Cobra 138XLR

[unit435]

That freq counter is just an example. I have had decent luck with that type, but keep in mind they have different models that cover different freq ranges if my memory serves me.

Thanks. That looks perfect. Good price too. Probably will not be using it much past my 138xlr. I will do a search for probes and hopefully get that ordered tonight.

I did pick up the 2.2uf caps yesterday. I will probably have time later in the day to get that intalled then will hopefully get some additional instructions from LC until I can get the freq counter. The electronic store where I got the cap does not offer any freq counters that will help with my project.

 

[loosecannon]

my advice is to try to get a signal in to the radio.

without a freq counter, you won't be able to verify that all the required signals for receiving are there, but you can get set up with another radio in the same room so you can try transmitting from it and see if you can pick up anything in the 138.

try to determine whether or not there is a signal coming in to the radio, and just not making any sound, or if the signal is just not making it in to the radio.

make sure you eliminate all the external factors before diving into the radio itself.
confirm that the mic on the 138 works 100% properly.

try to make sure you are dealing with a NO receive, NO transmit issue, rather than a weak receive, weak transmit issue, or even one or the other.

you can do this by putting the radio on AM mode (once you replace C179 that is) and transmitting with your external watt meter set to SWR CAL with the knob turned to full sensitivity.
this way you can see if your transmitter is putting out anything at all.
try this on SSB while speaking into the mic as well.

if none of this leads you anywhere, then it's time to start looking for correct voltages.

check each pin on the PLL chip.
Check the VCO setting.

check the input and output voltages of each 2SC1419 transistor in the radio.
(they are the ones that look like the driver and final).

what you are looking for is a DC voltage that is either low, or not there.
after that, you'll be searching for other components on that trace that could be leaking or shorted and dragging the voltage down.

we are going to need a service manual in order to do this, and i think CBtricks is down, so you might have to get one from the Howard W. SAMS company.

they will sell you a PDF of any of their SAMS photofact CB series for about 22.00.
i think the one you want is #126. someone correct me if im wrong on that.

If budget is an issue there are usually cheap and dirty ways around having to get expensive test equipment.

for example, you can get one of these:
https://www.ebay.com/itm/124155705733?hash=item1ce841b985:g:P08AAOSwICpaAEjV

which will work for a frequency counter for you, and you can just make the probe yourself.
no they are not the greatest, but it will get the job done.

all that being said, i still think a re-cap is in your future, and may take less time and expense than troubleshooting the problem first, and then replacing all the caps.
LC

 

[unit435]

my advice is to try to get a signal in to the radio.

without a freq counter, you won't be able to verify that all the required signals for receiving are there, but you can get set up with another radio in the same room so you can try transmitting from it and see if you can pick up anything in the 138.

try to determine whether or not there is a signal coming in to the radio, and just not making any sound, or if the signal is just not making it in to the radio.

make sure you eliminate all the external factors before diving into the radio itself.
confirm that the mic on the 138 works 100% properly.

try to make sure you are dealing with a NO receive, NO transmit issue, rather than a weak receive, weak transmit issue, or even one or the other.

you can do this by putting the radio on AM mode (once you replace C179 that is) and transmitting with your external watt meter set to SWR CAL with the knob turned to full sensitivity.
this way you can see if your transmitter is putting out anything at all.
try this on SSB while speaking into the mic as well.

if none of this leads you anywhere, then it's time to start looking for correct voltages.

check each pin on the PLL chip.
Check the VCO setting.

check the input and output voltages of each 2SC1419 transistor in the radio.
(they are the ones that look like the driver and final).

what you are looking for is a DC voltage that is either low, or not there.
after that, you'll be searching for other components on that trace that could be leaking or shorted and dragging the voltage down.

we are going to need a service manual in order to do this, and i think CBtricks is down, so you might have to get one from the Howard W. SAMS company.

they will sell you a PDF of any of their SAMS photofact CB series for about 22.00.
i think the one you want is #126. someone correct me if im wrong on that.

If budget is an issue there are usually cheap and dirty ways around having to get expensive test equipment.

for example, you can get one of these:
https://www.ebay.com/itm/124155705733?hash=item1ce841b985:g:P08AAOSwICpaAEjV

which will work for a frequency counter for you, and you can just make the probe yourself.
no they are not the greatest, but it will get the job done.

all that being said, i still think a re-cap is in your future, and may take less time and expense than troubleshooting the problem first, and then replacing all the caps.
LC

I will perform all those checks. I do believe that in the case of transmit that all is completely dead. I have used my meter with a dumby load that does swr, modulation and power. No meter movement on any meter on side band or am. The meter is good I used it to install my 29ltd in the motorhome. I cannot say if the receive mode is completely dead but will check both receive and transmit as you suggested after replacing C179.

I will not be installing the new C179 cap tonight. I will tackle it tomorrow, do the additional testing and post the results.

Thanks for the help.

 

[Dmans]

I'm just going to leave these here.

I also have about 10 Technical Service Bulletins for this model if needed.

73
David

 

[Holydvr]

I forgot to mention that I do have the SAMS for this model. Its paper copy though.

