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Cobra 139xlr good power out but no mod.

im glad it worked!

the other parts are to make the radio work better.


for example, tonight, when you have had the radio on for a while DXing; pull the top cover off and look on the right hand side of the PC board for that big resistor with the teflon sock on it like we discussed earlier.

touch it if you dare, or just hold a finger close enough to feel the heat.

that is the reason for one of the transistor changes.

just replace the other parts at your leisure one at a time.

good luck with the rest,
LC
 
loosecannon said:
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
Click to expand...
I have a 139xlr with no mod on AM only keys up and has mod on ssb and receive on all.. any ideas? Thanks for any help
 
ok, there is a difference between no modulation and weak modulation.
it sounds like you have weak AM modulation.

with a radio of this age, there could be a bad electrolytic cap dragging the AM audio to ground, or the actual trimmer resistor that raises and lowers the modulation level for AM might have gone bad from being turned too much, among many other things.

you might check VR6 and VR7 to see if they make any difference.

here is a link to a service manual that is for a different radio, but they are the same on the inside:
http://www.cbtricks.com/radios/realistic/trc_449/graphics/realistic_trc_449_sm.pdf

scroll down to page 13 and read about how the audio is produced in this chassis, that should get you started.
if that info seems over your head then im afraid you are probably going to have to send it out for repair.
LC
 

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