Cobra 138XLR

[unit435]

435, my link should still work.
LC

Downloaded the service manual for the trc449 with the parts list.

I looked through this thread and did not see a Sams link for the service manual. Holydvr said he has a paper copy. Is it worth getting the Sams service manual or is the Tandy Corp good enough ?

 

[kopcicle]

@unit_399 , Jim , I'm mildly surprised you haven't found your way here ;-)

Since Dave passed the Service Manual to you there isn't much more documentation other than the PhotoFact that would help.
I'd pass that along if I had it and any member that does will approach you via PM. Things like that just seem to happen off camera.

Just because this is one of the few classic radios that deserves it , take Mike up on the offer and re-cap the radio. This will sometimes fix things you didn't know were broken.

 

[Dmans]

I do have the Sams photofact for the 139XLR (same radio but with an AC power supply) but it has some mistakes (or omissions) in it and I do not have a way to scan it. The TRC449 is a better guide in my opinion.

+1 on the KlondikeMike recommendation.

73
David

 

[unit435]

10-4. Looks like I have enough documentation. A counter and some time and I will be off and running.

Are there any members that work on this chassis in the Houston area?

Thanks

435

 

[Holydvr]

If you find you need anything from the sams I have, I dont mind screen shotting (so to speak) pics from it. Scanning properly would mean I would have to rip the book itself apart. Let me know if you need anything, though you may be good to go. Between Sunday thru Tues I can take voltages if they are really needed on the three that I have that are functioning. But, loosecannon was schooling me a bit over a decade ago on this very radio. He is a wealth of knowledge, as is 399.

 

[kc8mob]

Man this is reading like a vintage aircraft restoration.

138xlr huh. Kinda feel like I need to buy one now, seeing all the determination here.

Mind you I only design/build tube guitar amps to feed myself and wipe my ass...

Recap the bloody rig.

There is never a reason to be this obstinate over 40+ year old electrolytics.

The whole thing about recapping it was actually annoying. Like watching a baby get a slow motion full fingered swipe of poo and then rush in for the taste test.

And to think most of today's caps are real crappy and don't stand a chance of making it 20 years.

 

[loosecannon]

While i agree that re-capping is a must do, and normally my advice would be to do it after finding a problem and before trying to troubleshoot around a bunch of vintage electrolytics, our OP is in a somewhat unique situation.

When i try to formulate a plan of attack for someone troubleshooting a problem, i try to take in to account their means, skills, and whatever other factors might change the advice given.

in this situation, he is out on the road, sleeping in a motor home, with a voltmeter and a limited budget, as well as not much experience soldering inside a radio.

for him, doing a re-cap first might do more harm than good.
LC

 

[unit435]

While i agree that re-capping is a must do, and normally my advice would be to do it after finding a problem and before trying to troubleshoot around a bunch of vintage electrolytics, our OP is in a somewhat unique situation.

When i try to formulate a plan of attack for someone troubleshooting a problem, i try to take in to account their means, skills, and whatever other factors might change the advice given.

in this situation, he is out on the road, sleeping in a motor home, with a voltmeter and a limited budget, as well as not much experience soldering inside a radio.

for him, doing a re-cap first might do more harm than good.
LC

Thanks LC. Thats pretty accurate. I hope to start working on the radio after the holidays. Is there anyone in the Houston area that would like to do the work on this radio for a trade on my 148 and some cash ? I would consider that as this will save me lots of time. Just the recap is going to be a big effort for me. I would need to make a jig to hold the radio and proper lighting, magnification etc. Lots of caps. I did get one in without issue though !!!! Maybe there is hope !

Have a great holiday season everyone and thanks for all the help and input. Could not do any of this without all this help. Thanks again.

435

 

[nomadradio]

And if you fancy the partial "whack-a-mole" solution, starting with the caps rated at ten Volts or less will target the ones we see fail first in a radio that old. Might get by for a week, or a month, maybe longer. By the time 16-Volt rated caps start to fail, you're on the downhill slide.

And changing all the electrolytic caps creates some risk. Any complex procedure does. But taking it one part at a time, noting carefully the polarity of the part you pull out improves your odds. Might agree with screen-print markings on the circuit board. Might not. Believe the part, not the artwork when they disagree.

73

 

[TM86]

And changing all the electrolytic caps creates some risk. Any complex procedure does. But taking it one part at a time, noting carefully the polarity of the part you pull out improves your odds. Might agree with screen-print markings on the circuit board. Might not. Believe the part, not the artwork when they disagree.
73

This is why I mark the caps for orientation before I start swapping. Little bit of time with a sharpie saves a lot of time trying to figure out which way the old part was mounted on the board. Especially when it falls out as the solder is removed.

 

[Enterprise312ok]

I hate to revive an old thread but all the advice here helped me bring my 138xlr back to life. I bought the sams manual wich made the recap much easier, the radio still had a couple issues and neded aligned so I sent it to nedlebender 35 Jon Beck I think he found 4 burned resistors and replaced them then did a great tune and alignment. The only thing left to troubleshoot is the extension speaker jack but radio is perfect other than that. Thanks to Klondike Mike for the capacitors. And all of the members who shared their knowledge.

 

[nomadradio]

The burned resistors suggest that they were feeding juice to shorted caps, maybe?

Leads me to wonder how many resistors a 'cap kit' for some of these radios would need to include? I don't give it much thought since I keep resistors on hand in quantity. But Joe weekend would have a different experience finding burned parts.

Hadn't considered it until just now.

73

 

[Holydvr]

Glad to hear another one is back on the road!!

 

[nomadradio]

Did have one last thought. Seems to me the three 9.1-Volt zener diodes in this radio each had a half-Watt resistor feeding it. One of them is 39 ohms, the other two are 47 ohms.

If those are the resistors that got hot, it's no surprise. Just means somebody ran the radio a lot of hours some time in the last 45 years. Consider that those zeners probably got just as hot.

I'm ready for someone (else) to work out a quick way to remove that half-Watt resistor and each of the three zeners, to replace each with a 7809T 3-terminal regulator chip.

Would probably serve to reduce or stop any tendency to 'warble' or change frequency in step with the audio level.

Hmm.

73

 

[Enterprise312ok]

Did have one last thought. Seems to me the three 9.1-Volt zener diodes in this radio each had a half-Watt resistor feeding it. One of them is 39 ohms, the other two are 47 ohms.

If those are the resistors that got hot, it's no surprise. Just means somebody ran the radio a lot of hours some time in the last 45 years. Consider that those zeners probably got just as hot.

I'm ready for someone (else) to work out a quick way to remove that half-Watt resistor and each of the three zeners, to replace each with a 7809T 3-terminal regulator chip.

Would probably serve to reduce or stop any tendency to 'warble' or change frequency in step with the audio level.

Hmm.

73

When I got the radio It would autokey anytime It could find dc ground through the coax or case wich Im sure caused some long keydowns sometime in the past, the mounting scew on the final had turned back lol.
I need to just get an assortment of resistors becaus I am always ordering specific ones. I really think If I keep getting good advice on this forum I think I will be able to do more work on these old radios. My local tech has asked me abut doing restorations on old radios and I told him I still have much too learn. You can search the internet for hours and not find what your looking for but you can ask a question here and search the minds of many great techs and tinkerers.

Thanks and 73 from enterprise 312 aka
World Radio 497 Oklahoma