Every machine I buy I meticulously go through all assemblies and compares them to the manual to see if there are missing parts or any ugly hacks made. On my ToM trapdoor assembly I realized It missed a spring (should be two, there were only one). Was thinking, "ok, it's an easy fix, let's do this now". Checked my spareparts boxes, no correct spring. Checked pinball shops in Sweden where I live, spring not in inventory. Checked shop in Germany, nope not in stock. Checked Marco's, nope out of stock. Finally found it in a shop in Netherlands I think it was and ordered. Waited about a week to get it. Time to put it in, this should go fast, it's only a spring right...

Opened game and studied the assembly. Realized that I had to remove the whole assembly from the bottom of the playfield to get proper access to add the spring. Removed all screws that hold it in place and then realized it was hanging on the trapdoor itself also. Remove another screw with some hassle as it's hard to reach properly. Finally assembly is loose, but the switch cables are really short so hardly any slack to rotate the assembly. Finally able to remove the coil and add the missing spring. Put everything back and turned game on. Directly at start game started pushing the trapdoor up over and over again, f*ck! Turn off game and check switch, looks good. Never had this problem before so had to be related to my recent doings. Looked like the switch actuator arm was getting stuck in the first spring for the VUK so tried to adjust the screws holding the switch to make it swivel away from the spring. Turn on game, nope nope, trapdoor still messing up. Got fed up with the switch and removed it completely, took a plier and bent the whole metal piece the switch was fastened to so to make sure the switch arm won't get stuck in the spring. Reassembled all and tested, fuuuu*k! Trapdoor still think it's a busy elevator in a two story building. Time to calm down and think.. why did it work before but not now... Finally realized that the second spring I had added were probably forcing the plunger further down than before, making the switch being activated from start when playfield was swiveled down in play position. Verified my theory with my arm through the coin door, yep, switch actuated from start by gravity + second spring. Damn.... Raise playfield for the 2345 time this evening and thinking how to fix it. At this time I was getting really annoyed to put in so much time to fix something that was already working before I started messing with it and seriously thought about removing the spring again. The thought of going through all the hassle of removing the assembly AGAIN got me really pissed, but thankfully I then realized that if I just made the new spring a tiny bit weaker it would work as before but still have the second spring mounted, as intended from factory. Got my nipper and cut away a few turns of the spring making it weaker and tested again and finally it worked as it should.

TLDR; I waited a week for a part, then spend about 3 hours cursing and sweating before I "fixed it" and my pin got back to working exactly as it was before I started messing around

This one is almost embarrassing to tell.

Recently I bought an original MM that I renovated thoroughly (pf swap and much more). Finally almost everything was working again, only thing left was the lamps in the start and launch buttons who didn't light up at all. Well, this will be easy, let's fix it now (we all know where this is heading)!

I had recently replaced the whole launch ball button assembly so I knew that one was brand new and thus most probably not faulty. Started measure voltage at the lamps, there were voltage present but as I only have a DMM (and not a logical probe) the voltage looked a bit low in my eyes.

Looked at the schematics and started tracing the cables to the coin door interface board, through the board and finally up to the backbox while measuring for continuity. Everything looked OK.

Removed coin door board, inspected solder and contacts, measured diodes. Still found no problems, weird...

Removed power driver board and inspected the involved row and column connectors, measured connectivity, all seems fine, what the hell?!?

By this time I've spent several hours and basically tested the whole circuit for the two involved lamps (and as all the other controlled lamps in the same column and rows worked I knew the involved transistors and such were good too).

Took a break to gather my findings so far. Really stumped, then it hit me: the only thing I hadn't tested yet was the actual lamps inside the buttons, oh no...

Opened up the brand new launch ball button and saw it was a LED...no, could it be a polarity problem? Switched cables on the two lamp connectors and yep, it worked! I'm an idiot...

Ok, now let's have a look at the start button lamp then. Removed it from cabinet and opened it up. What the heck, some previous owner have removed the lamp completely?!? fuuuuuuuuuu... Put a lamp in and of course it worked right away.

Spent 3-4 hours over two evenings trying to find the problem before realizing the issues were two 1 minute fixes. I know what I will check first the next time a lamp won't work, the actual lamp!

Didn't know this was a thing. Is this only applicable for WPC-S games or any SS?