Cobra 29 LTD Chrome PLL Issue

[DTB Radio]

I have a strange one on my hands. the PLL is unlocked due to a low signal to the PLL chip from pin 6 of the TA7310 IC. I have tried multiple 7310's with the same result. I have replaced every component in the pin 6 circuit, no luck. Even swapped PLL/mixer chips with a known working 29 and no go. the ones pulled from the failed radio work fine in another 29. I can think of nothing else to check/try. As for clues beyond the low signal, the DC voltage seems to be high at about 6.3 volts (varies a bit from one 7310 to the other, with the lowest seen being at about 4.3). All other signals/voltages seem to be normal, and the other power supply points on the board read normal. Any thoughts appreciated on this one!

 

[brandon7861]

I'd like to know why the service manual says to diddle the VCO slug as part of the noise blanker alignment. And why L1 is part of the RX sensitivity procedure when it is the noise blanker slug. Unless there is a different 29LTD schematic that I haven't seen, I think this part of the procedure should look like this:
If I had to guess, I'd bet someone followed that procedure and, in the process, took the VCO so far off frequency that it fell out of lock.

edit to add internal bias diodes to VCO drawing
I have not looked at the rest of that alignment section.

 

[TM86]

Unlikely, but given your part swapping results may be worth a try. Check for cracked traces along the path and on components adjacent to it even if they don’t look like they’re involved. A hairline fracture at a component pad or a cold solder joint at a board via, that sort of thing.

Good luck and I hope you find the root cause soon.

 

[nomadradio]

Does L22 have a well-defined peak? I have had my share of aggravations with a bad L22. Beware of the slug being dead-flush with the rim of the hole when it appears to show a peak. That would indicate L22's internal capacitor has failed open.

73