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Cobra 29 - R56 smoking

Moleculo

Moleculo

Ham Radio Nerd
Apr 14, 2002
9,200
1,685
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I got a radio to look at today...It has almost no output and it seems that the final is out. Also, when you key for a few seconds, R56 next to the final starts smoking and is clearly burnt. I'm guessing this is because the final is shorted and drawing too much current? (already looked at the schematic) Are there any other reasons why this could happen?
 

The final is shorted causing R56 to overheat.
Click to expand...
Oh yeah, it's definately shorted...I was just curious if anything could cause that resistor to burn...after looking at the schematic, I can see that's the only reason :)
 
Actually, replace it with a 3.3 ohm 1/2 watt resistor with a ferrite bead on it. This will bring the input impedance closer to what it needs to be, and also bring the transistor deeper into a class "C" stage, which will cause it to run cooler. Becuase of how the stage is modulated, there is no concern for linearlity being needed here.

EDIT: Oh also I noticed you had asked why this happened. It's because of a short in the final. Basically the final went out. Also check C56 to make sure it hasn't been tampered with and that it's still within tolerance. If D10 has been jumpered (anti spike diode) remove the jumper. If the final was shorted, it might be a good idea to replace this diode as well. It probably had quite the load on it for a few seconds.
 
Man, this radio has more than a few problems. I put it down for a few days but need to get back to it.

After replacing the final, I power on the radio for a bit and smoke starts pouring out of the area around the audio transformer and coil. The transformer is hotter than a pistol and starts melting the plastic. So I figure the transformer is fubared and replace it. I also see that the coil is hot, so I replace that while i'm there. No help. Then i notice that the solder around D8 is bubbling...I replace that, although it tested OK. Something is causing this this part of the board to draw waaay too much current apparently. I'm looking at the schematic and nothing is jumping out at me although I have a few ideas. Anyone encountered this problem before that can point me in the right direction?
 
what about the voltage regulator?

that one that is a TO-220 style but is not chassis mounted.
i think its a 1096 or something like that.

is that getting really hot?
does it test good using the diode test?

im just guessing and trying to learn something.
LC
 
As a follow up, picked this radio up from mol for parts, but couldn't resist trying to figure it out. The problem ended up being the low quality insulator on the final. Casual inspection would lead you to believe the insulator was still OK, but it wasn't, dead short lead to high current drain and smoke. After replacing the insulator the radio works 100%.

This was an easy to overlook problem, probably made even easier by frustration.
 
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MisterFatty said:
As a follow up, picked this radio up from mol for parts, but couldn't resist trying to figure it out. The problem ended up being the low quality insulator on the final. Casual inspection would lead you to believe the insulator was still OK, but it wasn't, dead short lead to high current drain and smoke. After replacing the insulator the radio works 100%.

This was an easy to overlook problem, probably made even easier by frustration.
Click to expand...

Wished I had seen this 3 weeks ago!
Had one in doing the same thing. liked to pull my hair out!.
After an hour on the bench I pulled the final for the second time and saw a small hole in the mica insulator. Boy I felt dumb.
 
So I have the same problem....the final checks good an replaced the resistor and insulator..still the same thing transformer gets hot and smokes
 
xXxWonderer7xXx said:
So I have the same problem....the final checks good an replaced the resistor and insulator..still the same thing transformer gets hot and smokes
Click to expand...

Was d8 jumped at some point? Just a guess, but I'd pull the mod transformer and check for shorts against the schematic.
 

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