 

[unit435]

Installed the new C179 cap today and all is fine no chattering no problem and seems to be back to the original problem. The relay is working fine.

I followed up on that low voltage reading in the TX mode on IC6 by checking the voltage coming into the radio at the power plug.
13.42 volts in RX and 13.04 in TX so I think the supply issue has been eliminated.

Re-tested the original microphone using the 29ltd in the motorhome and it works perfect.

I checked both TX and RX for any sign of a signal input or power output and got zero in both modes. This was done using the 29ltd and the swr/modulation/power meter.

I plugged in an extension speaker to see if possibly it was in the PA mode and it is not in PA mode and the wires from that switch look fine and soldered.

I then took voltage readings on all 858 pins in the receive mode on CH19.

1) zero
2) 2.28
3) 2.28
4) 2.74
5) 2.40
6) 2.40
7) 4.78
8) 2.25
9) 2.05
10) 2.55
11) 1.45
12) 4.78
13) 4.78
14) 4.78
15) zero
16) zero
17) 4.78
18) zero
19) zero
20) zero
21) 4.78
22) zero
23) zero
24) 4.64

Found three 1419 transistors. One was board soldered and mounted on the back panel. The other two were wired and mounted one on the back panel and the other mounted on a side panel near the main transformer. I also looked at the two transistors in the finals area 1306 & 1307. I did not look up the schematic identifications for them as of yet. Voltages during both TX and RX are below and are listed facing forward with the source on the right.

1306 RX 0 0 0 TX 0 7.5 .71
1307 RX 0 0 0 TX 0 7.5 .67

1419 board mounted RX 7.65 13.29 8.25 TX 7.65 12.78 8.20

1419 wired by antenna plug RX 0 0 0 TX 0 7.5 .67

1419 wired by main transformer
RX 3.6 13.25 3.0 TX 3.53 12.72 2.93


Thats it for me today. I cannot tell if anything looks bad as I am not very good with this stuff but it looks like 1306 and 1307 are not right. I have not got to ordering the freq counter yet. I will get to that in the morning I am out of time today.

Thanks for all the help

 

[TheRealPorkchop]

LC is hitting bullseye’s here, as usual.

Mike hit the nail on the head, recap.

I skimmed so I might have missed it but - corrosive/conductive glue. Look for it. Might not have a “bad” crystal but one where the leg has been eat off due to that glue.

I’ll stress - recap - because I have two of these radios and both were DOA, a recap started things on the road to recovery. Removing that glue and finding damaged parts, replacing them, radios work perfect now. Get a capacitor tester and start pulling them, you’ll likely find many are way out of spec.

 

[unit435]

Agree with the recap. I would like to solve the problem or at least identify the problem before a recap of the radio. This would be easier for me to do everything all at once and be done with it as I do not have proper facilities to work on the radio. I am out on the road in a motorhome setting up things to work on the radio using the dinette then putting everything away and so forth so it would be nice to do it all together. If that is not an option (I am not qualified to determine that) then I guess I will need to figure out how to get the job done.

 

[Holydvr]

One issue is that a recap could potentially fix the radio. I know I have not chimed in too much, but there is always the chance that a single (or more by now) cap has failed and taken out the lot..

 

[Holydvr]

https://www.ebay.com/itm/New-Transi...2349624.m46890.l49286&mkrid=711-127632-2357-0


Not saying this is a lab grade meter, but it does measure capacitors and transistors etc..

 

[unit435]

One issue is that a recap could potentially fix the radio. I know I have not chimed in too much, but there is always the chance that a single (or more by now) cap has failed and taken out the lot..

The radio quit in the middle 90s maybe early 90s so under 20 years of sporadic use. After some reading, I suspect that the caps are up for replacement but it seems unlikely to be the root of the problem but possible and I guess I could find new problems if I do not replace the caps but fix the tx and rx problem.

I do have the schematic but did not have the parts list. Thanks. I should probably get the SAMS. Can you send or post it ?

 

[Lkaskel]

The radio quit in the middle 90s maybe early 90s so under 20 years of sporadic use. After some reading, I suspect that the caps are up for replacement but it seems unlikely to be the root of the problem but possible and I guess I could find new problems if I do not replace the caps but fix the tx and rx problem.

I do have the schematic but did not have the parts list. Thanks. I should probably get the SAMS. Can you send or post it ?

Here is a link on a CBTricks mirror site http://robco.nitwitz.org/cb_tricks/radios/cobra/138xlr/index.htm

I will say this, I am unsure exactly what your final desired outcome is but I would recommend that you "listen" to what the knowledgeable folks say on here. The troubleshooting process is one that can make or break a repair. If you are hoping to have the radio back up and running like day 1 and learn a lot in the process then these men here know the right way to do that. If you are hoping to just get it to transmit and/or receive and are not so interested in a complete repair then your method may be worse than the issue itself. I understand that you are saying that the radio had issues before the timeframe that the caps should have been bad so there is some other reason for its not working. The problem is, the caps are NOW at a timeframe where they could be bad (and are likely in some cases) and if they are not replaced then you will not be able to fully troubleshoot the radio otherwise. I recently had a same generation President radio that had sat since 2001 when the owner passed. I bought it, brought it home and all it would do is light up (meter and display). No transmit, no receive. I did a full re-cap and cleaned the radio. Guess what, it worked 100%. All it needed was a full alignment after that. I would suggest, if your end game is a fully restored/functioning radio, do the re-cap now. The voltages you showed could be impacted by an open, shorted or a cap that is turning into a resistor as it dries out. The troubleshooting process is sometimes more important than the fix/repair for folks like us that are a little knowledge handicapped.

