Cobra 29 LTD Cl,assic R58 keeps burning? ( Replaced L7 ) 3 Times

[loosecannon]

well my guess is that the driver cracked from shorting against that heatsink, but without the radio in front of me, i can't be sure.

if you are saying that you now have a working transmit, have replaced that cracked driver, and the resistor no longer gets hot or burns, then it sounds like you found your problem.

with the transmit that is...
LC

 

[Mark A Bradley]

well my guess is that the driver cracked from shorting against that heatsink, but without the radio in front of me, i can't be sure.

if you are saying that you now have a working transmit, have replaced that cracked driver, and the resistor no longer gets hot or burns, then it sounds like you found your problem.

with the transmit that is...
LC

This point no,I still need to give it a tune, ran out of time today,11:45,im going to bed, .First thing tomorrow morning, will give it a tune, and keep you posted. Days are to short,comon Spring. Right. Here is a pic of the Resistor placed correctly., Have the correct final heat absorbent on the way.Have a goodnight .

 

[Handy Andy]

Nice re-work - much better.

We hope this turns out well for you.

 

[Mark A Bradley]

well my guess is that the driver cracked from shorting against that heatsink, but without the radio in front of me, i can't be sure.

if you are saying that you now have a working transmit, have replaced that cracked driver, and the resistor no longer gets hot or burns, then it sounds like you found your problem.

with the transmit that is...
LC

Good morning,no,still need to go over the radio, no transmit or receive, voltages are fluctuating all over. I am thinking either Voltage Regulator or Final. What would be your guess, as this voltage fluctuating is going from 13v to 0v to any volt, all over,but not passed 14v. Going to lift center leg of final, and go from there.

 

[Mark A Bradley]

Nice re-work - much better.

We hope this turns out well for you.

Thanks, made sure it was straight, and no touching,so far going in the right direction. Never had a issue with the wrong Resistor value before, as I guess it could happen, now I know if and when I have possibly the wro g value, but correct colors on the part. ( Resiator )Thank you sir, and Happy Holidays to you.

 

[Lkaskel]

Good morning,no,still need to go over the radio, no transmit or receive, voltages are fluctuating all over. I am thinking either Voltage Regulator or Final. What would be your guess, as this voltage fluctuating is going from 13v to 0v to any volt, all over,but not passed 14v. Going to lift center leg of final, and go from there.

Hey Mark,
I would say that it may make sense to start with removing (at least de-soldering) the final completely. Then recheck the voltages. Also, where are you measuring those voltages that they are fluctuating like that? One other thought, start your measurements at the power supply. If they fluctuate disconnect the feed wire to the main board. If they become stable reconnect the wire and maybe disconnect the regulator. The direction is one that you start closest to the power supply and begin disconnecting some of the main points in the radio to see if you can determine what "stage" is causing the fluctuation. Someone here with more Cobra 29 knowledge should be able to tell us what areas make the most sense to disconnect. I hope that makes sense. Of course, if disconnecting the final solves it then you know.

All the best!!

 

[loosecannon]

when i hear people say that they see DC voltages fluctuating wildly on a digital meter, it usually means they have the meter set to read AC voltage, or are trying to use the chassis of the radio as the ground connection.

i only mention this just in case, because we would be chasing our tails.
so make sure you have the meter set to DC, and the negative lead goes to PC board ground, not chassis ground.

I agree with unsoldering the legs of the driver and final and then starting to check voltages.

start right at the power plug positive, make sure the voltage coming in to the radio is steady, then key the mic.
does the voltage drop on TX? it shouldn't, especially since the driver and final are not connected.
Be sure to check the RX/TX light on the front of the radio, that it is green on receive, and turns red on TX.

steady DC in to the radio and no voltage drop on TX?
then start following that voltage on that trace and see where you lose it or it starts going crazy.

follow it through T2, past the On/OFF switch, through R104, and then to the collector of TR23. (might be mounted on the chassis)
now check the emitter of TR23. you should see 8 volts DC here on RX and TX. do you?

use this schematic:
https://web.archive.org/web/2019110...29wx_st/graphics/cobra_29_ltd_classic_sch.pdf

we will stop there for now.
LC

 

[Mark A Bradley]

Will look in to soon, having a family issue, nothing bad, thanks