From the wiki:

Short on the MPU causing F115 to blow
There is one more way for the 12VDC power to be lost and for F115 to blow. A shorted IC on the MPU that uses 12VDC can cause F115 to blow. To help isolate the problem to the MPU, replace F115, disconnect power going to the MPU at J210, and turn power to the game on. If the fuse does not blow, the problem is almost definitely on the MPU board. Any of the LM339s (theoretically) or the ULN2803 (definitely) can fail and short 12V to ground, causing F115 to blow.

Both fail quite often, hard to isolate to one chip making troubleshooting a pain.

When using the multimeter to measure 'current' , it effectively creates a new path in the circuit -overruling fuse F115- and generates a maximum pull from the 7812 regulator and give huge stress to the already blown 2803.
I would advice NEVER to use this method to locate a problem in the circuit.
With DMM set to 'resistance' and machine 'OFF' it is just as easy to find the problem.

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I definitely agree that measuring current as you described is a problem. Additionally, it wouldn't work if the fuse was intact.

To measure current, you should lift one end of the fuse, and put the multimeter between the fuse and the fuse clip. That leaves the fuse in the circuit.

I think you missed his point. You should not measure current ever by bypassing the fuse as you no longer have overcurrent protection and a short can easily blow more components. It's the same problem whether you measure across an installed blown fuse or if you pull one end of a good fuse out of the holder.

The 2803 is such a common failure that WMS started socketing it from the factory on later boards. I socket it on CPU repairs if I get a board in that wasn't socketed from the factory.

If you aren't comfortable with troubleshooting the CPU problem, I repair these - but yours could already be socketed - it's worth checking!

Nice fix.

Just to get back on your fuse thingy and the fact that the circuit 'is still protected while measuring with a multimeter'. It's not. Well not with the correct fuse anyway. There's a reason there's a 0,75 amp fuse there. There should never go anymore than that amount of amps in the circuit. With your fused multimeter, you might be getting an accurate measurement of just 'how much' extra juice goes into the circuit, but the fuse in that meter is probably a 10A. Theoretically, the short circuit could have been 10 amps then, and it could have made the damages even more.

Oh shoot, missed that you put in the correct fuse. Good thinking that way.

picture update with 3/4A fuse inline.
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