I've been collecting for 17 years, but I'm always learning. System 11 games seem to give me more learning opportunities than other pins. This month's installment is:
"Special Solenoids: special when lit (on fire)"
I was playing a nice game of Space Station a few weeks ago, and suddenly the knocker went off about 5 times. Shortly after that, a few other coils locked on. I quickly turned the game off and opened the backbox. There was a faint burning smell, but nothing obvious. I turned the game back on, and all seemed well until Q77 on the MPU board caught fire. I made some notes in my repair log, and tried not to think about it. I had new games that needed my attention.
Last night I decided to tackle this. First, I reverted to newb mode and tried swapping in a known good system 11B MPU board from my Whirlwind. Same result: Q77 caught fire on that board too! I grabbed a beer and my schematics. Q77 is one of 6 transistors that drive the "Special Solenoids". What makes these coils "special" is that they can be driven directly by a switch, not just by software. The pop bumpers and slingshots are examples. I checked, and sure enough one of my pop bumper switches was closed. I adjusted it and turned the game on with the coil fuses removed. This allowed me to go into test mode and confirm that everything else was working - displays, switches, feature lamps, etc. A quick check confirmed that several coils were now locked on - indicating that the PIA at U10 is probably toast. Tonight I'll get out my Hakko and try to fix all the damage.
What I learned: First, always try to understand the root cause first, before trying a shotgun approach (replace and pray). Second, the six Special Solenoids are a weak point in System 11 reliability. I'll get some fuse holders and put 1A fuses inline with the output of the transistor drives for each of these. I need to do this in Whirlwind too, when I get back to that.