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Cobra 139xlr good power out but no mod.

KB1TPX

W1MBZ
Jul 22, 2008
493
10
48
71
Warwick, RI 02818
Just picked up an "as is" Cobra 139XLR, I took my chances.
It receives well on AM and SSB, transmits a strong signal on AM with practically no modulation. On SSB the relay buzzes when keying, with no signal, no modulation.
Finals?:confused:
 

Obviously can't say with 100% certainty the following is your problem, but is a known issue with the radio... Worth a shot....


Change C179 2.2uf 25 volt small blue tantalum electrolytic. Change this to a standard 2.2uf 35 Volt or better electrolytic capacitor.... make sure observe polarity...
 
Obviously can't say with 100% certainty the following is your problem, but is a known issue with the radio... Worth a shot....


Change C179 2.2uf 25 volt small blue tantalum electrolytic. Change this to a standard 2.2uf 35 Volt or better electrolytic capacitor.... make sure observe polarity...[/quote

Thanks Mr. Fatty,
It figures that you would be the first to reply, you're right on the ball.
Your suggestion is worth a shot.
BTW it also just blew the fuse. I think I should send it out to a tech, instead of trying it myself. Any recomendations?
 
7732,

mister fatty's suggestion is most likely right on the money.

this was a very common fault with this chassis, and it causes these exact symptoms.

you can fix this yourself!

for cheaper than the shipping to send the radio to a tech, you can make a run to radio shack and probably fix your problem.
if it doesnt fix it, then go ahead and send it out for repair, as the troubleshooting list gets long.

go to radio shack, buy yourself their 30 watt soldering iron kit, which should come with some solder.

they should have the 2.2uF electrolytic capacitor too. just make sure its rated for 25volts or more.

also buy a roll of "solder wick" also known as desoldering braid.
the kid at the radio shack wont even know he has it, so dont bother asking. LOL

just look near the soldering stuff for something that looks like a roll of copper braided ground strap. probably an eighth or a quarter of an inch wide.

inside the radio, look near the front left of the circuit board, (front of radio facing you) about an inch to the right of the driver and final, and about two inches in front of the relay, you will find a tiny blue tantalum capacitor. C179

its just a little lump of a thing so it can be hard to find the first time you look for it. use a magnifying glass if needed.

turn the radio over (oh yeah, heat your soldering iron up now) on its side.

shine a flashlight on the capacitor and look on the solder side of the PC board to match up the two solder leads that are for that capacitor.
make a mark so you dont lose them.

now turn the radio upside down and put your solder wick on one of the capacitor leads.
now touch the soldering iron to the solder wick and let it heat for a few seconds.
the solder wick should "suck up" the solder and leave you with just a hole with a small capacitor lead in it.

do the other one just the same way.
try not to leave the soldering iron on there for too long. if after about 5 seconds or so, its still not "wicking" the solder onto the braid; heat the lead of the cap to melt the solder and then put the braid back on and try again.

should be pretty easy, just try not to over heat things.

now, using some needle nose pliers or whatever you have, gently pull the cap out of the board if it hasnt fallen out already.


now you are ready to put your new cap in.
there is a positive and a negative lead, and the negative lead is the one with the black stripe running down the side.

looking at the PC board, you should see a little "+" sign near where the cap goes indicating which hole the positive lead goes in.

if you cant see the "+" sign; look at the solder side of the PC board, and one of the empty holes you just created should connect to the main ground foil trace.
this is the big trace you see running all over the PC board.
the hole that connects to this trace is where the NEGATIVE lead of the cap goes. (that makes sense right? negative lead of cap to ground?)

once you have the new cap in and you have double checked that you have the polarity correct; bend the leads away from eachother a little bit so the cap doesnt fall out when you turn the radio upside down.

turn the radio upside down.
place your soldering iron at about a 45* angle to the board, touching both the cap lead and the foil trace. hold it there for about 2-3 seconds, and touch the solder to the lead, the trace and the soldering iron tip for a second.
the solder should flow nicely and should look like the rest of the ones on the board except shinier.


once you have both leads soldered, cut the excess lead length off and give the radio a test.

oh yeah, dont forget to replace the fuse first or you may start cussing. LOL

good luck,
i do hope you try this, as it only gets easier the more you do it.
LC
 
Considering the age of the radio I would guess a few of the electrolytic caps need replacement. might be a good idea to have an alignment done as well. As for who to send it to, I'll leave some others chime in with opinions.

Good Luck on the radio
 
I would also agree with loosecannon, if there is a chance to do it yourself do give it a try, it isn't a real difficult job.
 
