I Want to Keep This One

[Sammitch]

Hi everybody,
I just got hold of an early model Cobra 139xlr. Serial number 7001xxxx. It has its share of problems and I am trying to work them out. When I first got it, it had very little receive on ssb and am, mostly just some white noise. It shows modulation on the meter but not on the RF meter, I get about 1 to 2 watts out on the RF meter on ssb and nothing on am, with the exception of the modulation meter working. So I worked the controls some and it seemed to start getting better. I sprayed Deoxit on the am-ssb switch and Bang ssb jumped up real loud, but I am only hearing the loudest signals but still no movement on the RF meter, and am is still real quiet until one of the big boys at the truck stop about 5 miles from me keys up.
I have only had it a few days, but have replaced every electrolytic in it, including the power supply, but it did not help a bit.
I do know that the receiver was overloaded before I got it, guy fired up another cb in the same room and really made it squeal.
Can anyone point me to what diodes may have blown when he did this? I am waiting for a Sam's Book and am hoping it is not TR5 that went, the 1730's are not around anymore. I am going to pull this one today and check it, but am hoping for something simple like some diodes. So if someone could tell me which ones are the most likely to be affected by an overload, I would really appreciate it.
This radio is in looks a 8+ inside and out. It has some of the factory updates, at least up till my serial number.
I do not plan to mod it, just make it talk and receive like it did back in the day. Since it does not transmit, I suspect the driver or final, but I hope for better.
Any suggestions or info you can pass my way will be greatly appreciated. This is one good looking radio and I want it to work as good as it looks.

Thanks, Sammitch

 

[357]

keying up a radio next to it wont kill the RX, unless its got 600w.
Is the rf gain all the way up?
Are you sure what channel the radio is REALLY on, does it have extras?

First place I'd look is the relay.
Try tapping it firmly while RX and see if anything changes.

Check and clean the ant jack in the back.
Dont try cleaning the relay with anything else but a deoxit soaked piece of paper, even paper will take the gold plating off.

If this radio has never been screw drivered, the RX should be really quiet anyway, they arent like galaxies with a room full of white noise with no antenna connected.


good luck

 

[M0GVZ]

This is where test gear comes in. A RF signal generator and an oscilloscope make fault finding such a fault easy. Without them you just waste time replacing components randomly until you find the one which has taken the hit for the team.

With those two bits of test gear you just hook up the radio to the RF signal generator and set the sig gen to chuck out a -73dBm signal (the strength for S9 on amateur gear) at the frequency say for Ch1, get the probe for the scope and start working backwards through the receive path from the antenna socket.

 

[Robb]

http://unit399.wixsite.com/858ssb

 

[sonoma]

here is a bunch of tech bulletins for your radio. check them out. reliable 399 has some good info for the 858 radios.

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

 

[2RT307]

Good luck and I hope you figure it out. I have a 139XLR as well, and it's a great rig. And stock, too!

73,
Brett

 

[Sammitch]

keying up a radio next to it wont kill the RX, unless its got 600w.
Is the rf gain all the way up?
Are you sure what channel the radio is REALLY on, does it have extras?

First place I'd look is the relay.
Try tapping it firmly while RX and see if anything changes.

Check and clean the ant jack in the back.
Dont try cleaning the relay with anything else but a deoxit soaked piece of paper, even paper will take the gold plating off.

If this radio has never been screw drivered, the RX should be really quiet anyway, they arent like galaxies with a room full of white noise with no antenna connected.


good luck

 

[Sammitch]

OK, I have a new relay to install and also a copy of the Sams Manual.
Now I have a another question, is the stock relay mounted in a socket? The original sits a lot taller than the replacement and has a wire bail over the top of the relay, and I can see a little split for the lack of a better term at about the point of where the new relay is at the bottom, almost like it might unplug from the tall black base mounted to the circuit board.
I am still trouble shooting this radio, none of the VRs for S meter or power seem to have any affect on anything. The wax is still in the coils and does not look disturbed.
I just got the Sams and am going to take some voltage readings this weekend, replace the relay, if someone can tell me if it is in a socket or was just made with a taller base, they are both OMron, same parts numbers, just the base on the new one will sit flat on the circuit board and no wire bail.
I found new 1730 transistors for replacing TR5, waiting on them now, I ordered a bunch so I could have some extras. I ran a deoxit soaked paper thru the relay contacts and it did not help, but the contacts look awful dark and black.
I keep hoping at least one thing I do will show a little improvment to give me some hope, but nothing seems to change anything, still no transmit and very low receive.
I am going to have a lot of time this weekend, can anybody tell me anything in particular to check or try to adjust? I would sure appreciate it. Maybe the new relay will make a little difference and give some glimmer of hope for the old girl.

Thanks, Sammitch

 

[LeapFrog]

OK, I have a new relay to install and also a copy of the Sams Manual.
Now I have a another question, is the stock relay mounted in a socket? The original sits a lot taller than the replacement and has a wire bail over the top of the relay, and I can see a little split for the lack of a better term at about the point of where the new relay is at the bottom, almost like it might unplug from the tall black base mounted to the circuit board.
I am still trouble shooting this radio, none of the VRs for S meter or power seem to have any affect on anything. The wax is still in the coils and does not look disturbed.
I just got the Sams and am going to take some voltage readings this weekend, replace the relay, if someone can tell me if it is in a socket or was just made with a taller base, they are both OMron, same parts numbers, just the base on the new one will sit flat on the circuit board and no wire bail.
I found new 1730 transistors for replacing TR5, waiting on them now, I ordered a bunch so I could have some extras. I ran a deoxit soaked paper thru the relay contacts and it did not help, but the contacts look awful dark and black.
I keep hoping at least one thing I do will show a little improvment to give me some hope, but nothing seems to change anything, still no transmit and very low receive.
I am going to have a lot of time this weekend, can anybody tell me anything in particular to check or try to adjust? I would sure appreciate it. Maybe the new relay will make a little difference and give some glimmer of hope for the old girl.

Thanks, Sammitch

You might try a known good external s-meter yours may be stuck, if a "spring latch" is on top of the old relay, id' bet it's in a socket!
Best Regards
-Leap

 

[Sammitch]

You might try a known good external s-meter yours may be stuck, if a "spring latch" is on top of the old relay, id' bet it's in a socket!
Best Regards
-Leap

Thanks, I will try gently to remove the relay from what does appear to be a socket.
I have seen the "S" meter move twice, but only on super strong signals from the "BIG BOYS" down on I-70.
I am going to check voltages tonight, there is no voltage to even check the bias at the driver or final, so there is a bad component somewhere.
Thanks for the info, Sammitch