Cobra 29LTD Classic PLL/Display problem

[Unit 75]

I had a 2005 Cobra 29LTD classic brought to me.... initially to have the channel display replaced. The original was missing a few of segments. Prior to doing any work I bench tested the radio into my dummy load and it seemed to transmit just fine and on frequency. I replaced the channel display with a stock one from a 29LTD. Once it was put all back together, I turned it on and flipped thru every channel to make sure the display read correctly and had no drop offs. Everything looked great. I then hooked my coax from my dummy load up to it and turned it back on to transmit into the dummy load. As soon as I screwed the collar from the PL259 on to the SO239 the display read ch28 when I was on ch20. The radio keys and puts out power but on ch40. I thought what the heck. I undid the coax thinking something was causing the radio the short...so as a test...I hooked a alligator test lead from the center of the SO239 to ground thinking it would do the same thing but it didn't. The display read correctly. I have never EVER seen a Cobra29 do this before and have no idea why its doing that. Does anybody have some insight into what may be causing this "possession" ?

 

[loosecannon]

just spitballing here, but you might try putting an ohmmeter from chassis ground to PC board ground, with and without your coax to the dummy load connected.

in both cases, you should have no DC connection between them.
LC

 

[nomadradio]

A principle called "Occam's razor" frequently applies to faults like this one. It states that all other things being equal the simplest answer it the most likely. When a fault appears following a repair procedure, a side effect of the repair is probably how the new fault occurred.

In this case, I think some part of your display board has a foil trace touching grounded chassis metal. Any time hooking a grounded coax to the radio changes its behavior, a fault connection to chassis metal is likely. I suspect that removing the plastic bezel and a VERY close look at the channel digit board may reveal something touching chassis metal that shouldn't be touching.

Maybe.

73

 

[Unit 75]

LC...no connection between chassis and board ground.
Nomad......I'm pretty sure it came in that way. I pulled the whole selector/display back out and everything is good. All small diodes were checked on the selector board and all are ok...so no Occam's razor happening here. Also, in my initial post, I said I purposely shorted the rear connector to both chassis and pc ground with a alligator test lead and it did not mess it up. The display read correctly. It was only when I screwed the coax in the back that things were messing up. I also hooked my omni and beam antennas (seperate coax cables) to the back and it messes up. This is not an easy problem...lol

 

[brandon7861]

If connecting the coax shield to the chassis is what does it, check for a potential difference between the two. Insert the coax center pin, but keep the shield disconnected and test for AC and DC between the chassis and coax shield.

 

[Unit 75]

Problem fixed. There was a short in the AWI circuit. All is well. Thanks for all your input. Brandon7861...I did do that and had no readings between chassis ground and DC board ground. I even undid all them 103 caps between the two thinking something was leaking but no.