cobra 29 LTD problems

[longhaireddwb]

I got this at a yard sale along with another one that works fine. The one with problems wont tx or rx. Ive checked the audio chip by touching pin 4 and i get noise plus when radio is in pa mode it has audio. Please let me know if you agree with me this confirms audio is good.
Next i did is checked with another rig and no rx or tx. Not even a carrier coming through. I checked for 10.240 at pin 2 of ic1 and gotnothing. Checked pin 10 on ic1 and did get 5.120.

I did some looking at the schematics and it looks to me that the problem must be between X1 and ic1.

Am i on the right track here or could it be ic1 that is making it not tx or rx.
Im learning this stuff but dont have a grip on this section of the radio yet. Thanks for any help with this.

 

[longhaireddwb]

No replys. I thought i explained it ok but maybe not. Ill get back to his rig in a couple days and try a couple things and post up.

 

[Robb]

Just had a similar problem with a Superstar radio.
It was the electrolytic caps in the PLL area.
If it is old and you want to keep/fix it; then replace all of the electrolytic caps in the radio.

 

[longhaireddwb]

Thanks for the reply robb. Now if i can get someone to agree or disagree with my therorys about checking the audio chip. Im not real good at reading or making sense out of schmatics. I was hoping someone would let me know if i was reading the area of theproblem correctly. Thanks again robb as i will check the elec caps in the area first.

 

[longhaireddwb]

Well I was thinking WRONG!
I was compairing the test points of a 25 to a 29 and made a mistake.
I did find the 10.240 and 5.120 at the PLL. I even replaced it with one from a working radio and that didn't help.
I pulled every electr. cap one by one and tested them and they are all good.
I compaired the frequency reading at TP3 from the good radio to the bad radio and they did not match!
I turned the channel selector while checking the frequency on the good radio at TP3 and it would change when the channel selector was changed BUT when I did this with the not working radio it didn't change when I turned the channel selector.

So, Anyone have an idea about where and how to locate this problem? I know I'm getting close but just not too sure on what to do next. Remember I have no TX or RX but audio circuit seems to be OK.

 

[longhaireddwb]

Getting closer.
I got a short between D35 and D36. But not on the good radio.

Any Ideas?

 

[Robb]

You didn't need to change all of the caps at first; but it is just as well that you did. At least you know the caps are all right - and that is a definite plus. Just the caps in/around the PLL would have been enough for troubleshooting - IMO.

When the ch selector was turned, one radio showed a change on your test freq counter from the test point. That means that the PLL is getting a voltage and the VCO voltage is correct. Do both radios have the proper VCO voltage set? If it will not set; then there is still a fault in the VCO somewhere and the TP freq won't show a stable freq. I would concentrate on that. Check the transistors in that circuit too; just use the schematic to find them. Did you check the caps for capacitance AND resistance too? If a cap shows a resistance value/continuity, it is shorted and will drag the voltage down and that circuit won't be able to operate.

 

[longhaireddwb]

I used a cap tester to check the caps so I'm guessing it was just checking capacitance. If the resistances were wrong wouldn't that throw the capacitance off too?

I think I compared the voltages at the PLL on all the legs between the good and bad radios but now I'm not sure. I'll do that just to be sure they are the same. At least I know that the PLL is good and not the problem. I believe I went through all four IC's checking voltages.

I did go through most all the TR's but I'll do it again. BTW, I didn't replace all the caps, just took them out and tested them then put them back in. I think their was only 5 caps in that area.

The last post I put up I had found a short between 2 diodes (D35 and D36) that are in the VCO section. This don't seem right as I check the good radio and there is no short between them. I checked the schematic and they are not connected in anyway. That was the last thing I did before going to bed. I'll do some more testing today after I drop the wife at the airport, as it is my day off.

Thanks for your help with this Robb. Nice to have someone else to give opinions and ideas because after a while this radio might just go into the parts bin but I think I'm so close to finding it I don't want to give up just yet.

 

[Robb]

Buy some 1N4148 diodes. For $1, I bought more than 30. Take out and test any diode; or any part for that matter. There is a voltage chart available for this radio. In fact, there is a LOT of info available for this radio on CB Tricks. Having another radio to scavenge parts from is convenient. Don't give up; persistence and determination wins out. Having fixed two SSB radios in as many weeks has gotten me started with a better working knowledge of how a radio operates. Got help from this forum too.

