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Board Repair Cobra 29

2FB327

2FB327

Active Member
Oct 31, 2012
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2fb327-albums-cobra-29-picture4084-final2.jpg
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Could someone explain to me how the circuit board functions as far as the layout of the traces. Is it possible to view it from the solder side and follow a path? How is it built to seperate the +and-?

I'm asking because I picked up a 29 and where the gate pin from the mosfet is suppose to be soldered, but the green layer is lifted and burnt off. Is it posible to solder a jumper and if so to where?

The radio receives, but transmit is dead.
The rx/tx light changes when the keyed.
It has the resistor on the pa jack and you can hear yourself thru the speaker.
The needle does not move on transmit and there is no carrier out of the radio.

Could someone point me to a post on troubleshooting or where to start.
Thanks
 
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This is a problem when wanna be techs try to do some internet mod on a working radio. It's now a parts radio or you need to find someone that is very good at soldering, knows circuit boards, and has a giant telescopic magnifying glass. It gets even worse with surface mount boards in many of the newer radio's.
 
Sure, repair the trace with wire and glue after soldering everything up. Replacing the MOSFET with an OEM can also be done and then see if it works or not. IIRC, replacing the original MOSFET with an IRF-510 can be done, but the Gate/Drain/Source pins are different from each other. Not sure if it was a drop-in replacement, but I don't see why not either - except for the G/D/S pinout. It will also need to be insulated from the frame as well. Don't remember if there is a diode across the MOSFET (probably); but if there is I would surely test and replace if found faulty. Probably needs replacement. Don't know what the gate voltage is for the OEM part; but if replacing it with a IRF-520, it should be set at 3.5-4v max.

That should get you started.
 
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Thanks again

Thanks Rob,I'm trying to figure it out by looking at some old boards and the circuit diagrams on the net, it doesn't look like that leg of the mosfet should be touching the R123, I'll try undersoldering it and running a jumper over to R56 and hope.
It has the en 1230 in the R56, if that doesn't do it I'll start from scratch with a different final.
 
I do not know the history of the radio, I got it in a stack of "as is, non working" radios.
It's a Cobra 29 LTD front mic, made in China 2012, it looks brand new, except for the two parts and the amount of heat they used on the board, it was hot enough to lift the green foil.
I'm just trying to find out where to put the jumper, it's the third leg of the mosfet, the furtherst leg away from the so239, with the radio solder side up and the knobs facing you.
I have a lot of old parts and was hoping to switch out the mosfet or put it back to stock with a bipolar final.
Thanks
 
I have one of them and I can't find the final, I also found the block diagram and from what I can make out is that the transmit leaves tr14 and thru the listed coils to the antenna, so I could start there and work back, but I can't find the final or the three paths.
Thanks
 
so someone tried a MOSFET mod and ruined a few traces.

if you just want to solder the mosfet in and see if it works, here's where everything goes:

Gate: to R123

Drain: to L15, C60, and C61

Source: to C142 and PC board ground.

if it were me, i'd go to cbtricks.com and look at the mosfet mods page to make sure that this person did the mod right before keying up the radio.

good luck with it,
LC
 
Thank you so much, now I have a place to start.
I have a photo in my albums of the mosfet to the board, but for some reason it didn't post in the orginal.
I found the mod on cb tricks and it like that was all that was done.
 
2fb327-albums-cobra-29-picture4084-final2.jpg
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I soldered the gain to r123 and now have continuity, but not audio.
Maybe someone could point me to a troubleshooting section, I know a while back I remember reading a step by step post by a member on diag/repair.

But here's what I have.
Transmit dead
If you key it near a bench radio, nothing on transmit/rec
TXRx light does change from red to green when keying mic but the meter needle goes to zero when keyed(opposite of what it should)
It has the talkback resistor and you can hear youself thru the speaker.
No rf out of radio. Nothing with a freq counter except at crystal, all the switches and wires look ok and they weren't fooled with. I was able to down load all the info charts, diagrams and pin voltages.

Once again thanks to all that have helped, tomorrow is another day.
 
you are going to have to use the schematic diagram to follow the RF path through the transmit section.

google the term "RF probe" or "RF voltmeter" to find a diagram of how to build a little probe that you can attach to your multimeter (in DC volts mode) that will allow you to see relative RF power.

the numbers wont mean anything, but you should be able to see increases in power all the way out to the collector of the final.

start at the output pin of the TX mixer chip, and follow it through the pre-driver, driver, and final stages.
when you come to a transistor, the base will be its input, and the collector will be the output to the next stage. you should always see a bigger signal on the collector than on the base.

if you have 0 output, you will come to a point where your meter reads nothing, or "open line".
this will show you which components to check.

with all that said, unless you are absolutely sure that the mosfet mod was done right, you should put the radio back to stock before doing all this.
otherwise you might end up chasing your tail.
LC
 
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BTW, in your image it looks like there is component that should be soldered to the base lead of the final but isnt.

what component is that?
LC
 
Sorry I was away for a day but thanks again, you guys will end up making a repairman out of me yet.

In the photo the pin all the way to the right goes to r123 as you said, I soldered it and no go.

This weekend I hope to have some time, I'm going to put a stock transistor back in and remove the piece they have in R56 and get it back to stock.

I do have a scope a tech 2204-40 and I'll use that to follow the path if possible.

I'll let you know how it turns out.
 
Returned to stock with a 1969 final and repalced 55,56, and removed the cap on the back of 59. still the same(undid the mod).
I noticed that when I talk thru the bench radio, the receive on this thing is all hash, no voice and most of the cans are screwed out.
I'm off to bulding the RF probe and have the diagrams to follow as per

I have built the RF sniffer(thanks Robb) and can read carrier and mod but for some reason I can't use it to read voltage as listed above. If I hook a DVM to the pin points as in the repair quide the voltage is there on TX/RX
I tried a regular probe and a 10x to no avail. I must be missing something so simple.
 

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