Ok, so I posted on this radio before. I changed the final out with a new one. Radio worked for about 3 months. Now it has no audio but normal carrier. I tested it in PA mode with a speaker connected and nothing there either. I'm thinking the audio chip maybe? I tested the mic with my other 29 and it's not a mic problem. Where is the audio chip on the board? It's not the long IC chip covered in the pile of wax is it? I hope not, I have done enough shoveling this week!
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Another 29LTD classic problem
- Thread starter badfish
- Start date
it sure does sound like the audio chip.
its bolted to the chassis on your right hand side if you are looking down at the radio with the component side up and the knobs facing you.
look towards the back.
its got about 10 pins and they are all in one row.
when you go to remove it, dont lose the thin, clear mica insulator that isolates the metal tab of the IC from the metal chassis of the radio.
if that metal tab shorts to the chassis, then your new audio chip will blow also.
get some heatsink compound from radio shack, put a very thin coating on the back of the new IC, stick the mica insulator on, then put another thin coat on the back of the mica insulator.
use the same hardware that came off of it, and make sure the plastic shoulder washer (if its used instead of a nylon bolt) sits centered in the hole in the IC's tab.
also make note of the way the chip is oriented in the radio because you really dont want to put the new one in the wrong way.
IIRC the label on the IC faces the chassis wall.
good luck,
LC
its bolted to the chassis on your right hand side if you are looking down at the radio with the component side up and the knobs facing you.
look towards the back.
its got about 10 pins and they are all in one row.
when you go to remove it, dont lose the thin, clear mica insulator that isolates the metal tab of the IC from the metal chassis of the radio.
if that metal tab shorts to the chassis, then your new audio chip will blow also.
get some heatsink compound from radio shack, put a very thin coating on the back of the new IC, stick the mica insulator on, then put another thin coat on the back of the mica insulator.
use the same hardware that came off of it, and make sure the plastic shoulder washer (if its used instead of a nylon bolt) sits centered in the hole in the IC's tab.
also make note of the way the chip is oriented in the radio because you really dont want to put the new one in the wrong way.
IIRC the label on the IC faces the chassis wall.
good luck,
LC
i have to apologize for jumping to the audio chip conclusion.
upon re-reading your original post today, i see that you did not mention whether or not you had receive audio.
im guessing that you do.
if so, that means the audio chip is not at fault. sorry for not reading this closer and asking you this question before offering advice.
assuming you do indeed have receive audio, then the next place to look is in the mic amp circuit.
check TR17 which is a 2SC458 transistor. you can check to see if it has voltage going to its base leg by putting a DC voltmeter on it while the radio is on. (negative lead from the voltmeter to PC board ground foil of course)
the legs of this transistor are ECB looking at the flat side of the transistor, so you would be wanting to test the leg on the right.
you should see voltage on this leg. about 1.5 volts in RX and TX.
if you do not see any voltage there, check R70, and you may find that it is open, meaning that if you tried to test it with your ohmmeter, it would show no connection between the two leads. you should remove it from the PC board to test it.
if you did see the 1.5 volts on the Base leg, then check the middle leg which is the Collector. you should see about 4.8 volts on RX, and 7.7 volts on TX.
if the base volts were there and the collector volts are not there, or are way off from what they should be, you can either remove TR17 from the board and test it using the diode function of your multimeter (google this), or you can just replace it if you have a sub on hand. you can use an NTE 85 if you cant find a 2SC458.
if the collector volts are good, then check the Emitter leg on the left.
should be .9 volts on RX and 1.5 volts on TX.
again, if way off or missing, test or replace the transistor.
if all goes well with TR17, start checking the caps and resistors near it and post back what you find.
LC
upon re-reading your original post today, i see that you did not mention whether or not you had receive audio.
im guessing that you do.
if so, that means the audio chip is not at fault. sorry for not reading this closer and asking you this question before offering advice.
assuming you do indeed have receive audio, then the next place to look is in the mic amp circuit.
check TR17 which is a 2SC458 transistor. you can check to see if it has voltage going to its base leg by putting a DC voltmeter on it while the radio is on. (negative lead from the voltmeter to PC board ground foil of course)
the legs of this transistor are ECB looking at the flat side of the transistor, so you would be wanting to test the leg on the right.
you should see voltage on this leg. about 1.5 volts in RX and TX.
if you do not see any voltage there, check R70, and you may find that it is open, meaning that if you tried to test it with your ohmmeter, it would show no connection between the two leads. you should remove it from the PC board to test it.
if you did see the 1.5 volts on the Base leg, then check the middle leg which is the Collector. you should see about 4.8 volts on RX, and 7.7 volts on TX.
if the base volts were there and the collector volts are not there, or are way off from what they should be, you can either remove TR17 from the board and test it using the diode function of your multimeter (google this), or you can just replace it if you have a sub on hand. you can use an NTE 85 if you cant find a 2SC458.
if the collector volts are good, then check the Emitter leg on the left.
should be .9 volts on RX and 1.5 volts on TX.
again, if way off or missing, test or replace the transistor.
if all goes well with TR17, start checking the caps and resistors near it and post back what you find.
