The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

Ok guy's I'm in a little pinch here. Cobra 29ltd classic. 1989 philippines. The radio lights up. Has normal meter movement for receive, and keys up at nomal watts. Problem is there's no sound for receive, and when keyed up no audio or swing. Also tried external speaker still no sound. Pluged in speaker to pa and the pa don't work. ( no sound). Need some help please. Thanks Bill

Audio chip

Thanks for the reply marveylus 22. I thought the same thing too. I'll change it out and see what happens. Let you know how it worked out. Have a good one. BILL

Your welcome Bill.

If that doesn`t work try removing C112. If that works you will need to find a replacement cap.

Hey there!
I had already pulled the audio chip before I read your last post. I still pulled the c112 and checked it. The cap is a 470uf 10v. It showed it was 498uf on the meter. There allowed + or - (20) are'nt they or am I wrong.
Just in case I ordered caps and audio chips. Thanks again for the reply.
Bill

Check this out it may help you find the fault. I change all the 10 volt caps to 16 volt for better reliable.
http://www.cbtricks.com/radios/cobra/29 ... ic_sch.pdf

Have you replaced the audio chip? if so is the PA now working?

Rick

Hey Rick how you doing?
Thank you for the imput. No I did'nt get the audio chip in yet I'm waiting for it to be shipped. Hopefuly tomorrow or the next day. I wish I had new about the 16v I would had ordered those. But once I get it back up and running I can order some 16v and put them in. I had already ordered the 10v. Once I get the chip in I'll let you know if the pa comes back or the hole radio for that fact. Thanks again. Have a good one.
Bill

hello there guys.i have noticed most if not all the time.when the audio chip shorts out.you will find a small pin hole burned out in the center of the audio chip.side of chip facing the inside of the radio..tell tell sign it is toast.....take care ramblingman

Well replaced audio chip and c112 still no audio or swing or pa. If you turn the volume all the way up you faintly here talking. I mean faintly. The needle still shows receive just no sound. Well back to the bench. Any other imput would be great. Thanks guys.
Bill

Check the voltages around TR13, TR17, and TR23. Let us know what you find.

Hi Rick! Thanks for the help.
tr23 8.50, 12.66, 7.86 volts
tr13 0.00, 0.01, 0.01 volts
tr17 1.53, 1.53, 0.68 volts

I take it tr13 is no good? Thanks again for your help.
Bill

It looks like you have a few faults.

It looks like TR17 is short, but can you do the test below first.

Check the voltages at each side of R65 and R70 in RX and TX, and let us know what you find.

Check the voltages of TR11 and TR12 in RX with the squelch at min and max, and let us know what you find.

Also, try removing C37 see if the RX comes back.

Is the audio IC getting warm?

Also, check for cold solder joints around audio IC and TR17 and TR18.

Hey Rick I got some numbers for you.

R65 rx one side 8.63 volt. Other side 1.53 volt
tx one side 0.18 volt Other side 0.84 volt

R70 rx one side 7.85 volt Other side 7.80 volt
tx one side 7.87 volt Other side 7.46 volt

tr11 rx squelch min. 0.69, 0.01, 0.00
max. 0.05, 4.26, 0.00

tr12 rx squelch min. 0.02, 0.69, 0.00
max.0.63, 0.05, 0.00
Removed C37 receive didn't come back.
pulled tr17 it showed good. But when I put it back in I double checked volts and came up with something else. 1.53, 7.81, 1.59
Nothing else seemes to be getting hot. The only one that gets warm is tr23. Hope I got these numbers from the right place. Thanks for your time
Bill

The voltages you have given me are ok. Can you give me the voltages of the audio chip pins starting with pin1, pin1 this is the one with half moon cut out on the chip.

Funny you should mention that. I did that also last night. Here they are.

1) 13.48
2) 6.47
3) 0.00
4) 0.01
5) 1.15
6) 1.16
7)ground
8)ground
9) 6.68
10) 12.78

Also pulled tr18 and checked that and it was good. Thanks again. Bill

Well that is ok. You have tried a PA check by plug in an ext speaker in to the PA socket and switch the PA and giving a shout in the mic, try this and see if you can hear any audio? the only thinks to check are C84 and the transformer T1 if you have a short in T1 you may just hear very low audio. Also check the PA and ext sockets one could be o/c.

Can you give me the low-down on the how fault manifested it?s self.

Thanks for you help and time Rick. As far as what started this problem I can't tell you. All I know is I got it and I was told the speaker was no good. Easy fix so I thought. All I can tell you is if you turn the volume all the way up and put the speaker by your ear you can here people talk. But it is realy faint As far as the pa , yes I did try the pa many times and it don't do anything. Like I had said the meter shows receive no problem, it keys up to 4 1/2 watts but no mod. No swing No nothing. Lights all work, turns green to red for tx. , rf gain works, dim switch for lights work and emerg. channel 9 switch works. I also hooked up freq. counter and everything is on freq. Here is a long shot, but if evrything else shows good could it be something as simple as in the mic jack or in the board where the mic jack is? But if the audio chip shows good then I guess that would blow that thought out. Hugh. I also went through and did some cold solder joints cause I know how that all works but also found nothing. Hey would you be interested in a boat anchor. Thanks again Rick for you time. Back to the bench.

Bill

Ok hear what you need to try, I am assuming that the speaker is good and capacitor C48 (470u) is good, just to make shore the speaker is good can you try it a second radio if possible.

Unsolder the positive side of capacitor C48 (470u) and isolate from the rest of the circuit the positive side of the cap is the one connected to transformer T1 this may not be easy as the caps legs of the are usual short, a second way to do this is to cut the PCB track between the cap and transformer T1. When you have isolate the positive side of the cap solder one wire from the speaker to the isolate the positive side of the cap and the other wire form the speaker to ground. Do not put the radio to TX this test is for RX only. Let us know how you get on.

I just remember a 29 I repaired about 8 years ago had on audio and it was C39 or C44 which had gone short, it maybe it would be worth check all the caps around the audio amp just in case one is bad.

Hey there Rick hope you are doing well!
I happen to have another 29 that I had hooked the other speaker to, and it seems to be fine. Speaker also not a problem. I haven't pulled C48 to check it yet. I did pull C44 and a few others with no luck. All good. I've been on this hard for the last few days, I think I better walk away for a day or two. I also did somemore solder touch ups too. I have also did some double checks on votages on this other 29 and so far came up with about the same. I've got to be walking all around the problem after all these checks. Anyway thank you very much for all your help and time.
Have a great one Bill

Well Rick you can ignore that last post. I couldn't leave well enough alone. Went back and pulled C48 to check it, the positive side was burnt off, the bottom of the cap was black. Stuck another one in and there she was,( receive) and (audio). Just think almost the last thing to check,and there it was. Go figure hugh! Well nonetheless it's done. Thank you a million. your good people Have a great one.
Bill

This is why it can be costly to have radios repaired, it easy when you know what as failed.
Well why did you not check C84 in the first place, just think of the time it would have saved you, only joking.
Well you got there in the end that all that counts.

rick, youre a pretty good hand... umm, but your picture scares my kids... and me a little bit