Cobra 139 XLR help

a2phil

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Aug 6, 2012, 10:33:24 PM8/6/12

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I recently bought a Cobra 139 XLR at a garage sale for $15. The guy didn't have an antenna for it, so I couldn't check TX/RX, right then, but it lit up and didn't LOOK like it's seen the "golden screwdriver" (no extra switches, no missing/stripped out screws or knobs, no holes in the case, etc...I've always been told to STAY AWAY from those radios unless you know the person who did it and you know they know what they're doing)and I've seen the "it lights up, but I don't know what's wrong with it" ones go on eBay for $40+, so I bought it. I took it home, and found that the radio receives GREAT (TONS of skip with an indoor antenna!!) but no transmit (meters dim and deflect momentarily, but no "dead key" even with whistling) No power out indicated on my watt meter. Is there an easy and/or cheap fix for this? Can you still find the needed part(s) for this 35+/- year old radio?

Channel Jumper

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Aug 9, 2012, 9:12:50 AM8/9/12

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The components are all surface mount.
If you can read the schematic's and if you can solder and if you have
the proper tools to do the proper measurements - you can repair it
yourself.
The problem is with a $15 radio - you probably aren't going to spend
$50.00 to repair it...

--
Channel Jumper

a2phil

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Aug 10, 2012, 10:28:30 PM8/10/12

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I have a friend that I could borrow a multimeter (and maybe a soldering iron) from, I'm just worried how much the parts could cost, and if IS the finals (I'm thinking most likely) can they be found?? For a 35 +/- year old radio, it's in pretty good shape otherwise.

morseelec...@gmail.com

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Sep 20, 2012, 9:31:52 AM9/20/12

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hi not sure if you have fixed the radio at all but it sounds like the final or driver or both have failed , this is what gives the rf power output , these to transistors are based usually on the rear of the radio chasis most common part numbers for driver is 2sc2166 / PA is 2sc1969 you might find one or both transistors have failed and pulling the postive rail to ground rail ( hence the dimming of the radio) parts should cost around $20.00

Channel Jumper

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Sep 21, 2012, 9:29:58 AM9/21/12

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The problem is - if a service tech charges $40.00 to repair it and
$20.00 of parts - you now have $100 into a $25.00 transceiver - with no
warranty.

THROW IT AWAY!
SELL IT
OR USE IT A A DONOR RADIO

There isn't a lot of high technology in a AM CB radio.
But for a person that does not even OWN a VSWR meter or a VOM meter - I
can't with a straight face - tell them how to repair them and guarentee
them that their repair - with no technicial experience or background -
can fix it.

You would want to use some type of heat sink when you soldered the
transistors in and you would want to replace what ever else it took out
when someone operated it with a bad match - which is usually what takes
most finals out. Or a near lightning strike - which fries more then
just the transistors.

http://www.cbtricks.com/radios/cobra/139/index.htm

When you take the 4 screws out of the back and the one big screw from
under the radio, it will slide out of the cabinet.

If you look on the one side - you will see 3 transistors - those are the
first place to look.

Cross reference them via online - see if they are PNP or NPN
transistors.
NPN are the most common - since they are cheaper to manufacturer.

USE A VOM METER that has a transistor checker and do a internet search
to find the gate, drain, source - to check them out.
Then I would throw them away and put new ones into it and see if it now
works.

With a radio of that age, it probably needs all the capacitors replaced
also - which is very expensive.

Even after all of that, usually one of the bulbs - (AM) goes out in the
display - since it was used the most.

The switches will need cleaned, the pots will need cleaned, the selector
will need cleaned and adjusted.

After all of that, if you are really lucky, it will work again, like it
did 30+ years ago when it was new.

--
Channel Jumper

joey8...@yahoo.cn

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Dec 27, 2012, 10:49:41 PM12/27/12

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these to transistors are based usually on the rear of the radio chasis most common part numbers for driver is 2sc2166 / PA is 2sc1969 .http://www.hqew.net/product-data/2SC1969

Vinnie S.

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Jan 25, 2013, 6:21:28 PM1/25/13

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C179 is blue tantalum cap. Replace it with a 2.2uf 50V electrolytic. Do not use a lower voltage rating. Match the polarities. If that don' fix it, then take it to a tech. But that is a quick and cheap fix. That is a notorious failure in that chassis.