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Message |
    
123upmichigan
| | Posted on Thursday, June 19, 2003 - 8:37 pm: | 
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hello 123 upper michigan here.. i have a cobra 139 xlr i just picked up and have a little problem.. this radio has had the rotary switch channel mod done to it, as well as an unlocked clarifier..
ok the problem... this radio will only recieve close signals on the receive.. a mobile in front of the house will bury the meter but will barely drag in enough skip to budge the meter while my washington will drag in 10 s units. i know the receive is messed up just not sure how. also for some reason i can only get this radio to tweak up to 6 watts. it has new finals in it, but that is all it seems to want to do. can anyone help me? |
2600
| | Posted on Friday, June 20, 2003 - 2:20 am: |
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If you have also "tweaked" the receiver alignment and all the slugs show a decent peak, take a look at L16. It's the slug-tuned coil nearest the front of the circuit board to the right (your right) of the 858 chip. If the slug appears to peak with the slug EXACTLY LEVEL with the top of the can, that's a clue. It should peak with the slug down inside the hole, NOT level with the top of the can. If this is what you find, there is a 'dirty trick' to avoid having to find a replacement for L16. If the slug in L16 DOES peak with the slug at least a couple of turns down inside the form, this isn't the root of the problem.
73 |
123upmichigan
| | Posted on Friday, June 20, 2003 - 2:23 pm: |
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well, i tried to peak l16 but to no avail.... it did not seem to help the recieve at all. it is almost like the rf gain is turned down compared to another radio. my washington is giving 9 sunits and the cobra will read one.. it is for sure not a meter adjustment.. is their anything else that can be done to help this old 139? |
Karatebutcher
| | Posted on Friday, June 20, 2003 - 5:29 pm: |
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They are a great radio if you get it up and running, I have mine in line right now while my other radio is being worked on. |
123upmichigan
| | Posted on Friday, June 20, 2003 - 9:41 pm: |
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i'm tryin my best, wanna be talkin on it.. like the looks a little better than the washington.. but i refuse to run as an alligator station all mouth and no ears...lol hopefully someone has got some help for me |
Clark514
| | Posted on Friday, June 20, 2003 - 11:44 pm: |
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on my 135 xlr the meter ajustment are under the am usb lsb switches on the circut board there are 2 white varible flat pods these 2 ajust the recieve meter hope this helps |
2600
| | Posted on Saturday, June 21, 2003 - 1:10 am: |
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Hi 123,
I got yer e-mail, but thought I'd answer back here.
If this was a radio that you had used, and had seen perform properly in the past, that would be a basis to try troubleshooting. Since it has never received right since you got it, the list of possible problems gets a LOT longer. Just guessing how many "golden screwdrivers" have been in it since it first broke down might help. every time someone "tries" to fix a radio and doesn't, more uncertainties get added to the picture. How many slugs got randomly twisted?
The usual advice for a "new-to-you" radio that was already lame is to attempt the alignment. Some folks will key the mike, focus their tunnel vision on the wattmeter needle, and start twisting. If they would just put the slug back where it was when it DOESN'T make the meter go higher, no harm would be done by trying each one in the radio. Very few folks take that much care. The result is a receiver that was 'ALIGNED WITH THE WATTMETER' with the slugs set randomly off their proper peaks. Usually not an effective method.
The one place you can find the procedure to see that all the receiver adjustments are set to their proper "peak" positions would be to buy a reprint from Howard Sams publishing for that radio.
This model was made by Uniden for Cobra. Numerous other brands and models used the very same circuit board. If you can find a book for the Robyn SB-520D, Radio Shack TRC-457 or 458, the original (4-pin mike plug) President Washington and Madison, the alignment procedure and adjustments are the same.
One more cheerful note: this radio was rather sensitive about being left hooked to an antenna during stormy weather. A lightning strike down the street could damage the receiver, if your beam was pointed that way, even from a half-block distance.
I don't know for sure if newer radios are more robust in this department, or if more people have learned to unplug their coax in the last 25 years. I just remember a lot of people learning that lesson with a radio like yours. The typical death toll would be a transistor, some diodes, one or two of the slug-tuned coils and the noise-blanker chip. And some of those parts are getting hard to find, unless you have a junker or two on hand.
Kinda reminds me of driving a 25-year old T-Bird to work. So long as it's runnin'
Too bad the tools for fixing a 1978 CB radio aren't as simple as the ones for the T-Bird. Doesn't hurt to have a junker or two in the back yard, either.
