Older cobra 29 ST with no modulation

[Cityboy]

I have a 1995 ST that I have replaced the final, driver, audio chip and inspected the board for bubbles and microphone wiring problems with no solution to my problem; which is no modulation. It keys up 5 1/2 w and will not swing. All soldering points look good and mic gain is good. Please help for I can not find the problem.

 

[dudmuck]

Test the public address function.

If it fails, then its problem with audio amplifier. This is the same circuit that provides modulation source.
If you have PA sound, then its problem between audio amplifier and interface to collector on final RF.

 

[9C1Driver]

Ouch, and you have already spent way more then that soundcrapper is worth. Hope you get it working, if your doing it yourself at least you save the labor cost.

 

[Cityboy]

Thanks for the help and will test when I have time

You are correct in that I have spent way too much time on this soundcrapper; but all the parts were laying around so no real money loss. I am fairly new to messing around with cbs. I went over this to learn and so far I'm just frustrated with it. The suggestions on the PA testing is something I thought of but haven't pulled out my external speaker to test. Thanks for the hint and at least I got the cb for free also; so the only thing I might out of is my mind in the end.

 

[9C1Driver]

I hear ya, thats perfect for learning and messing around. I did the same with an old Uniden SSB radio.

 

[Cityboy]

A couple of other things about this radio; someone blew the rectifier diode and charred it so bad it fell apart into dust. Then someone put a rectifier across the plug terminals to get it working, and someone had an echo board in it at one time because the delta tune is not original or hooked up and one of the tuning cans had a soldered ground wire on it. Thats all for now thanks for the replies.

 

[Tony]

did you try more than 1 mic?

 

[Cityboy]

Yes, I have a RK56 and an Astatic 636l that I tried with the same results. I still haven't tried the PA yet but will do so later tonight.

 

[Cityboy]

I have tested the PA has low volume white noise but when keyed up it will not come out of the external speaker, so please point me a direction to hopefully fix this old piece of junk and maybe I will learn something!!! Thanks

 

[SkipJunky]

I just had a 29 doing the same thing, it was a cold solder joint on the audio chip.

 

[Cityboy]

I just installed the audio chip, but I will go back over it with a fine tooth comb and make sure it definitely soldered correctly. Thanks for the help.

 

[Cityboy]

I did some more investigating on the soundcrapper and the audio chip has no cold solder connections, but I did some voltage checks on the rectifier diode before and after turning on the radio and something came up. The voltage across it before was a little low around .513v and when I turned the cb on it went way down to .112. I'm sure it lies in the problem there because the radio was definitely shorted and rect diode replaced and I did it again and the capacitor right behind it c124 or c174. Hope this will help in your replies. The volume contol and squelch has been looked over and all looks ok. Point me in the right direction please.

 

[Moleculo]

I wanted to follow up on this thread because I just got through troubleshooting a radio with a similar problem. In this radio, the problem was TR17. One of the pins of the transistor was shorted out to another pad for some reason. After I fixed that, I had modulation. What I did to find the general location of the problem was turn the radio so the solder side was up, then key the radio and use a wet finger to slide around the radio starting from the audio chip and working toward the final. On the scope you can see that this causes modulation. When I got around TF17, it stopped causing any spikes in the output, so I knew it was around there.

One thing that's interesting is that now on the air reports tell me that after a while the radio starts cutting out. Watching the meter, it seems that when it starts to get hot, the radio is cutting the power back. I guess I'll need to check the final and driver now, or perhaps TR17 is bad all together. Anyway, at least I have modulation and full power most of the time. Now it's on to troubleshooting the next problem.

 

[Cityboy]

I just looked back at this thread and that sounds like something I will try asap and I will let you know what I find. The wet finger trick I have not heard of; do you use it to trouble shoot other things? I really appreciate it, I would like to get this piece a junk working. Not because it is worth my time, but because it is bugging the heck out of me to figure out and maybe this will work. By the way I tried the wet finger and watched my scope and didn't see or didn't run my wet finger across the right parts. Do you talk into the mic while running your wet finger across the board; explain to me what I should exactly do and what am I looking for on the scope. Sorry about the ignorance, but I am trying to learn this on my own without anyone around here to show me hands on. Thx and thx to BC I will yank c45 and put a new one in there and get back to you when I am done. Thx again.

 

[Bionic Chicken]

I have seen C45 to go bad and cause no xmit audio. Pull it and see if that is the problem.

BC