LesComm crystal failure

[543_Dallas]

I wanted to start a thread to see if anyone else had issues with these boards. I have the lescomm channel board on a 2013 cobra 29 chrome. The 15.36 crystal fails about every 8 to 12 months. I think yesterday was the 3rd time. Has anyone else had an issue like this? I contacted the manufacturer and he said he's never heard of this.

The 15.36 crystal is for the normal 40 channels. The upper and lower channels are rarely used and no crystal failures yet. I made some rubber isolators to mount the board last time thinking it might be caused by shock.

 

[Captain Kilowatt]

Perhaps something in the oscillator is driving the crystal too hard and taking it out. It may not be the crystal itself going bad.

 

[ExitThirteen]

I haven't had a newer Cobra 29 opened up in a while... is the original crystal a 15.360? If so, swap out the LesComm 15.360 with the original OEM crystal that came out of the radio. That should fix the issue.

It is interesting that the LesComm crystals are failing that early... do they wander out of spec, or does it just quit altogether?

~Cheers~

 

[543_Dallas]

Perhaps something in the oscillator is driving the crystal too hard and taking it out. It may not be the crystal itself going bad.

I was thinking it had to bee something else causing it. After the original crystal failed I ordered a bag of them on ebay so I've had failures with different brands. The ones I have been using are branded "fox." I'm not all that savvy with oscillators yet.

I haven't had a newer Cobra 29 opened up in a while... is the original crystal a 15.360? If so, swap out the LesComm 15.360 with the original OEM crystal that came out of the radio. That should fix the issue.

It is interesting that the LesComm crystals are failing that early... do they wander out of spec, or does it just quit altogether?

~Cheers~

I don't know what the OEM crystal is off the top of my head. Good idea though. I usually notice it's failed because the RX has almost no noise floor, then I try to transmit and I have no output. The upper and lowers still work fine. It did fail when I was listening to the radio once. The RX started dropping out and getting distorted. It was dead in 5 minutes.

I went out of town Friday evening and listened to the radio on the drive and it was not working when I turned it on Sunday morning. I was driving on rough roads and over a few cattle guards and in the past I can usually recall hitting a big pothole or something before the failure. Could the crystal be running too hot and a rough ride finish it off?

 

[543_Dallas]

Looks like the OEM crystalike is a 10.24 mhz. This is some info I found on the CB Dr website.

"The reference crystal (10.240 MHz) is used to time the PLL and is used as a mixer frequency for the receiver. In addition the reference frequency is divided by two to produce a frequency of 5.12 MHz. This frequency is then multiplied by three to produce a frequency of 15.360 MHz"

Looks like the LesComm board is injecting 15.36 mhz after this oem tripler circuit for the regular 40.

 

[ExitThirteen]

No, I don't think so.... those crystals are pretty durable. I don't think the standard road vibration/pothole vibration will cause a crystal failure... if that were the case, 90% of all the CB used in semis would have failed, nothing beats up a CB worse than being in a semi.

I'd suspect a manufacturing fault on the board before anything else, to be honest. That's why i was wondering if it was wandering, or if it would just suddenly fail, with no warning.

It could also be a switching diode on the LesComm that is leaky, or has gone fault. Also, double check for any suspect solder joints.

I have an STL branded 15.360 crystal I could send to you to see if it's the crystal. However, I'm not real sure that the crystal is the problem.


~Cheers~

 

[Onelasttime]

Have you considered sending it to Lester to let him check it out. At this point it is like replacing a fuse that keep popping with out figuring out why. You are going to keep getting the same outcome no matter how many times you replace it. It could be the Lescom board or it could be the radio itself.

Unless you driving on Tank Trails in a Tractor Trailer of driving on a test loop at Ford or GM's proving grounds I doubt it is vibration.

Do you have all the voltage values for critical test points and a schematic of the radio and Lescom board? I take it you have already started at the crystal and traced each run back testing as you went and found nothing? Have you tried feeling the parts under operation to feel for oscillation?

Who did the installation?

Tap around the area with a plastic tool to look for cold solder joint issues.

 

[543_Dallas]

Have you considered sending it to Lester to let him check it out. At this point it is like replacing a fuse that keep popping with out figuring out why. You are going to keep getting the same outcome no matter how many times you replace it. It could be the Lescom board or it could be the radio itself.

Unless you driving on Tank Trails in a Tractor Trailer of driving on a test loop at Ford or GM's proving grounds I doubt it is vibration.

Do you have all the voltage values for critical test points and a schematic of the radio and Lescom board? I take it you have already started at the crystal and traced each run back testing as you went and found nothing? Have you tried feeling the parts under operation to feel for oscillation?

Who did the installation?

Tap around the area with a plastic tool to look for cold solder joint issues.

I'm not going to send it out and I agree the crystal isn't the issue. I haven't taken the time to do any real troubleshooting. The first time I replaced the crystal, second time I added the rubber pads thinking it might be vibration and this time I popped in a new crystal and verified it was on frequency just to get it going. I did the install and am confident all solder joints are fine.

I do have a service manual for the radio and the LesComm board is pretty simple so I think I can make it without a schematic. I just need to sit down and take the time to find the root cause but there are other irons on the fire. If crystals weren't so cheap I might be more motivated. Thanks for the tips.