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Power supply shuts off when i plug my radio in help!

King Mudduck

King Mudduck

FEAR THE DUCK!
May 6, 2005
864
17
28
285 South Western Virgina waving!
I have 2 radios. One is a Cobra 29 the other is a pc68. When i try and plug them in my power supply shuts down so im guessing that someone blew the capacitors that sit just above the power plug on the inside of the radio. I bought an assorted pack of these at radio shack but im not 100% sure which one to use. The ones i removed from the radio have imprinted on them the number 103M. In this pack i have i found some that have 104M on them but the others that have number 103 do not have the letter M. I don't know which ones to use,a little help please!
 

The 103 & 104 are actual values. The "M" is tolerance.

103= .01uf
104= .1uf
M = +/- 20%

Use a 103.

I'm guessing though...that you didn't just blow two capacitors in both your radios at the same time.

Did you connect in reverse polarity? Are you still connected in reverse polarity? Do the radios have a diode near or on the input terminals? Is it burnt? Can you check it in forward and reverse bias? It should work one way.....not both. If it does work both ways....replace it. Also, clipping the diode will work to get your radio(s) up again....but you will no longer have reverse polarity if you do so...
 
the caps are just decoupling caps rated at least 50v, they will never de damaged by connecting the power lead backwards,
the protection diode if its not been blown to pieces is your most likely culprit,

you may or may not be lucky and only taken the diode out,

if you dont see a diode connected across the socket then follow the wires/chokes to the board and you should find a shorted protection/idiot diode usually a small black barrel with a silver band connected across the power connections to the board,

good luck.
 
I do not have a radio to look at...is the board marked?

On the Cobra....find: D23

It should be very near the power in jack. Maybe on the main board, maybe right on the back side of the jack.

Should look something like this...but may be burnt:
diodes.JPG


Can't help you on the Uniden. I have no schematic....You should be looking for the same thing in the same area though.....
 
I give up! I replaced the caps and the diode and still the same thing. Apparently this problem is far behond my limited understanding of the inner workings of CB radios. I guess i will take them both to my local cb tech and have him show me what im doing wrong. I only have $25.00 in both of em and the 68 is less than 1 month old so im not out much if i have to spend a little more to get em fixed. Thanks for you help guys.
 
BTW....the diode is not like a resistor or ceramic disk capacitor. It can only go in one direction....you want it connected OPPOSITE the above pic...
 

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