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No voltage to Final Transistor,confused???HELP

B

baconater4749

Guest
Howdy,Cobra29LTD Classic, I have around 15+ Volts to one leg, it would be the outer leg that goes up, that has the power, same with the driver,only15+ volts to the left outside leg that goes up on the final and driver.
I cannot seem to find where the juice for the other 2 legs comes from, I do not have the correct schematics.
Can anybody look for me ,or can you give me a hint on where to go to fix this radio , I tried everything and cannot find where the power comes from to the Final and Driver other 2 legs. I replaced a few fuses, but not sure if they were the correct ones, they did check bad. I am not able to align the radio till I can get some juice back for the transmit side of the radio. it is massively out of tune, big time, my Daughters Husband thought he could fix it, so guess who adopts it after he has his fun turning the pots.
Any help would be appreciated 10 times over, I have been on this radio for 3 days, with it not fixed, I can fix a car, but a little slow on CB radios. Thank you, Mark
 

Hmmm. Where to start?

Usually I start with a book titled "The Screwdriver Expert's Guide to CB Radios" by Lou Franklin. Still the best explanation of this technology I have ever seen.

http://cbcintl.com/segbook.htm

But that's for someone seeking a wider body of knowledge. Your objective seems a bit more narrow, like just getting this radio to transmit again.

When a working radio that's performing normally quits, you can probably blame either a failed component or failed solder connections.

When a radio has an unsuccessful repair attempt, we normally expect to find two or more problems in the radio. The one that first prompted the attempt, and the accidental faults added to the radio since then. Tuning adjustments out of whack, for example.

The first scenario, where the story is "Worked every day until yesterday, now it won't transmit" is the best one to work with in an online forum. Odds are that there is only one fault to track down and fix. The normal troubleshooting process is to identify parts of the system that are not broke, until this process leads you to what IS broken.

That can get a bit tedious and frustrating along the way, but this process has been successful many times on this forum and others.

Just one problem. Your radio appears to fall into the second category. Once the original fault in this radio is fixed, it still won't work until the new, added faults are also taken care of. This is not quite so clear-cut a process as chasing down a single fault in just one circuit. For every one guy who had fingers in the radio we expect two or more added faults for each attempt. By the time three tries have been made to get a radio working, the laundry list of what needs to be fixed can get pretty long.

Best place to start here would be to find out just what tools you have to undertake this job. Dummy load?, Wattmeter? Frequency counter?

I would start by suggesting you make sure the radio is receiving on the channel that's selected. If it is, we can probably cross one set of problems off the list.

And if you are NOT hearing the channel number showing on the display this points to the radio's PLL synthesizer that selects the channel frequency. When this circuit lose its "lock" on the channel frequency the FCC rules require a circuit that will detect this and shut down the transmitter. To keep it from transmitting on a frequency that's not legal. This would direct focus away from the transmitter's RF-power circuits.

Quickest, easiest first test to make for radio with no transmit is to use a spare radio, even if it's a mobile on a power pack. Power pack doesn't need to have more than a 1 Amp rating, since you only need to power this radio for receive mode. Plug a coax jumper into this radio and set it to the same channel as your 29LTD and key the mike. If you hear a weak carrier, that tells us that the radio's low-level transmit carrier is being generated. You may need to back the shell of the coax jumper's far end down the cable and use the plug's center pin as an antenna, "sniffing" the inside of the 29 while it's keyed.

If you can't detect even a weak carrier in the monitor radio this way, that's a starting point.

If you can, we can take it from there.