73 |
2600
| | Posted on Saturday, June 21, 2003 - 1:15 am: |
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Uh Clark,
The 135XLR came from a completely different factory. That one and the Cobra 132 mobile radios came from the Toshiba factory. The 139XLR is a Uniden product. The adjustments won't match. The "GTL" line of radios that followed when the "XLR" models were dropped all came from Uniden, and resemble one another a lot more on the inside.
73 |
Kc0gxz
| | Posted on Saturday, June 21, 2003 - 1:51 pm: |
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123upmichigan
There are a couple of diodes in the receiver that will blow if it takes a hard RF hit from a transmitter that is close by. Like maybe a mobile going by your house. And I also recall a FET in the receiver that was known to blow from overloading the front end too.
I have been looking for the schematic to the 138-139XLR but can't seem to find it. And for the life of me, I do not remember the diode-placement numbers. Perhaps someone else has a schematic for that radio.
I had my doubts right away that it would have been a mis-tuned coil. Those slugs rarely turn by themselves unless someone deliberately turned them. But anyways, my money is on a couple of blown diodes.
I hope someone can help you. The 138-139XLR were great radios in there day. Far better than any 142-148GTL or Uniden Washington. Too bad the FCC put the kabosh on their PLL chip. Not too many CBers of this generation know much about them other than us (i'm speaking for myself of course) older guys.
73s
Jeff, kc0gxz. |
123upmichigan
| | Posted on Saturday, June 21, 2003 - 10:38 pm: |
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thank you.... i really really appreciate the help. i will start checking doide right away and keep you all posted... totally great.. thank you for all the support ... take care sam holmes 123 upmichigan |
Kc0gxz
| | Posted on Sunday, June 22, 2003 - 8:03 pm: |
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Hey 2600
I liked your explaination about "tuning the receiver" with a watt meter. It was very funny but also very true!! I see that you have had radios pass through your hands that were tuned that way also.
73s
Jeff, kc0gxz. |
123upmichigan
| | Posted on Monday, June 23, 2003 - 2:51 pm: |
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well i checked all the diode in the front end of the radio... and.. nothing they were all good.. i tested this radio against a washington and it is like running the rf gain turned back to ten oclock.. i marked all the cans, then tried adjusting them for amx noise but ended up putting them back where i found them cause it did no good.. i am fresh out of idea's on this radio so any input anyone else has is welcome to give me some suggestions.. thank you all once again 123 up michigan |
2600
| | Posted on Wednesday, June 25, 2003 - 11:34 pm: |
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Hi 123,
Don't know if you have just given up in disgust yet, or not. There are a couple of things worth checking, just because the odds favor them.
The noise blanker in this radio is a little primitive. It is fed right from the receiver side of the antenna relay. The receive signal coming in from there splits off two ways, feeding into L3 and L1. These are the 10mm slug-tuned cans almost directly between the relay and the rear panel. L3 is the one nearer the rear panel. It feeds into IC1, the uPC577 noise blanker chip. If there is a problem here, it could rob the receiver signals before they get to the rest of the receiver. Find C1, 39 pf disc that feeds into one pin of L1. Unsolder and clear one leg, or just pull it out, and try the receiver. If the blanker section is causing trouble, you will have more channel noise and chatter than before. There is no easy way to adjust the three slugs in the blanker section without a signal generator for sure, and a scope if possible. If someone twisted the L1 slug the wrong way, it could cripple the main receiver section. Pulling one or two legs from C1 out of the board would restore the main receiver, if this is causing some, or all of your trouble.
L3, the one nearer the relay is the first input coil into the receiver's preamp stage. Transistors TR4 and TR5 are vulnerable to any overload coming down the coax. TR4 is really easy to check here. Find C12, a .0047uf disc cap that is between two gray thin electrolytic caps, midway between the relay and L3. Unsolder one leg only, or just pull it out. If your receiver sensitivity returns, something is unwell with TR4 and/or the parts that control it. If this produces no change for the better, pull out TR5, the 2SC1730 transistor just to the right (your right) of L3 and test it.
I'm gonna skip the whole issue of how to test TR5, but if it's blown, you will only hear the very loudest ones, for sure. I gotta track down a handy all-purpose link to an explanation of the whole "How To Test Those 3-Legged Things With An Ohmmeter". Sure would be handy to refer folks to if they aren't familiar with that routine.
And if you decide to just chuck it and try again, you won't hear me criticize. At all.
73 |
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