find the problem. Most likely radio will end up as a parts radio soon LOL

 

[Handy Andy]

When I come across these, I usually tap the area around the Audio Chip - see if I get any buzz.

You seem to have audio, but do you get audio to the TX side,

Any voltage checks? Jp36, for TX, JP29 for TX BIAS to predriver/Driver...

TP 3? Any thing - sort of noise when you clip a lead to check...

Have you looked for "blown traces" and or parts along the Output coil and traps? The SWR bridge goes here too, if this opened - that would easily kill RX just as well as TX.

Also a "dead shorted part" on the Driver and or Final - would also deaden the RX side too...

To get RX and TX, you should also check PA and Ext Speaker jacks - never know if the mod transformer quit - goes back to checking the JP36 and JP29 areas for True TX voltage - not just switching.

 

[Dmans]

The 29 meter is seeing static noise but when I key the other radio the 29 meter goes back to zero.

PLL or Local Oscillator (tuning coil) fault???

73
David

 

[RoadRanger]

Just like Handy Andy Say's hit it with a hammer & show it who's boss.
Sorry that's all I got.

 

[nomadradio]

10.240 MHz crystal with the corrosive brown glue on it with one lead wire rotted away.

73

 

[Handy Andy]

@dss56 - hows the patient?

We've given a lot of suggestions and you even said...

The 29 meter is seeing static noise but when I key the other radio the 29 meter goes back to zero.
I have green light and red light when i transmit but no output.
Radio was working perfect si I know all the cans should be ok it has to be a transistor, Fet, or cap.

Tried many test and nothing.

So when you say you have a "Green and red light" - well that means you have the PLL working and it's Local OSC is oscillating. And that it is in Lock else you wouldn't have any form of toggling going on.

So we suggested various methods, how is progress?

When this condition you posted, happens, the easiest of methods to try; is to see if the PA works, if not, then it's most likely problem is in the Audio path, from the PA/Mic amp section on thru the main Audio Amp into the Speaker. Something failed from the either the Detector - into the Volume control - Squelch - even Mic wiring as simple as a broken wire or loose hardware can cause this condition - speaker seized up - onto corrosion and moisture entry ruining your day.

"Seeing static noise" may indicate loose hardware or broken antenna connector lead wire - even getting and seeing a signal but the audio detected from it - isn't getting to the speaker - lots of guesses but we don't know if you've even worked on the patient to see if it's still breathing...

 

[dss56]

Thanks for the replys.

So the radio has plenty of audio volume works and the meter shows some activity but not any true receive. There is no output either.

When I key up another radio the 29 meter will drop back to zero un key other radio and the 23 meter will move up some again not true receive. I talked into other radio and no sound on 29.

I will check the voltage final and pre driver jumpers and report back of what I find.
I checked the 5v regulator and its good, I get led channel display and voltages seem to be ok on the pll chip.

 

[dss56]

Just checked JP29 and it gets 8v when mic is keyed.
J36 14.5 volts and when keyed jumps to 17-19 volts like it fluctuates
I did change TR20 near L23 and still nothing

Radio pa works great plenty of audio.
Radio has plenty of up down audio loud.
Rf gain works
Squelch works
everything works but no transmit and receive.


I tried another 10.240 xtal nothing.

thanks again for helping.