Cobra 139 xlr no tx or rx relays key

[Staybolt]

I have no TX or rx but the relays key up. The orange resistor on the right side smokes, r 113.

 

[unit_399]

I have no TX or rx but the relays key up. The orange resistor on the right side smokes, r 113.

Check to see if D25 or C101 is shorted.

- 399

 

[nomadradio]

I need to find the animated GIF with Samuel L. Jackson and the caption "He's right, you know".

Zener diodes are a famous breakdown risk. When they fail, it's usually as a short circuit. R113 is the current-limit resistor feeding current to D25, a 7.5-Volt 1-Watt zener diode. If it shorts, R113 smokes.

Bearing in mind this radio is 45 years old, electrolytic capacitors begin breaking down for no reason but age alone around 25 or 30 years. C101 is in parallel with D25. If C101 shorts, same result. You need a new R113. And a new cap. C101 is shown as a cap rated for 10 Volts. They are famous for going bad first, before the other electrolytic caps marked with higher voltage ratings. Got in the habit of calling this the "Ten-Volt Blues" a coupla decades ago.

And when you hear people refer to "re-capping" a radio this old, this is how you prevent what I like to call "Electronic Whack-A-Mole", replacing each electrolytic cap in the radio one by three as they all eventually fail.

Naturally changing every, single electrolytic capacitor in the radio creates a risk factor. You have to put the right one in place of the old cap, and turn them ALL around the right way. Polarity matters. And don't leave any solder drips that short between foil traces.

Ain't nothin' risk free.

73

 

[TM86]

 

[Staybolt]

Yeah, ok..

 

[Staybolt]

Yup, the zener as well as the cap was bad. I replaced both and it's 10-08! Yes, nomad, I'm planning on a total recap. This radio is pretty much stock. The previous owner had nothing done to it, not even the vr pots are turned.

 

[nomadradio]

previous owner had nothing done to it

That's a pretty unique find, this many years down the road.

Definitely a good candidate to do the 45-year tuneup.

73

 

[unit_399]

I second what Nomad has to say. Finding an untouched 139XLR now-a-days is unheard of. I run one on my base, and feel it's the best CB radio ever made. You're a lucky guy. 73
- 399 J.J.

 

[Cable Guy]

Do y'all know of any service bulletins for that model? I usually start there when restoring a virgin.

 

[TM86]

Do y'all know of any service bulletins for that model? I usually start there when restoring a virgin.

Service/Technical bulletins for the 138XLR, which is the mobile version, can be found here:

Cobra 138XLR

Owner's Manual Links Page 1 to 18 Page 19 to 37 Parts List Diagrams Schematic Diagram Schematic Diagram Technical Bulletins Technical Bulletin #1208 Technical Bulletin #1208A Technical Bulletin #1209A Technical Bulletin #1209B Technical...

www.worldwidedx.com

Other good info on the Uniden 858 chassis can be found here:

HOME | 858ssb

unit399.wixsite.com

And, yes, that's the same Unit 399 that posts here.

 

[Cable Guy]

Other good info on the Uniden 858 chassis can be found here:

HOME | 858ssb

unit399.wixsite.com

And, yes, that's the same Unit 399 that posts here.

Thanks, I actually have a untouched 139xlr sitting on the shelf. Cabinet is in rough condition but no tweaks or mods. I am planning to work thru it in a few weeks which is why I asked of service bulletins. I am very familiar with unit_399's work on the 858 in that link.

 

[Staybolt]

That's a pretty unique find, this many years down the road.















Definitely a good candidate to do the 45-year tuneup.



That's a pretty unique find, this many years down the road.



Definitely a good candidate to do the 45-year tuneup.



73

Yup finished it last night.

 

[Staybolt]

I second what Nomad has to say. Finding an untouched 139XLR now-a-days is unheard of. I run one on my base, and feel it's the best CB radio ever made. You're a lucky guy. 73

- 399 J.J.