73
 
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Hello, thank you for the write back, as you are helping me with certain repair help, I have done all of that, second radio for receiving a transmit, I have all the equipment. Oscilloscope, Signal Generator, Another radio hooked to a Antron 99, with ground plane kit,known now as a A99, 40 feet up, 500 watt kicker, ( IF NEEDED ), tons of Resistors, Caps, Diodes, AMAZON is my friend. Everything I need. Actually I am new at this, I can repair a car, 30 years with Chrysler, but this is foreign to me. I am learning about the Loop Lock with the PLL, I have followed everything to a T in these books. I have had the Screwdrivers Book for 30 years or so, I also have the PLL book for 30+ years, I also have my Ham Ticket, need to upgrade, still at Novice, when I had to learn code. I just spoke with Lou Franklin himself for his other book, Understanding and Repairing CB Radios, he is at his Company, CBC International, he sells his CB Books, all kinds of CB upgrades, check it out, he personally called me to let me know about a book, but I spaced out on the name and forgot to have him sign it. He is a super nice guy, but he is very sick with a bunch of issues, asthma, he broke his ankle and his wife is driving him around, very nice person, but he will now answer to any CB questions, he will kindly ask you to not ask him at all. But now back to the radio, I have another one here, a Cobra 19 Ultra III,,tiny little sucker. This one has the same issues, it has receive, but no transmit, only 17.446, Volts on the center leg, the oth to ter cobra 29 ltd Classic is on the left upper leg. I am still learning the oscilloscope, just received a RF Sampler today from Amazon,I made one out of a PL259 Tee connector, now I have to learn how to use them..I will go through the Screwdrivers book again, and the Understanding and Repairing CB radio book is about 20 pounds of book, you have to realy know CB's to get into this 2 inch book, lots of good info. OK,Do you have any ideas were to go on these radios,I put a Final in the 29,and a driver, no good,still the same..OK,Talk soon,Thanks to all.Mark B...Cedar Lake ,Indiana..OH I am new here, nice to meet you all,,I will not be a pain, but I mat ask a bunch of questions,,,who know about Uniden SDS200 Scanner,,,JUST KIDDIN, into scanning also,,Have a good Sunday,Mark
 
His measurement is probably okay. There is a diode D8 between the modulation transformer's secondary and the driver/finals' collector circuits. If the driver/final are drawing the normal current, the voltage on the center pin of these two transistors will follow the audio waveform, and rest around 13 Volts DC or so with no modulation.

But if there is no drive to the driver/final, or if one of them fails they won't draw down the voltage at this spot. The DC voltage coming out of D8 charges up to the peak audio voltage and "hangs" at 18 or 24 Volts since the two RF transistors are turned off, not drawing current, and not discharging capacitors in the circuit.

That high reading tells you that either the driver transistor has failed.

-- OR --,

More likely that there is no drive to the base of the driver transistor. This is where the 'scope pays for itself. A peek at the base leg of the driver transistor with the 'scope will reveal this.

73
 
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Hello back, I am still trying to figure this out, as I am new at this and learning, I again tried to tune the cans, I get all of the MAX VOLTAGES PER CAN,and I still cannot get any transmit, ( with a CB next to me for listening and looking ) and no receive. I put a new Audio Amp Chip in, as I can now run a finger on it and it makes noise now, but still will not put out receive, and or transmit.
Is there a area I should look to, I noticed that checking the JP's, that JP37,this is near the D23 Diode, near the input of the power source,there is no Voltage at that point. I will have to start digging. But is there any area to pay attention to, this is all I am asking, just to get a foot or a toe in the door. Thanks all, talk soon,Mark
 
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nomadradio said:
That high reading tells you that either the driver transistor has failed.

-- OR --,

More likely that there is no drive to the base of the driver transistor. This is where the 'scope pays for itself. A peek at the base leg of the driver transistor with the 'scope will reveal this.
Click to expand...

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Above from a Master!

73
David
 
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baconater357 said:
I again tried to tune the cans, I get all of the MAX VOLTAGES PER CAN,and I still cannot get any transmit, ( with a CB next to me for listening and looking ) and no receive.
Click to expand...
tenor.gif

I've had days like that...

Even now...
​
How are you checking for "MAX VOLTAGES PER CAN"?
 
YES EVERY CAN AS PER PHOTOFACTS I BOUGHT LAST NIGHT????????????? OK, I can read, and I have tuned and aligned 4 radios so far, but this one was played with by a 65 year old child, and it seems this radio does not want to tune, no noise out of the ext speaker, and no receive on my extra radio hooked to the tower, and the radio being work on,is on a dummy load,and I switch it to a mobile antenna to check to see if I am getting any output,,NONE AT ALL...I have no juice to the final. I retuned these cans till I was blue in the face, and they seem to not have much voltage to tune with?
 
no no - oh geez, we are looking to help you but need to know what you have so we can directly and correctly - GUIDE you - not done as an insult - we're trying to offer assistance

By the response, you have tried several attempts using DC measurements - so you'll need to locate a schematic to the radio...

Got a Board number, for Cobra is an enigma in itself - we have little to go on and even less when someone new doesn't realize how difficult it is to give them the right information. Especially when it comes to Cobra and the sheer number of radios they produce that are different from each other - don't want to send you to the wrong Rabbit hole, there are several Foxes loose here in the Forums that have been known to creep in a snatch the thread and take it in the wrong direction.

@BJ radionut provided several links - did they help you?

IF so, which ones helped(?), so we can start, or do you need more info to help?
 
Tried 10 mikes,nothing. Going to trace back from that Final, gonna light it up and TRACE...................... To be Continued.................
 

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