Cobra 138XLR

[Dmans]

unit435,
Attached is the TRC-449 service manual LC mentioned above. It is a pretty good manual and has some troubleshooting information in it that may be helpful to you.

Best of luck.
73
David

 

[unit435]

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.

Does the 148GTL use the same crystal ? I have a stock 148 with me that I purchased out on the road for cheap and it works. The guy I bought it from wouldn't sell me the 5 pin microphone he had when I tested it as he was short on microphones so I do not have a microphone as the 138 is a 4 pin and the same for the 29.

Thanks to everyone for the help. Its been very helpful as I slowly figure this thing out.

I have not purchased a counter yet but I should get that resolved this weekend. I will also download the manuals. I have some things coming up next week that may not be helpful as far as how much time to spend on the 138 and this weekend will be busy.

LC I will test with an extension speaker this weekend. I used headphones for the last test but the stereo jack is different than the mono jack so I will try it another way to make sure nothing is there on both PA and CB modes.

Thanks everyone for the manual links. I will shortly follow all of that up and get the counter here at the RV park where it looks like I will be for another 2 months.

 

[loosecannon]

yes a 10.240 crystal can be found in most PLL driven radios.

you can swap the two crystals in your radios and see if the problem follows the crystal.

you might just try this since you have all the parts already.
LC

 

[unit435]

yes a 10.240 crystal can be found in most PLL driven radios.

you can swap the two crystals in your radios and see if the problem follows the crystal.

you might just try this since you have all the parts already.
LC

Sounds good. Would it be better to put the 138 crystal in the 148 or the other way around? I dont want to cause any more problems if it can be avoided.

I dont have a 5 pin mic for the 148 and I think that unit may need a mic plugged in to receive. I cannot remember. I could pick up a 5 pin connecter and rewire one of my mics or possibly stop in at a truck stop and see if i can buy a 5 pin mic or jumper the 148 without a mic. I will be near a Travel America truck stop tomorrow. I will stop in to see what they have.

Thanks

 

[Lkaskel]

Sounds good. Would it be better to put the 138 crystal in the 148 or the other way around? I dont want to cause any more problems if it can be avoided.

I dont have a 5 pin mic for the 148 and I think that unit may need a mic plugged in to receive. I cannot remember. I could pick up a 5 pin connecter and rewire one of my mics or possibly stop in at a truck stop and see if i can buy a 5 pin mic or jumper the 148 without a mic. I will be near a Travel America truck stop tomorrow. I will stop in to see what they have.

Thanks

One other option for the 5 pin mic jack is to get a 5 pin to 4 pin adapter. You can find the on Amazon cheap. Look for Workman C4P5C. This way you can use the same mic on both radios.

Regarding the crystal, put the 148 in the 138.

Where are you BTW, it would be cool if you were near one of the techs.....

 

[loosecannon]

just take the 10.240 crystal out of the 148 and put it in the 138XLR.

if you turn on the radio and now have RX and TX then you know you need to order or find another crystal for the 148.
LC

EDIT: there is a chance that the crystals in your 138XLR are the plug in type that have fatter legs on them, and the one coming out of the 148 will have the skinny solder type legs on it.

if this is the case, then just remove the socket from the 138XLR and solder the crystal from the 148 right to the board.

 

[unit435]

I have a long day today but will see if I can pull out the 138 and the 148 to see what I need to do later tonight. Sunday will be mostly accounted for also. Either way it looks like I may get some answers doing the crystal swap. Thanks for all the help.

I will order the mic adapter and a couple 5 pin plugs probably when I order the counter which needs to be pretty soon by the looks of things.

If it turns out to be the crystal, I will need to find a replacement and get the cap kit from KM. After those repairs, I will need to get the unit aligned so it may be necessary to get the radio to someone who can do that as I am not sure I could do it myself. I have not looked at what's involved.

Thanks everyone.

I am currently about 10 miles south of Houston TX.

 

[Klondike Mike]

... and get the cap kit from KM.

I made up a kit specifically for the Cobra 138 XLR. Use this one instead of my link posted earlier. See link below;
https://klondikemikescapkits.com/products/cobra-138-xlr-electrolytic-capacitor-kit

 

[Dmans]

Hope all the caps are 12v or less!!

73
David

 

[Holydvr]

If you end up needing a 10.240 crystal let me know once you have run your tests.

 

[Lkaskel]

Hope all the caps are 12v or less!!

73
David

Ok, that was a little bit funny......

 

[unit435]

I swapped the 10.240 crystal with the 148 and no change. Later tonight I will download all service related manuals, locate and order a frequency counter and start looking around.

I have not placed the radio in the PA mode which will require some rewiring. I may have indicated earlier that I did but I did not. That test may provide some useful information as I think there's circuitry that is in common with the CB mode. That is on my list for the next round.

The only other item worth mentioning is that this being the first time I had the 148gtl open, I did notice that the power was peaked on it. Everything else looks untouched. I bought the 148 many years ago out on the road near Knoxville and never opened it to look inside. Who ever peaked the radio wrote the date and service underside of one of the covers.

Thanks for all the help. Hopefully I can restore the 138xlr back to working order. Nothing is easy in this world. Just need to keep at it.

 

[unit435]

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


This link is dead. Does anyone have a copy of this document or a good link ?

Thanks

435

 

[unit435]

I screwed up on my last post. The link the Lkaskel posted from CB tricks is dead. Is there any other way to get that document ?

 

[loosecannon]

435, my link should still work.
LC