COBRA 29LTD NO TRANSMIT

[Alabama Trainman]

I have looked at this Cobra 29LTD. I found the problem at L23 (tx & 2nd IF peaking). I have my 10.24MHz going into L23, but do not have it coming out to go to pin 4 of IC3. I removed L23 and it has checked out ok. I pulled a L23 from a working 29 and soldered it into the radio I'm repairing. I still have no 10.24MHz on the output of L23 . I peaked L23 with my scope and nadda.
Ic3 has tested fine..just no 10.24MHz to mix with the 16+MHz on Pin 1 to give me my 27MHz out on Pin 9.
Has anyone had this problem or have any suggestions? Traces where L23 solders to the board has been checked for a discontinuity.

 

[nomadradio]

Does the radio receive?

L23 feeds into IC3 and into the source of FET2 the second receiver mixer.

If L23 is not doing its job, there should be no receive signals either.

Just trying to get the whole picture.

One last thought.

I peaked L23 with my scope and nadda.

How did you peak L23 if nothing is coming out of the secondary winding?

73

 

[Handy Andy]

It may not be L23 - but the cap to the TX mixer.



Test the L23 to TX mixer line including that cap - be sure the Cap C106 is good - the trace from C106 to TX mixer - check it's voltage - if it's 8 volts - then the IC is shot - the input to the mixer is damaged - no signal can get past that 8V DC power to make it to the mixer.

​

 

[Alabama Trainman]

Chris, the radio does receive. L23 was peaked in the working radio before it was removed. I figured it would be close enough to fire up that way when installed. Andy, I measured IC 3 pins. Was missing 10.240MHz on Pin 4 giving no output on Pin 9. All voltages on IC 3 are correct . Something must be loading down that line. Will check it some more tomorrow and report my findings. Thanks for the help guys!

 

[Handy Andy]

Then it's possible that C106 is broken - I've known that to happen.

Fingers crossed...Good luck!

 

[nomadradio]

Not one I have seen yet.

73

 

[nomadradio]

Or the tip of the 'scope probe is loading down the output side of C106, but not the input side?

Please confirm you are using the "X10" setting of the probe. Makes a difference for this kind of measurement.

73

 

[Alabama Trainman]

Or the tip of the 'scope probe is loading down the output side of C106, but not the input side?

Please confirm you are using the "X10" setting of the probe. Makes a difference for this kind of measurement.

73

I'm using X1 setting. I believe your correct on the probe loading down that part of the circuit. It's evident on IC3 pin 4 as well. My counter toggles when I depress the mic and I momentarily see the frequency displayed before it returns to zero. These readings are from a working unit that I'm using to troubleshoot with.
At the moment, I have lost my oscillator (tr20) on the troubled unit. It's one thing after the other. Meaning now my 10.240MHz is missing from the PLL pins 12/13.
Too much coffee and Murphys law happening at once. Decided to put it aside for a day to work on something else. I will get back to it and update the thread. Andy, problem is not with C106.

 

[Handy Andy]

I see...

You have your hands full, take a break - it's not going anywhere.

To see TX side try TP4 - which is BARE lead of R61 by TR16 - but if no RF, then the issue is between the L23 and Mixer. No signal from L23, then the Mixer can only produce the 16MHz side - which is out of the passband of L21 - so it appears empty.

Also verify the channel selector detent is working - try Channel 1 and Channel 40 to see if any TX - RF can be found.

We'll be here...to help...

 

[nomadradio]

Gotta use the X10 probe. The X1 option connects way too much capacitance to the tip of the probe. Adding a (for example) 200pf cap from pin 4 of IC3 to ground will shunt your 10.24 signal to ground, but only while the probe tip is connected to pin 4.

Really makes it sound as if your transmit signal is getting clobbered downstream from there.

73