Wierd Cobra 29 problem

[Moleculo]

I've got a new 29 that is exhibiting very wierd behavior. I'm hoping some of you techs can point me in the right direction for troubleshooting so I don't have to do this from the ground up...I don't have the time.

When the radio is working, it works GREAT. I've put it on my bench which includes a very sensitive freq counter, o-scope and sig generator.

Here's the problem:

The radio is off frequency by just a little until it warms up. I just chalked this up to normal CB build quality. However, after warming up, it had a problem where it would randomly switch back and forth between frequencies, all on it's own. For example, if I'm monitoring 27.285, it will all the sudden switch to 27.275 (rounding off here) It would always switch down. I inspected the radio real close, and found a solder blob that was shorting out a couple of resistors and a cap that was part of the PLL circuit. I didn't look at the schematic yet to see what this part of the circuit does. It looks like the solder blob had been there from the factory, or at least a very long time. I removed the solder blob, but now the radio works for a while, then all the sudden TX & RX just quits. No carrier, no RX, no nothing. Turn the radio off, after a few minutes it comes back like nothing is wrong.

Anyone ever seen something like this, or have an idea on where to start troubleshooting? I'm hoping for something more difinitive than "look at the PLL", like maybe someone has seen this behavior before.

Moleculo

 

[Switch Kit]

I had a 25 LTD Classic a while back that had simlar problems to your 29 , it wasn't new out of the box though , but it was in very good condition. I honestly can't tell you why but it turned out to be a faulty TA7310P .......good luck to you.

 

[Switch Kit]

I personally don't know why this is ? but I've had a few 25s and 29s over the years that had what looked like 1/4 of the regular sized 10.240s in them , I only needed to find this out on one of the radios before figuring out why the others would do the same thing ,meaning that you really couldn't get anything out of them as far as power was concerned. OH they worked just fine ! but you weren't going to get anymore more then 4 watts out of them with very little modulated swing , IM guessing it was either cost cutting or FCC had something to do with it ? but these quarter sized 10.240s only would allow the radios so much concerning power , everything else would work OK , so just changing these baby 10.240s to the regular sized ones would bring these radios back to life as far as showing some decent power was concerned. I always thought if it were a FCC thing ? they would have just kept the smaller crystals in the radios ? Go figure .......but I do remember in the early 90s that there was talk of them making these cb radios harder to modify as time went by , well , other then seeing those smaller 10.240s and the goodbye's to the Grant XLs and LTs and the not as good as they used to be 148s ......I personally didn't see that much change in cb's , but it seems like a very big percentage of cb radio operators use 10 meter converts anyway . At least the newer schooled cb'ers.

 

[DTB Radio]

If you can, when it drops out next time, see if the 10.240 is still oscillating at roughly normal level. If not, touch up joints and see if that helps. If not, replace it. if none of taht helps, you'll need to check voltage/signal levels on the PLL chip when its down and compare them to when its up.

 

[Moleculo]

OK, thanks for the replies. The trick on this one will be to catch it when it does it again to troubleshoot...

 

[loosecannon]

switch kit,

i may have misunderstood you, but are you saying that the radios with the smaller sized 10.240mhz reference crystal put out less power when tuned up than radios with the bigger sized crystal?
and that the difference in the crystal size was the reason?

im certainly no expert, but this statement raises one of my eyebrows.
this crystal is only used to create a signal for the PLL circuit to work with.
i cant see how this crystal would have any bearing on the TX power output.

if there is something i am missing here; someone please explain it to me. :?

loosecannon

 

[Switch Kit]

this would mean we are both are missing something here ? As I said , the radios worked OK , they just would not tune up and had very little modulated swing after all was said and done to the radios , after the cyrstals were changed to regular sized ones , the radios now did what they were pretty much known to do as far as power and modualtion was concerned. The small ones on these radio limited tuning them up .......but I said that already . Why ? scientifically .........I couldn't tell you. They just did.