radio has good receive 5 watts on key up just no modulation output.had been working great but now it has me stumped.what has been tried all new caps check all tr transistors ,regulator, predriver driver final also replaced ic1 2 3 4 with new ones.mic gain rf gain pots checks fine.i tried a different mic two antennas.but still no luck.pa works good.you could say i been shot gunning this one .hope someone may have had this problem and solve it....would have junked this 96 model if not been so clean and all soldering joints so shinny.
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
cobra 29 ltd no modulation output.
- Thread starter lost bandit
- Start date
you said PA works.
can you just confirm that you meant that in PA mode, when you talk into the microphone, sound comes out of a speaker that would be plugged into the PA jack on the back of the radio?
LC
can you just confirm that you meant that in PA mode, when you talk into the microphone, sound comes out of a speaker that would be plugged into the PA jack on the back of the radio?
LC
BTW - 5w is a bit too high for you unmodulated power. The final transistor in that radio is a 10w peak output rated part. Better to set it at 3w and give it some room to swing up.
sorry pa was working before this happen but now no pa that was one reason i put a new audio chip in.all voltage on new chip is ok pin 4 of chip. has great affect also.have tried new diodes checked for bad traces etc but no luck in finding this problem.maybe its time to travel to a cb shop just hate that long drive.
Last edited:
check C48 I think it is right between the audio IC and the transformer. it gets shorted and you lose all audio.
ok, so i have to assume a couple of things based on what you have posted here, and if those things turn out not to be the case, then everything else i post will be meaningless.
so, you said you have tried different microphones. this means i have to assume that you are 100% certain that the mic you plug into the radio works 100% of the time on another radio or a mic tester that you can hear yourself talk on.
that's verified?
moving on.
you say that you have good receive, and that the radio transmits a 5 watt carrier.
that's verified?
ok.
we are going to follow your transmit audio path from the mic to the driver and final transistors.
your mic audio comes in on pin 2 of the mic jack. from there it connects to the center pin of the DYNAMIKE control. one of the outer pins of this control goes to PC board ground, and the other outer pin connects to C74 and R122.
If C74 shorted, then it will pull all your mic audio to ground.
If R122 is open, your mic audio would not move past this point.
the other end of R122 connects to the collector of TR18, and also to C68 and C69.
TR18 is your AMC control and is taking a sample of your audio and feeding back anything over 100% (or wherever you have VR4 set) out of phase back into your audio path.
this has the effect of cancelling out the "overmodulation" while allowing the audio through at the preset level.
this is very much an oversimplification but i want you to know what the parts are doing.
If TR18 was shorted to ground then all your audio would be shunted and you would have no modulation out.
If C68 was open, or if C69 was shorted, same thing. no modulation out.
i have to skip a few parts in the path just to keep this on point, but know that if i skip them, it's because the chances of them being culprits is pretty low, and we can go back to them if you still can't find your problem.
next in line is TR17, your mic amp transistor. your audio goes in the base, and out the collector. you can check C125 for a short, but again, it's not a likely cuplprit.
If TR17 is open or shorted, you will have no modulation, so check it.
After the audio is amplified by TR17, it goes through C37 to R48 and then in to pin 4 of the audio chip.
then it comes out of the audio amp, through C48, then into the transformer, then out of D8 to the collectors of the driver and final transistors. the reason i don't think your problem lies after the audio amp is that if C48 was open, you would have no receive audio. If the transformer was bad, you would have no voltage to the driver and final, and thus no deadkey wattage. same with D8. if it's open, then no voltage to driver and final. you have a deadkey so i think these guys are all ok.
if you have receive, then the chances are that your audio amp is all good and the problem lies somewhere in the path that i laid out.
in general, when it comes to troubleshooting electronics, you suspect the semiconductors and active devices first, then the peripheral passive components like capacitors, resistors, and coils.
my first guesses would be TR17 and/ or TR18. i know you said you checked them, but you might try just replacing them as they are cheap and easy to come by.
remember that TR18 is a PNP transistor, and TR17 is an NPN.
all this is of course assuming that your dynamike control is all the way to the right. LOL
just had to put that one in there!
best of luck, and please post back your results for all to see so that others may learn.
here is a link to the schematic i am using for this: http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29_ltd_classic_sch.pdf
LC
so, you said you have tried different microphones. this means i have to assume that you are 100% certain that the mic you plug into the radio works 100% of the time on another radio or a mic tester that you can hear yourself talk on.
that's verified?
moving on.
you say that you have good receive, and that the radio transmits a 5 watt carrier.
that's verified?
ok.
we are going to follow your transmit audio path from the mic to the driver and final transistors.
your mic audio comes in on pin 2 of the mic jack. from there it connects to the center pin of the DYNAMIKE control. one of the outer pins of this control goes to PC board ground, and the other outer pin connects to C74 and R122.