Thanks loosecannon for your excellently detailed instructions!
I will try it myself. I'm not totally inept. I do have a soldering iron already and I'm sure I'm capeable. I have done such things before. I just thought that a well equipped tech could use an oscilloscope, etc. and do it up nicely. But as stated, it would certainly be cheaper to try it for myself.
Also, thanks everybody for the imediate responses.
 
Last edited:
you are most welcome.
im glad to help.

good luck with it.

i have owned 4 or 5 of these radios, and i love them.

there are lots of improvements that can be done to this radio, and if the bug bites you, you may decide to replace some other parts and then send the radio to a tech to have them give it a full alignment.

the two main things are, as was mentioned before, the age of this chassis and the fact that electrolytic capacitors dry out over the years and can cause all kinds of crazy symptoms that will make you scratch your head. LOL

i just got done restoring mine a few months ago, and i replaced all of the electrolytic capacitors with brand new ones.
only cost about 20 bucks worth of parts from mouser electronics.

fun project, time consuming, but very satisfying when you are done.
just remove the old cap, plug in the new one, solder and move on.

when you are inside the radio, take a look at the back of the PLL section (that little walled off area with the UPD858 chip in it)
you will see a transistor mounted to the wall that looks like your driver and final.

the thing about this transistor is it gets very hot, as does that big resistor behind it that has a teflon sock on it.

when they get hot, they heat those crystals that are next to them and that causes the SSB to drift in freq.

on some later models, this transistor has already been moved to the back of the chassis where the audio chip is mounted, and wires run to the spot where it used to be.

if yours is not already moved, you should move it, and if the part number on that transistor says 2SC1419, you should replace it, and the other ones of the same number as they were underrated from the factory.

good replacements are the NTE152 or a 2N6487.

lots more stuff you can do, PM me if you ever get the urge to mod it.

here's a really cool simple mod you can do at the same time of your cap replacement that will really improve the ANL.

just replace R43 with a 1 Meg ohm resistor.

its in front of the relay.

have fun,
LC
 
Wow, great! I'm going to have to jot down a shopping list of caps and resistors, order the parts and get started.
BTW, I also own a 139 (23 ch) and an 89XLR. I guess they both could use updating, but I plan on selling them as soon as I get the 139XLR up to snuff.
 
you should be able to get the cap and resistor at radio shack.

the transistors you might have to go to an electronics store for.

if you do decide to order the parts, mouser.com is a great source.

they have everything, prices are great because they are so huge, and they have no minimum order!

i find myself searching the catalog for things to order just to justify the shipping costs of 10 resistors. LOL

the old cobra 139 you wont get much for on the used market, if it were my radio i would keep it as a parts radio.

the driver and final in that 139 are the 2SC1306 and the 2SC1307, both very desirable and hard to come by.
the meters in that radio will come in handy if or when the meters in your 139XLR go out.

heck, you might even find a 1 meg ohm resistor in there for your ANL mod.
(brown, black, green)

the 89xlr is also a pretty cool radio, but if you would rather have the cash, that one might snag a few bucks.

good luck with everything,
if you have any questions, feel free to ask them here or PM me if you want.
LC
 
cool!

keep us posted.
do the C179 repair first and make sure the radio works normally before you start with the other upgrades.

a few things to remember while doing your first radio mods/ repairs:

1. set yourself up a nice work area with all the things you might need close at hand, and make sure you have a good light source that you can move around.

2. dont hurry through anything. we all get excited towards the end and want to see the results. this leads to stupid mistakes. avoid the compulsion. LOL

3. do your mods/ repairs one at a time! this is very important!
if you start by taking out all the parts for all the different things you are doing, when you go to turn it on after you're all done if it doesnt work; you wont know where to start looking for the problem.

replace one part, then turn the radio on (after triple checking that its ok to do so) and check for normal operation. this way if it doesnt work right, you know right where to start looking for a bad solder connection or a faulty component, etc...

4. practice your soldering on an old junk VCR or something. im sure you have at least one piece of obsolete electronics lying in a closet somewhere. of you dont, you're probably the only one. LOL

just open it up so you can get at both sides of the PC board, and practice removing the solder from a component, and practice both removing the component, and re-soldering it in place. you will find that after doing it 10+ times, you get a feel for it, and you will have much more confidence when you start working on your baby.

good luck, feel free to ask any questions,
LC
 
You bet!

I'll take that advice seriously, as it is obviously spoken by "the voice of experience".

Needless to say, this is a very helpful website, with many knowlegeable members who are always willing to help. Thanks. (y)
 
Done

I replaced C179 first, then powered it up. I guess that was the only problem to get it working. Of course without test equipment, I don't know if it is perfect, but my power/SWR meter and frequency counter indicate that I was successful. I'll do some Dx'ing tonight.
That's all I had time for today. Nice to know that I have the transistors, resistors and extra capacitors on hand.
Thanks again for the help guys.(y)
 

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