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?

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...

On Thursday, August 9, 2012 9:12:50 AM UTC-4, Channel Jumper wrote:
a2phil;794670 Wrote:

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?



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

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.

Keep it for parts, keep it for what ever - even if you just keep the microphone.

This is called a life's lesson.

People do not usually get rid of a good working CB radio unless it is JUNK!

A indoor antenna is not going to work properly.

It needs to be a certain segment of a wavelenght in size 1/4 - 5/8 ths and it needs to be at least 30' off the ground...

If your antenna system was ok - and the SWR of the antenna was ok, I would suspect the radio was faulty.

Unless you have another transceiver to check the antenna - the SWR meter is not going to do you any good.

I highly doubt if it is going to be as simple as just using an OHM meter and a soldering iron to repair - especially since you have no electronic's background.

You can check the TX if you have another microphone.

Some CB's will not receive unless there is a microphone attached.
There is no way for me to know if it is the microphone or the transceiver until you find a microphone that actually works.

If you knew of someone who had a electronic's background, you could check the final transistors and replace them...
Probably - the person who you bought it off of had it connected to a crappy antenna and it fried the finals....

Sometimes you can start at the front end and work your way back - but it is going to cost you more to fix it yourself then what it is worth.

A CB shop will probably charge you $65 to repair it - not worth the bother, when you can buy a brand new one at Pilot for about $89.00 with a warranty

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

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.

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.