If C74 shorted, then it will pull all your mic audio to ground.
If R122 is open, your mic audio would not move past this point.
the other end of R122 connects to the collector of TR18, and also to C68 and C69.
TR18 is your AMC control and is taking a sample of your audio and feeding back anything over 100% (or wherever you have VR4 set) out of phase back into your audio path.
this has the effect of cancelling out the "overmodulation" while allowing the audio through at the preset level.
this is very much an oversimplification but i want you to know what the parts are doing.
If TR18 was shorted to ground then all your audio would be shunted and you would have no modulation out.
If C68 was open, or if C69 was shorted, same thing. no modulation out.
i have to skip a few parts in the path just to keep this on point, but know that if i skip them, it's because the chances of them being culprits is pretty low, and we can go back to them if you still can't find your problem.
next in line is TR17, your mic amp transistor. your audio goes in the base, and out the collector. you can check C125 for a short, but again, it's not a likely cuplprit.
If TR17 is open or shorted, you will have no modulation, so check it.
After the audio is amplified by TR17, it goes through C37 to R48 and then in to pin 4 of the audio chip.
then it comes out of the audio amp, through C48, then into the transformer, then out of D8 to the collectors of the driver and final transistors. the reason i don't think your problem lies after the audio amp is that if C48 was open, you would have no receive audio. If the transformer was bad, you would have no voltage to the driver and final, and thus no deadkey wattage. same with D8. if it's open, then no voltage to driver and final. you have a deadkey so i think these guys are all ok.
if you have receive, then the chances are that your audio amp is all good and the problem lies somewhere in the path that i laid out.
in general, when it comes to troubleshooting electronics, you suspect the semiconductors and active devices first, then the peripheral passive components like capacitors, resistors, and coils.
my first guesses would be TR17 and/ or TR18. i know you said you checked them, but you might try just replacing them as they are cheap and easy to come by.
remember that TR18 is a PNP transistor, and TR17 is an NPN.
all this is of course assuming that your dynamike control is all the way to the right. LOL
just had to put that one in there!
best of luck, and please post back your results for all to see so that others may learn.
here is a link to the schematic i am using for this: http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29_ltd_classic_sch.pdf
LC
thanks guys.
sure would have been nice if lost bandit showed up so we could find out what the issue was, but i guess it's good to have these types of threads out here for others to learn from.
LC
sure would have been nice if lost bandit showed up so we could find out what the issue was, but i guess it's good to have these types of threads out here for others to learn from.
LC
I don't know if anyone is still following this old thread, but I'm here to try. LC great job walking through the path. Picked up this 29 ltd classic, the Hong Kong Flavor. Had no RX Or TX. When keyed the display lights would go out, and no response on the key. I did see when some had this it was bad solder joints, Checked and reflowed a few but no luck. I Found a shorted cap, believe it was C120 near TR23. Replaced it and had great receive, and a 4 watt D.K. No Mod, No PA. Mic good, known good, and yes made sure Dynamic was turned up as well as VR4 to CC. Made my way through the path Thanks to LC, replaced TR17, 18. C74, 68, 69, 125, 37, and 48.as well as R122, and 48, and D8 for good measures. still no Mod, or Pa. Oh also replace audio chip. Any help or advice would be great. I don't think I pulled all my hair out yet so there is still room to move forward. You guys are appreciated, DR.
Shadetree Mechanic
Delaware Base Station 808
Welcome to WWDX! I am sure someone will stop by before too long to give you a hand. Although I have learned a lot in my short time here, I am not up to speed in this area yet.
Chris
Chris
Thanks Chris
I am not up to speed yet either, but I can follow orders
I am not up to speed yet either, but I can follow orders
Welcome!
Did you happen to read thru this message thread?
https://www.worldwidedx.com/threads/cobra-29-ltd-classic-not-transmit.228821
You say you have no audio - now lets' see if some of those tests in that thread can help you locate your issues with Mic audio on this radio. Remember to look for and read thru on R70 to locate the voltages and any bad parts in that area.
:+> Andy <+:
Did you happen to read thru this message thread?
https://www.worldwidedx.com/threads/cobra-29-ltd-classic-not-transmit.228821
You say you have no audio - now lets' see if some of those tests in that thread can help you locate your issues with Mic audio on this radio. Remember to look for and read thru on R70 to locate the voltages and any bad parts in that area.
:+> Andy <+:
Thanks Andy, I will read through it, but seems to be a different monster. I do have a 4 watt dead key, that is picked up by other radios. There is output just no Mod.. This thing is virgin all cans are still covered.