Cobra 139xlr

[Flyfisher]

Hi all
i was in the middle of using this radio last night when all of a sudden my
power on AM and SSB dropped from 5 wattts am and 10 Watts SSB
to about a 1 1/2 watts total? No smoke no nothing changed? So i started
poking around and the only thing that seems to have changed is that the
"heat" tracker on one of the finals is loose. they have a little metal band
that holds them tight. ipushed it back up to the same spot as the other one
but the metal band is still not tight. I am not sure id i should crimp it on tight
or a drop of glue? Could that cause the drop of power? I just plugged it in after
setting over night and its still the same. So i guess thats not the problem. Anyone
have any ideas?

 

[tramd]

Could be quite a few things in a radio that old. The early Cobra 138XLR and 139XLR had some issues, and bulletins were put out on them for the fixes. They can be found on CBtricks. Since it's been going this long and all you have is an output issue, it could be the final rf transistor has failed and the power you see is just from the rf driver. Could also be one of the components feeding the final that has failed. If you can read schematics and can solder well, then you should be able to find and fix the problem. Otherwise it's off to a tech.

 

[nomadradio]

The "heat tracker" serves to reduce the transistor's tendency to pull more current as it gets hotter. The diode inside that thing has a voltage drop that falls as temperature rises. Using that falling voltage to control the driver or final transistor's idle current serves to prevent what they call thermal runaway. Transistor draws current, gets hot. Higher temp makes it pull more current. More current gets it hotter still, until something melts.

If the final transistor overheated and was damaged, the bias diode that came loose may also have been damaged.

A test for both the final and the bias diode would be next.

73

 

[groundwire]

Could be quite a few things in a radio that old. The early Cobra 138XLR and 139XLR had some issues, and bulletins were put out on them for the fixes. They can be found on CBtricks. Since it's been going this long and all you have is an output issue, it could be the final rf transistor has failed and the power you see is just from the rf driver. Could also be one of the components feeding the final that has failed. If you can read schematics and can solder well, then you should be able to find and fix the problem. Otherwise it's off to a tech.

I know right. Even though they are cool radios, i would consider them recall radios. They had sooooo many service bulletins on them for design flaws. Makes me think they were rushed before enough testing was done. The 138xlr is a nice looking old school radio.

 

[Shadetree Mechanic]

I believe this radio has the 858 pll chip in it like my President Madison. Made by Uniden, excellent quality. Definitely worth fixing, awesome radios even by today's standard. The tech bulitens were mostly to make the transistor bias so it would not blow the final and driver when the modulation limiter was cranked up.

 

[Holydvr]

I have a couple of these radios. I work on them little by little, but two of them do work. I love hearing a relay click when I key a radio! I do agree that they are very much worth fixing!

 

[Low_Boy]

I am in the process of trying to get one of the exact same radios up and running. First Check C179 a Tantalian Cap. The one I am working on also had a bad driver due to the Cap frying.

 

[Holydvr]

On my third unit (had a bad band switch that I cleaned) that very cap blew up when I had the top off and a large chunk of it bounced off the ceiling! Its been awhile, but its got no AM receive. I need to track that issue down.

 

[unit_399]

Sounds like the final opened up, and you're running on the driver only.

- 399