LC
LC, I should have informed you from the get go....I DO NOT have any receive audio. I also checked the legs of tr17 and it does have 1.5v on the base. However the collector is off. on transmit it drops down to 1.7v.
Dont sweat it about the audio chip, $5 out of pocket from my local CB shop is no big deal. I want to thank you for helping me this far. I could easily take it in and get it fixed but I like to tinker. Besides....I have 2 of these radios and this one im working on is just for shits & giggles.
Dont sweat it about the audio chip, $5 out of pocket from my local CB shop is no big deal. I want to thank you for helping me this far. I could easily take it in and get it fixed but I like to tinker. Besides....I have 2 of these radios and this one im working on is just for shits & giggles.
the 945 is basically the same thing as the 458.
lets back up for a minute, and try to trace down where the problem lies.
give me some history of the radio.
has it ever been hooked up backwards and blown the reverse polarity diode?
this can wreak havoc with the audio chip and related parts if there was a high enough amperage fuse in line.
how old is this radio?
does the S meter move normally with incoming signals?
how many watts of a deadkey are you putting out?
if you touch around pin 4 of the audio chip with your finger, do you get a loud buzz from the speaker? (mic must be plugged in to get RX audio)
here is a link to a page that has schematics and the service manual for your radio. Cobra 29LTD ST, 29WX ST
download them and towards the back of the service manual, you will find a chart that gives the proper voltages for all the IC chips.
find the one for IC4 and check your measurements on the audio chip pins against what the chart says.
use the schematics to trace the signal through the radio.
when you said that the voltage dropped on the collector of TR17 on TX, that made me think you might want to check C37 for a short.
remove it and make sure there is no continuity between the leads with your ohmmeter.
do some testing and some educated poking around, get the data you need to answer my questions and post back.
if it takes a few days to get to it, just PM me to check the thread.
maybe we'll both learn something!
LC
lets back up for a minute, and try to trace down where the problem lies.
give me some history of the radio.
has it ever been hooked up backwards and blown the reverse polarity diode?
this can wreak havoc with the audio chip and related parts if there was a high enough amperage fuse in line.
how old is this radio?
does the S meter move normally with incoming signals?
how many watts of a deadkey are you putting out?
if you touch around pin 4 of the audio chip with your finger, do you get a loud buzz from the speaker? (mic must be plugged in to get RX audio)
here is a link to a page that has schematics and the service manual for your radio. Cobra 29LTD ST, 29WX ST
download them and towards the back of the service manual, you will find a chart that gives the proper voltages for all the IC chips.
find the one for IC4 and check your measurements on the audio chip pins against what the chart says.
use the schematics to trace the signal through the radio.
when you said that the voltage dropped on the collector of TR17 on TX, that made me think you might want to check C37 for a short.
remove it and make sure there is no continuity between the leads with your ohmmeter.
do some testing and some educated poking around, get the data you need to answer my questions and post back.
if it takes a few days to get to it, just PM me to check the thread.
maybe we'll both learn something!
LC
1. I dont know if it has ever been hooked up backwards. It hasn't by me anyway.
2. It's a 04' model (china made)
3. It shows roughly 12 deadkey on my cheesey RS meter.
4. Touching pin 4 with my finger does nothing. The only sound I get is when I turn the radio on and off. I get a pop in the speaker when powering on and when powering off.
5. c37 has no continuity.
2. It's a 04' model (china made)
3. It shows roughly 12 deadkey on my cheesey RS meter.
4. Touching pin 4 with my finger does nothing. The only sound I get is when I turn the radio on and off. I get a pop in the speaker when powering on and when powering off.
5. c37 has no continuity.
is shows a 12 watt deadkey?
if that is true, you might have a power supply issue.
just as a check, touch the audio transformer after the radio has been on for a while, and see if its hot.
also try this after keying the mic for a while.
pin 2 of the audio chip is the ripple filter. very strange to be seeing 6 volts on it.
check C44 which is a 4.7uF electrolytic cap, or just try replacing it.
this could also be an issue with the external speaker and PA jacks.
study the schematic, and try jumpering around the jacks to see if that restores the receive audio.
LC
if that is true, you might have a power supply issue.
just as a check, touch the audio transformer after the radio has been on for a while, and see if its hot.
also try this after keying the mic for a while.
pin 2 of the audio chip is the ripple filter. very strange to be seeing 6 volts on it.
check C44 which is a 4.7uF electrolytic cap, or just try replacing it.
this could also be an issue with the external speaker and PA jacks.
study the schematic, and try jumpering around the jacks to see if that restores the receive audio.
LC
I'm getting close to jumpering into it with a sledge hammer! :headbang
c44 checked spot on with my fluke.
The 12 watts is...like i said....on my "CHEESY" RS meter.
Who knows. I'm close to taking the thing into the local shop. Once he figures it out I'll have him do a ekl2030 mod.
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