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

 

[DTB Radio]

Thank you all for the replies.

Nomad, L22 appears to be fine. That was one of the first things I looked at when presented with the unlocked condition, followed by the VCO coil. both of those appear to be perfectly fine. I can adjust the VCO coil to get received signal on any given CB channel, and that tuning range is well between the mic/max adjustment range of that coil. The problem is a VERY low signal into pin 24 of the PLL cip (Cobra's newer PLL IC used in this 2021 model) from pin 6 of the 7310, along with incorrect DC readings on pin 6 of the 7310.

Brandon7861, I'm using the Sams Photofact manual (#217) for general reference, although the 7310 circuit in that differs from the modern ones a bit; for example, in the Sam's schematic, there is no rf choke in the pin 6 circuit, where-as there is a choke in this unit (I already checked/replaced that choke, no go).

I did check for continuity in the traces of that circuit and found no breaks, but it never hurts to check again.

Early in this process I plugged the problem into Google's AI, and it keeps talking about the bias of the pin 6 circuit, and since the DC voltage is NOT correct on pin 6 (based on the Sams manual), that may indeed be the problem. However, I've replaced all the external pin-6 components (as mentioned in the OP) with no change. I do NOT have and brand-new 7310's, but I did install one from a working 29 and had the same issues.

I'll keep you all posted as to results.

 

[brandon7861]

Pin 6 and pin 8 get their voltage from a 5.2v zener. To have 6.3v there suggests the zener is shot.

And although the chrome schematic on CBTricks is missing a trace, the PLL is also powered from this 5v zener. If the zener let 9v through, a CMOS PLL chip (typically with an absolute maximum of 8v) may not be too happy.