cobra 29 ltd no modulation output.

[loosecannon]

welcome to the forum David!

im guessing here, but what is the voltage on the PC board trace where TR23 connects to D21?

you should read 8 volts DC here in both RX and TX modes.

also, while doing your testing, set VR4 at about halfway. that way you are sure that some modulation should be getting through. you can re-set it once you get it fixed.

another test you can try is to put your voltmeter on the cathode (banded end) of D8, and key the mic. you will read about 13 volts DC or thereabouts.

now, with the voltmeter still connected, whistle or yell into the mic and see if the voltage starts jumping around.

you can also measure the DC voltage on each leg of TR17 and TR18, and compare them to the charts in the service manual here:
http://www.cbtricks.com/radios/cobr...st/graphics/29ltd-st_29wx-st_sm_pg12_pg34.pdf

I know these are kind of random tests, but their results will help track down your issue.

while you are diligently staring at the radio waiting for forum replies, try to confirm all of the things in the radio that DO work. does the squelch operate normally?
does the radio receive actual signals or just static? etc...

you can also use that voltage chart to start going through the radio comparing voltages on the audio amp, and any other transistor you decide is worth testing.
this is to help keep you moving and sometimes doing these things can help you stumble on your culprit, even if by accident.

do you have an external SWR meter?

if so, put it in line between the radio and the antenna/dummy load and set it to the CAL (calibrate) position. now turn the knob on the meter all the way to the left and key the mic.
while TXing, turn the knob until the needle sits about center of the meter.
now start yelling and caterwauling into the mic (dynamike all the way up of course) while watching the needle.
did it waver at all? any movement is a good sign.

post back after you mess with it a bit and try to give us a detailed description of what you encountered. the more detailed you are, the better responses you will get.

i am hopeful that something you notice will perk an ear of one of the gurus here who might be able to lend a hand.

good luck!
LC

 

[Handy Andy]

Virgin cans? Well that doesn't tell me much. It means the radio was sealed - by someone. Whether the radios is tuned right or not is still a question.

You tell us that you've replaced several parts...ok...
Including the audio chip...ok,
With all the looking around you did, did you see any burnt traces, where? Burnt resistors? Where?

We need a little more help - for if the radio doesn't RX - and no PA - yet virgin - ok - did someone leave it behind at a Truck Stop when done using it for a wheel chock? Failure to follow label directions can void the warranty...

Wired up wrong - possibly...

The front panel, WHAT ACTUALLY STILL WORKS ON IT? With all those parts replaced, you have not told us anything about RX or TX lights - WAS THAT CORRECTED? Does - channel 9/19 or BRT DIM function even work? A lot of these functions require voltages from other points on the board - can help us to determine what still functions and what is still dead...

Check for voltage on BOTH sides of R70 when you TX into a Dummy Load - and let us know what you find.

R70 is the buffer resistor from the TX switch line - shows no voltage nearly zero - in RX
but 1 side (towards side panel) will show 8 volts or so when you TX, other side will run about 6 volts.

IF R70 shows the same voltage on both sides - (8V) - then the circuit is not biased right, should show some drop across it (R70 one side to the other) when the radio is in TX. Else it will need to be investigated. Something in the circuit is affecting the operation - bias of it.

The same applies to the R70 having 8 volts on ones side with ZERO on the other - mic circuit has a dead short in it's line of parts...

If the above test passes, R70 measures 8V one side 6V or so other....you'll still need to review the mic wiring.

You get 4 watts, which means the Radio is producing power thru the Audio and BIAS line - so audio is not getting imposed on it. Yet, if you get receive - means the speaker is working and the circuit seems ok. It also indicates that the PA / CB switch seems to be working correctly.

When Mic amp checks out, the only other alternatives rely on wiring, Audio amp or route to audio amp FROM mic amp. Audio amp looks to be ruled out - you have replaced it.

The above is why I feel I'm repeating myself - said pretty much the same thing on the other thread.

TR18 (AMC Limiter) and TR17 (Mic Amp) are involved here.

Does R70 even get a voltage, then look into the support components - like those shown in another earlier thread - (8V Regulator Power supply) ,TR21 (TX Switch) and TR22 (BRT-DIM).

Along with W37, Dynamike wiring and Pin 2 Audio routing.

Again - Cobra 29 ltd classic not transmit please review the linked message AT THAT POINT.

For re-tuning Cobra 29 ltd classic not transmit this one - can help you even with tuning up the radio.

Does squelch even work? EXT Speaker - any receive?

