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Message |
    
Troyota
New member
Username: Troyota
Post Number: 4
Registered: 4-2004
| | Posted on Friday, September 29, 2006 - 11:41 pm: | 
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this is a friends radio. it has a great carrier, but no modulation. no tinkering inside has been done. already tried all the obvious things. changed mic, antennas, tried it in 3 different vehicles, and bench test. checked for loose or broken wires from the mic socket. he uses it in his rig. he said it sometimes would squeak, then one day it let out a loud squeal and ever since no mod. radio worked perfect before that. any ideas??maybe the final let go?? thanks guys |
Hotwire
Senior Member
Username: Hotwire
Post Number: 1829
Registered: 1-2005
| | Posted on Saturday, September 30, 2006 - 3:01 pm: |
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On the bench I'll assume it was conected to a 50 ohm dummy load. Mic cord and mic plug checked ok? If it has a carrier with low or no modulation it may need an alignment or replacement of RF final and or driver transistor.
Squealing when keyed? Mic may need rf filtering. Check for good rf grounding.
If your tests was done with an antenna and coax attached be sure your swr is acceptable. You may have tried different antennas but if you had bad coax with possibly a short then yeah your rf final/driver transistor may have let go. |
Troyota
New member
Username: Troyota
Post Number: 5
Registered: 4-2004
| | Posted on Sunday, October 01, 2006 - 1:00 am: |
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thank you hotwire, i tried many coax and antenna combos. unhooked my connex from my truck and hooked his up and no mod. my connex works perfect before and after. my bench has a antron 99 hooked up when i test. i suspected the final/driver just wanted to get a professional opinion. i guess i will pop it open and look around in there. you think cb tricks has a schematic for this? or can you point me in the right location of the driver and final? pretty sure of the general location, just want to make sure its the right ones. its always best to ask before prodding around in there.
thanks again |
Hootyal
Intermediate Member
Username: Hootyal
Post Number: 173
Registered: 9-2002
| | Posted on Sunday, October 01, 2006 - 7:37 am: |
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If you have a carrier(good)...I doubt it would be final/driver...Does it have receive audio? Maybe the audio chip if not...just a quess |
Troyota
New member
Username: Troyota
Post Number: 6
Registered: 4-2004
| | Posted on Sunday, October 01, 2006 - 12:47 pm: |
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yes, i have great carrier. and also great receive. just no modulation. it acts like a mic but i tried 3 different ones. even one off my connex that i know works. checked all wires around mic plug and checked for burnt traces. i will check again.
thanks |
Hotwire
Senior Member
Username: Hotwire
Post Number: 1837
Registered: 1-2005
| | Posted on Sunday, October 01, 2006 - 4:37 pm: |
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Like Hootyal says if you have a good carrier then the final is ok. But how do you know ? Best way is to have a friend go out a little distance and see if he can recieve your strong "dead key". If a recieving radio has to get closer than normal to pick you up then the final is most likely dead and its the little signal of the driver your recieving.
If you do have a strong dead key that can be picked up for miles with no modulation then its not the final. So you have good recieve audio? With most radios the some amp circuit that powers the radios speaker is the same circuit that produces your transmit audio.
The problem you are describing is the most typical and easiest to fix cb problem there is.
"Throws dead carrier but no modulation.". It is a mic problem 99% of the time. You really need a dummy load with a modulation light but another way to know the mic is working is this; Key the radio and watch the meter. The needle should go far over to the right. Now speak into the mic. The needle should move forward and back while speaking with almost all cb radios. If it does not do this then in almost all cases you have a mic/mic wiring problem.
I would double check the mic. Inspect the inside of the radio. Check the mic wires on each end to see if their soldered connections broke off. On the mic connector end and where they connect to the board.
The final in that radio should be bolted on the rear of the radio. The back of the radio acts as a heat sink. The final(TO-220 device)is a small black square shaped transistor with a metal tab on top for attaching it to the heat sink. It has 3 prongs sticking out from its bottom soldered to the board. They are labeled with a model number like c1096, c2166, c1969,c1237,c1306,c1307, c1678, many others.
Once again if it has a good strong dead carrier, the needle moves when you speak and the recieve audio is fine but no transmit audio , you are positive the mic checks ok, it sounds like you should take it to a good tech to run some tests on it. Good luck and hope I was able to hope a bit. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 2121
Registered: 9-2004
| | Posted on Monday, October 02, 2006 - 12:19 am: |
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on some radios, if the AM power regulator goes out, it will kill the modulation and raise the dead key wattage to dangerous levels.
if your radio is dead keying much over 6 watts, i would suspect this.
i dont know if this is an issue with this particular chassis or not, but its a place to check.
in this chassis, its TR23, and its a KTD880AY. replace it with an NTE152, as they are easier to come by and handle more current.
i cant say that this is for sure, but its a 3.00 part, and its worth a try if you dont want to send it to a tech.
good luck,
matt |
