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Message |
    
Mopedfreak
New member
Username: Mopedfreak
Post Number: 2
Registered: 11-2007
| | Posted on Wednesday, March 12, 2008 - 7:37 pm: | 
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I have a nice 1998 model Cobra 29 LTD ST radio.
Recieves fine, but on transmit it produces a good carrier signal that is picked up by other radios, but no audio. I have tried another mic. no change.
Any ideas?
I bought it this way, (for next to nothing) so I'm not sure what caused it. |
Romstar
Intermediate Member
Username: Romstar
Post Number: 130
Registered: 3-2007
| | Posted on Thursday, March 13, 2008 - 2:49 am: |
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Well, taking a look at the block diagram gives us a pretty good idea of where to start.
From the mic, the signal travels to the dynamic control (a variable resistor) then into the mic amp, which is TR17 and associated parts and the AMC control, which is TR18 and it's associated parts.
From there, it jumps to the AF power amp. The "audio-chip", most commonly the TA7222AP.
From there, the audio chip dumps the signal through a diode, D8 in most cases.
Then it feeds to the driver and final.
So, since you are getting audio on receive, it's probably a safe bet that the audio chip is okay. Not a guarantee, but a safe bet.
This leaves you looking at TR17, TR18, D8 and those local components.
I'm willing to bet that the driver and final are good because you are getting a carrier, and no modulation, and that the audio chip is good, because you are getting sound on receive.
Give those a check, and let us know.
Frank |
852
Intermediate Member
Username: 852
Post Number: 468
Registered: 3-2006
| | Posted on Thursday, March 13, 2008 - 6:34 am: |
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Just a thought, but have you checked to see that the mic. gain is turned up??
Tommy~852~CEF 750
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Captian_radio
Intermediate Member
Username: Captian_radio
Post Number: 374
Registered: 9-2004
| | Posted on Thursday, March 13, 2008 - 11:30 am: |
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Try it in PA mode,if nothing in PA mode could still be the audio IC.
Bob CEF451/VE1CZ
Robert L. Spicer The days of radio are just beginning!
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Romstar
Intermediate Member
Username: Romstar
Post Number: 131
Registered: 3-2007
| | Posted on Thursday, March 13, 2008 - 2:20 pm: |
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Bob's right,
It is possible that PART of the audio IC is gone bad. If so, you need to find one of those, remove the old one and solder in the new one.
Tommy has a good comment as well, since the dynamic control is basically a variable resistor, if its turned down, that would cause problems.
Personally, I never really understood the need for the dynamic. RF Gain sure, but dynamic should be re-labled. It serves no useful purpose at all, and in many cases most people tie the wires together, and use the pot for something else.
As much as I hate saying it, check your electrolytic capacitors. These things are notorious for "dry cap" syndrome after a few years.
Good luck to you in any case.
Frank |
Mopedfreak
New member
Username: Mopedfreak
Post Number: 3
Registered: 11-2007
| | Posted on Thursday, March 13, 2008 - 6:43 pm: |
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It appears that the audio IC must be partly cooked. I tried to Key up with a headphone hooked up to the PA.. No audio at all.
Still recievves fine, so I'm guessing I should try the audio IC.
Well, Does anyone know if this part is availible? should I call cobra and ask?
Also, can anyone tell me what may have caused this failure? Like I said before, I bought the radio like this so I don't want to put a new chip in, and have it pop!
thanks for all the help so far. Now sadly I have to go over to the base station tech section and see if I can get some help with my Uniden washington that just quit. |
Mopedfreak
New member
Username: Mopedfreak
Post Number: 5
Registered: 11-2007
| | Posted on Friday, March 14, 2008 - 12:31 pm: |
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Oh boy, I feel like an idiot!
I must have spent 2 hours with my multi meter poking around with this radio last night.
I gave up around 11pm, and decided to look at a little midland 1000 I have in my wifes car. I had put power mike on it and it had stated acting strange.
So I grabed my 2 other 4 pin power mics and headed out to the shed.
Well, BOTH the mics I had been using to test the cobra 29 were dead. and both had the same problem. both key up, but have to audio!
well, I put the stock mic back on the midland, and its back to normal, and then went back in and at close to midnight plugged in the power mic I use on my (now broken) base. It TALKS!
Thanks for all the help, and ideas. I'm glad it is all better, and now am moving on to getting my Uniden washington back up and running. |
Romstar
Intermediate Member
Username: Romstar
Post Number: 132
Registered: 3-2007
| | Posted on Friday, March 14, 2008 - 1:19 pm: |
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DUH!
You think you feel stupid?
I neglected the first rule of trouble shooting.
Be sure all cables are firmly connected, and check your mic.
Don't feel too bad. I am sure it happens to almost everyone sooner or later. I know that I have a few bad mics here, and that it has also caused me similar headaches.
Glad to hear that it wasn't anything serious.
Frank |
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