:+> Andy <+:

 

[David Randol]

Virgin cans? Well that doesn't tell me much. It means the radio was sealed - by someone. Whether the radios is tuned right or not is still a question.

You tell us that you've replaced several parts...ok...
Including the audio chip...ok,
With all the looking around you did, did you see any burnt traces, where? Burnt resistors? Where?

We need a little more help - for if the radio doesn't RX - and no PA - yet virgin - ok - did someone leave it behind at a Truck Stop when done using it for a wheel chock? Failure to follow label directions can void the warranty...

Wired up wrong - possibly...

The front panel, WHAT ACTUALLY STILL WORKS ON IT? With all those parts replaced, you have not told us anything about RX or TX lights - WAS THAT CORRECTED? Does - channel 9/19 or BRT DIM function even work? A lot of these functions require voltages from other points on the board - can help us to determine what still functions and what is still dead...

Check for voltage on BOTH sides of R70 when you TX into a Dummy Load - and let us know what you find.

R70 is the buffer resistor from the TX switch line - shows no voltage nearly zero - in RX
but 1 side (towards side panel) will show 8 volts or so when you TX, other side will run about 6 volts.

IF R70 shows the same voltage on both sides - (8V) - then the circuit is not biased right, should show some drop across it (R70 one side to the other) when the radio is in TX. Else it will need to be investigated. Something in the circuit is affecting the operation - bias of it.

The same applies to the R70 having 8 volts on ones side with ZERO on the other - mic circuit has a dead short in it's line of parts...

If the above test passes, R70 measures 8V one side 6V or so other....you'll still need to review the mic wiring.

You get 4 watts, which means the Radio is producing power thru the Audio and BIAS line - so audio is not getting imposed on it. Yet, if you get receive - means the speaker is working and the circuit seems ok. It also indicates that the PA / CB switch seems to be working correctly.

When Mic amp checks out, the only other alternatives rely on wiring, Audio amp or route to audio amp FROM mic amp. Audio amp looks to be ruled out - you have replaced it.

The above is why I feel I'm repeating myself - said pretty much the same thing on the other thread.

TR18 (AMC Limiter) and TR17 (Mic Amp) are involved here.

Does R70 even get a voltage, then look into the support components - like those shown in another earlier thread - (8V Regulator Power supply) ,TR21 (TX Switch) and TR22 (BRT-DIM).

Along with W37, Dynamike wiring and Pin 2 Audio routing.

Again - Cobra 29 ltd classic not transmit please review the linked message AT THAT POINT.

For re-tuning Cobra 29 ltd classic not transmit this one - can help you even with tuning up the radio.

Does squelch even work? EXT Speaker - any receive?

:+> Andy <+:

welcome to the forum David!

im guessing here, but what is the voltage on the PC board trace where TR23 connects to D21?

you should read 8 volts DC here in both RX and TX modes.

also, while doing your testing, set VR4 at about halfway. that way you are sure that some modulation should be getting through. you can re-set it once you get it fixed.

another test you can try is to put your voltmeter on the cathode (banded end) of D8, and key the mic. you will read about 13 volts DC or thereabouts.

now, with the voltmeter still connected, whistle or yell into the mic and see if the voltage starts jumping around.

you can also measure the DC voltage on each leg of TR17 and TR18, and compare them to the charts in the service manual here:
http://www.cbtricks.com/radios/cobr...st/graphics/29ltd-st_29wx-st_sm_pg12_pg34.pdf

I know these are kind of random tests, but their results will help track down your issue.

while you are diligently staring at the radio waiting for forum replies, try to confirm all of the things in the radio that DO work. does the squelch operate normally?
does the radio receive actual signals or just static? etc...

you can also use that voltage chart to start going through the radio comparing voltages on the audio amp, and any other transistor you decide is worth testing.
this is to help keep you moving and sometimes doing these things can help you stumble on your culprit, even if by accident.

do you have an external SWR meter?

if so, put it in line between the radio and the antenna/dummy load and set it to the CAL (calibrate) position. now turn the knob on the meter all the way to the left and key the mic.
while TXing, turn the knob until the needle sits about center of the meter.
now start yelling and caterwauling into the mic (dynamike all the way up of course) while watching the needle.
did it waver at all? any movement is a good sign.

post back after you mess with it a bit and try to give us a detailed description of what you encountered. the more detailed you are, the better responses you will get.

i am hopeful that something you notice will perk an ear of one of the gurus here who might be able to lend a hand.

good luck!
LC

Thanks for the response Loosecannon.

