I bought a Road Kings a couple weeks ago (first pinball machine) and had to do a couple easy fixes (resolder right flipper wires and left kickout wires) but it's been playing fine. Last night about an hour into playing, I noticed a burning smell so shut the machine off. After inspecting everything, it looks like the Q79 transistor fried/burnt up. Any ideas on what would cause this? I'm going to buy a new one (or a couple just in case) and replace it but don't want another one doing the same thing if I can prevent it.

Thanks,
Mike

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Awesome, thanks for the detailed response! The switch was stuck closed which seems to be the culprit. The coil didn't look melted and measured 3.6 ohms so I think it's still good. I also measured pin 9 of 1J18 and the ground braid and came out with 4.86k ohms so I think that's still good also? I'm going to buy a new Q79, Q78 and diode for the coil and replace them. Thanks again for the step by step run down!

Ok cool, thanks for the info. I'll go ahead and replace the coil while I'm at it. The other coils (same type) are measuring 3.8-3.9 ohms so they might be on their way out too.

I don't have another DMM, but I think mine might be reading low. The 9v battery in it is only measuring 7.5v but I'm not sure if that would cause the low readings. I measured all 11 and they're all about the same: 3.9, 3.8, 3.9, 3.8, 3.9, 3.8, 4, 4.1, 3.7, 3.9, 3.9. The one that previously measured 3.6 is now showing 3.7. I might just replace the diode on it along with Q78 and Q79 and see where that gets me.

Thanks again for your help GRUMPY! I replaced the coil diode, Q78 and Q79 (also adjusted the pop bumper switch) and I'm back in action. Quick question, is there anything that references which transistors correspond to which switches? I've looked at manual and schematics and can't figure it out. The schematic looks like it goes to STS6T, although I'm probably looking at it completely wrong.

Thanks GRUMPY, the info on page 2 is what I was looking for, I just skipped right past it because I thought it would be toward the back of the manual with the diagrams and schematics.

Sven, thanks for the info. I checked and sure enough, the fuse was wrong. It calls for a 2.5a but a 7a was installed. I probably should have checked all the fuses when I first got the machine so I'll chalk it up to a learning experience. I picked up a 2.5a and replaced it so hopefully won't run into any more trouble!