You are right about the caps; but if a cap is marginal it may also show continuity - and that is a fail. If a cap is marginal and doesn't show continuity; then it may at some point soon. I would just replace it. I also replace any cap that has a 10v rating with one with a 16v rating. That was the group that seemed to have the greatest failures - BTW. Probably just a coincidence. When I recapped this SS 360FM; used a greater voltage capacity on all of the replacements if space permitted it. No necessary; but can't hurt it either.

Fix both radios; I would.


Schematic::
http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29ltd-st_29ltd_sch.pdf

Voltage Chart:
http://www.cbtricks.com/radios/cobr...st/graphics/29ltd-st_29wx-st_sm_pg12_pg34.pdf

 

[longhaireddwb]

Scratch what I said about the diodes. I tested again and I must not have been on the leads right as its now not happening. And sense there is no connection between them there shouldn't be any continuity.

I went back and pulled the five caps again. Tested them for capacitance and resistance and they checked out fine. I replaced them anyway.

I tested all the TR's and pins on the PLL and here is where things don't match up.

pins 15, 16, 17, 18, 19 on the PLL don't show the right voltages. Should show a little voltage but they are showing nothing.
As far as TR's: TR1, TR2, TR3, TR4 and TR16 don't show the right voltages. Not even close!

 

[Robb]

http://www.cbtricks.com/radios/cobr..._st/graphics/29ltd-st_29wx-st_sm_pg1_pg11.pdf

Find the block diagram on this page. Ya might want to right-click on it and turn it clockwise and then make it bigger so it is easier to see/understand. Find the area of the diagram that shows IC 1, 2, and 3. TR20 oscillator as well. D16, 17, and 18. This whole area - IIRC - is a series circuit. If one link isn't working; then the whole PLL circuit won't operate. I would check all of the electrolytic caps, diodes, and transistors in this loop. You may want to test FET1 & 2 just to make sure.

Check TR23/voltage regulator and related parts to be sure that the voltage feed is correct.
Are you pulling the transistors/diodes out before you check them?

Like I said, I am new to doing this troubleshooting - too.
But that is where I would poke around until I found the fault . . .

 

[longhaireddwb]

I just dont get it!

I tested the voltages on IC2 and it was fine. Looked good. I went ahead and pulled it and put one in from an old Uniden pc66xl (same part number) and now the radio works.

I guess you need more than the proper voltage readings on these IC chips.

Live and learn.

Thanks for the help Robb. Good luck in your next radio repair.

 

[RFEverywhere]

http://www.cbtricks.com/radios/cobr..._st/graphics/29ltd-st_29wx-st_sm_pg1_pg11.pdf

Find the block diagram on this page. Ya might want to right-click on it and turn it clockwise and then make it bigger so it is easier to see/understand. Find the area of the diagram that shows IC 1, 2, and 3. TR20 oscillator as well. D16, 17, and 18. This whole area - IIRC - is a series circuit. If one link isn't working; then the whole PLL circuit won't operate. I would check all of the electrolytic caps, diodes, and transistors in this loop. You may want to test FET1 & 2 just to make sure.

Check TR23/voltage regulator and related parts to be sure that the voltage feed is correct.
Are you pulling the transistors/diodes out before you check them?

Like I said, I am new to doing this troubleshooting - too.
But that is where I would poke around until I found the fault . . .

I've been poking around every website known to CB land trying to fix my three C29 radios. All have the same problem, but different issues. Anyway, after a month and half of poking around, this post was the most helpful. I already had the service manuals downloaded, so I did exactly what you said and within about 2 hours I had it narrowed down and repaired.

One radio down, two to go. The issue of the other two are different, but I'm sure I'll figure it out. Trying to work on the second one, I accidentally hooked up the final backwards (Yeah, I know..... how can anyone possibly do that, right? I had hooked it up to the solder side of the board for testing purposes.) and now it seems like I have two problems. I was getting the 27MHz frequency at the predriver and TX light would come on (but no power output to ANT), but after the incident, I am now missing 10.24MHz and TR20 is all off voltage and the TX light wont come on. I'll have to hit the schematics again tomorrow and get it fixed.

Just out of curiosity have you guys ever seen the ANT warning light be on at all times -Both during TX and RX-??

That was my original problem along with no TX, but now it's a double whammy.

Oh well..... I guess I'll be a self certified semi-pro technician after I finish fixing the 3rd one.

Then I'll be off to repairing a few 25 models and doing some 520 and 1969 upgrades.