Could not have done it without your help. As i was going through checking voltages I found another shorted cap at C67. All is fine now. RX/TX is as it should be. Thanks for other responses too, regardless of the sarcasm, and condescending attitudes. ., .

 

[OldTech03]

Thanks for the response Loosecannon.

Could not have done it without your help. As i was going through checking voltages I found another shorted cap at C67. All is fine now. RX/TX is as it should be. Thanks for other responses too, regardless of the sarcasm, and condescending attitudes. ., .

That'll be $69.95 , LOL

 

[Handy Andy]

Congratulations!

Well now... give credit where credit is due - Nomad might appreciate it more if you visited here [URL="https://www.worldwidedx.com/threads/cobra-29-ltd-classic-not-transmit.228821/page-3#post-624942"]Cobra 29 ltd classic not transmit[/URL] and thanked him after reading THAT answer at the link.

All the best!

 

[David Randol]

Congratulations!

Well now... give credit where credit is due - Nomad might appreciate it more if you visited here Cobra 29 ltd classic not transmit and thanked him after reading THAT answer at the link.

All the best!

Not sure who Nomad is. And the check is in the mail OldTech03

 

[606KentuckyWaving]

I have same problem as David but the cb tricks link above for testing voltages will not load could someone help with a new link for me please

 

[nomadradio]

Here is a 'new' copy of the old site. Some files are missing.

http://www.cbradioinfo.com/cbtricksoldsite/

It's in the Wayback Machine, but that site is also missing some content.

https://web.archive.org/web/20180808010933/http://cbtricks.com/

73

 

[606KentuckyWaving]

Thank you, I will check it out.
73's

 

[kopcicle]

10.240MHz ÷ 1240 = 10 KHz (channel spacing)
10.240MHz ÷ 2 = 5.15MHz x 3 = 15.360 MHz
Channel 1 = .91(MHz)
Channel 40 = 1.35(MHz)
Transmit oscillator is 10.695MHz
10.695MHz + .91 MHz+ 15.360MHz = 26.965MHz
10.695MHz - 10.240MHz = 455KHz

 

[606KentuckyWaving]

Thank you nomadradio after checking voltages my modulation started working again must have had some resistors touching or something? now its really acting funny when dead key is at 1-2 watt or less it swings to about 15 watt but if i lay my hand on the radio or hold it it will swing to about 25 - 30 watts ?? any body got any ideas on this haunted radio situation ?

 

[Handy Andy]

Hmm. Did you make sure the antenna connector is tight to the chassis - back panel of the radio?

I think you use a 19MM or 18MM Flat open end wrench and tighten the nut - and check the solder side- you may want to loosen then RETIGHTEN the chassis screws that mount the board to the frame/chassis - do this also for that big heat sink panel too - make sure all screws are tight.

When you grab the radio and suddenly jumps up in power - not sure of the setup you're using to check this but it may mean that you are capacitively coupling yourself as part of an antenna to the radio and the radios is responding to that. IF that is the case (SIC) then your board inside may have come loose or your antenna connector and it's connection to the board may be loose and intermittent.

Might need to re-flow some solder back there...

 

[606KentuckyWaving]

Hello, Thank you its just a old cobra 29 ltd classic that i hooked up backwards with a old GE power cord that was reversed in polarity and blew my diode I changed it and then I had no modulation. so i traced the modulation trail as LC described in beginning of post and found nothing wrong then checked voltage spots that nomadradio gave me new link to. Everything read fine so I tried it out again and it was modulating again so I must have separated some touching resistors or something checking the components. Maybe I smashed something together removing the echoboard and channel board or replacing diode? but now this weird problem it may have blown a ground joint or something when diode blew i will check it out. thank you for the help Handy Andy

 

[606KentuckyWaving]

UPDATE
I found out the problem its the power outlet in back of radio, If I run power to the inside of plug from power supply its doesn't do that. everything on this one is back to normal. I really appreciate every ones Help, Knowledge, and Expertise. I would not have been able to accomplish this repair without you guys, so thanks again you guys help many people and make it possible for new guys to the hobby like me to accomplish intimidating goals in repair on their radios and that builds confidence to tackle other projects. I hope someday I can hold the knowledge to help others the same way you guys do, I have learned a lot on this project so thank you guys again.
73's

 

[Bigjoe77]

I have a vobra 29 ltd classic yhat has a connex board in it j was using the radio had it in cig ligter and it just quit making modulation if u turn on the weather setting it sounds like the guy is off frequency and through my talk back I can hear myself but it sounds like its off frequency bit it puts out signal when u key up