Why do MCR boards (Tron, Kick/man) smoke the +12V 10uH coil (L116)?

Pac-Fan

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Why do MCR boards (Tron, Kick/man) smoke the +12V 10uH coil (L116)?

This is about the 3rd or 4th time this has happened, and there is no consistency to the cause.

I was fixing up a Tron with original PS, metered all of the voltages, checked the ribbons, connected power and suddenly L116, a coil on the +12V line on the Super CPU board smoked. This is the last sealed one on the row that usually looks like a black bullet (similar to a large diode but with a tapered end), or a super-sized cyan-colored resistor-looking package. Not the open face wire wound coils on the +5V lines.

Years earlier, one or two of Kick/Kickman boards I was testing on a switcher had the +12V line toast the coil as well.


Then, when benchtesting a new batch of Kick boards, along with the Tron board that toasted itself in the machine, with a new switcher, one of them also promptly toasted the +12V coil.

All voltages, interconnect cables, etc.. have been double and triple checked. I am completing bench testing of the boards without the +12V line connected now to prevent any others from being damaged. Am scared to ever reconnect it. The boards (video output only at least) appear unaffected after the damage? What explicitly does the board use the +12V for -- I haven't had time to trace through the schems/board yet.

Interestingly, the two caps leading to ground along side the coil (.01uF C121/C122) had been clipped off the board on this line only--just on the Tron board.
 
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looks like it goes to pins 11 and 12 of J4 which would lead to the 1 player optical encoder. Stops at Jw3. At least thats how it looks visually on my board
 
oooooh ooooooh ooooooh
I know! I know! (raising hand frantically)

Something on the I/O top board has shorted from +12V to ground.

I fried the exact same coil on two CPU boards while mixing and matching boards.
The switching power supply put out enough juice before blowing the fuse to swell up one of those black capacitors on the top board. If it hadn't been visibly altered, I wouldn't have known how to find it.

I clipped out the offending cap, measured reasonably high resistance between +12V and ground, found that the coil still had low DC resistance, jumpered the fuse in the switching supply and it fired back up. Worked like a charm

I did replace the fuse later too.

Lots of times, the only way you can find a shorted component on a board is to hook up a beefy supply that can handle the load and look for the hottest component. Smoke is a good sign too.

Good luck-
Kerry
 
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oooooh ooooooh ooooooh
I know! I know! (raising hand frantically)

Something on the I/O top board has shorted from +12V to ground.

I fried the exact same coil on two CPU boards while mixing and matching boards.
The switching power supply put out enough juice before blowing the fuse to swell up one of those black capacitors on the top board. If it hadn't been visibly altered, I wouldn't have known how to find it.

I clipped out the offending cap, measured reasonably high resistance between +12V and ground, found that the coil still had low DC resistance, jumpered the fuse in the switching supply and it fired back up. Worked like a charm

I did replace the fuse later too.

Lots of times, the only way you can find a shorted component on a board is to hook up a beefy supply that can handle the load and look for the hottest component. Smoke is a good sign too.

Good luck-
Kerry

I've seen a Midway service bullitin about one of those tantalum caps being installed BACKWARDS at the factory! Might be the same one.

Edward
 
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