Best Wishes for your repair.

 

[unit435]

Here is a link on a CBTricks mirror site http://robco.nitwitz.org/cb_tricks/radios/cobra/138xlr/index.htm

I will say this, I am unsure exactly what your final desired outcome is but I would recommend that you "listen" to what the knowledgeable folks say on here. The troubleshooting process is one that can make or break a repair. If you are hoping to have the radio back up and running like day 1 and learn a lot in the process then these men here know the right way to do that. If you are hoping to just get it to transmit and/or receive and are not so interested in a complete repair then your method may be worse than the issue itself. I understand that you are saying that the radio had issues before the timeframe that the caps should have been bad so there is some other reason for its not working. The problem is, the caps are NOW at a timeframe where they could be bad (and are likely in some cases) and if they are not replaced then you will not be able to fully troubleshoot the radio otherwise. I recently had a same generation President radio that had sat since 2001 when the owner passed. I bought it, brought it home and all it would do is light up (meter and display). No transmit, no receive. I did a full re-cap and cleaned the radio. Guess what, it worked 100%. All it needed was a full alignment after that. I would suggest, if your end game is a fully restored/functioning radio, do the re-cap now. The voltages you showed could be impacted by an open, shorted or a cap that is turning into a resistor as it dries out. The troubleshooting process is sometimes more important than the fix/repair for folks like us that are a little knowledge handicapped.

Best Wishes for your repair.

Thanks. I cannot compare any of the voltages I posted as I do not have the service manual or any other reference other than what members are telling me here. You are the first to imply that there are voltage problems and I was hoping to get some feedback so I thank you for that. I listen to everyone with a high level of respect and would not have it any other way. A re-cap is not in my immediate future so I was hoping to gain as much info as I can while I have some time to spend. If everyone feels that there is nothing else to be done until a recap is completed then I guess I am done until I do the re-cap. Thanks

 

[Lkaskel]

Thanks. I cannot compare any of the voltages I posted as I do not have the service manual or any other reference other than what members are telling me here. You are the first to imply that there are voltage problems and I was hoping to get some feedback so I thank you for that. I listen to everyone with a high level of respect and would not have it any other way. A re-cap is not in my immediate future so I was hoping to gain as much info as I can while I have some time to spend. If everyone feels that there is nothing else to be done until a recap is completed then I guess I am done until I do the re-cap. Thanks

I am not saying that you should give up. I am however suggesting that the troubleshooting process is the the best way to a repair. You and I may need to follow the process to diagnose something where a more experienced tech may be able to short cut the troubleshooting because of experience or knowledge. Also, I am not saying that you have a voltage problem either as I did not compare your findings with any documentation I have.

Try this, there is a sticky note at the top of the CB General Discussion Forum labeled Sams Photo Facts Collection. Dan McClain is the one who posted to it. Message him and see if he can get you the documentation for your radio. He is a really good guy!!! It's in Sam's 126 I think. Don't come to a complete stop if you can't do a complete re-cap right now. I would suggest to get one of the re-cap kits for that radio though. At least you will have the correct replacement cap if you find a bad one in your troubleshooting. I have repaired a couple of those 138XLR's and in my opinion they are one of the better older radios to have. My main radios are all pre 1980. PM me if you need more info that I may help you with.

 

[loosecannon]

here is a service manual for that chassis.
it's for the TRC 449 but its the same as your radio.

https://web.archive.org/web/2020122...tic/trc_449/graphics/realistic_trc_449_sm.pdf

in here you will find troubleshooting charts, and a block diagram that should prove helpful.
i didn't see any voltage charts though, which is a bummer.
i might have this SAMS at home, so i'll try to remember to look for it.

for right now, you might try plugging a speaker in to the PA jack, put the radio in AM mode, and in PA mode.
mic gain up.

when you key the mic and talk, can you hear yourself through the PA speaker?
this won't solve your problem, but will help in knowing what does work.

the other thing to check is the VCO voltage, which you can check with your DC voltmeter.
put your voltmeter on TP7 which is pin 5 of the VCO chip (IC5) and set the radio to channel 1.
you should see 2 volts here.
switch to channel 40. the voltage should be higher.

if it's below 2 volts, adjust L17 to set it there.

you should also read 9.5 volts on pin 1 of the VCO if that helps identify pin 5.
LC


EDIT: the schematic nomad radio provided on CBtricks does have voltages on the PLL.
your voltages on pins 9 and 10 seem off, which could mean a problem with the 10.240 mhz crystal.

you need a freq counter to check it, but if you have another, you can change it out and see if anything changes.