I want to be a teacher too ! Already back at work since tuesday 
gotta love those school holidays
anyway, just came back from my mother's place. first thing i did was put the capped chassis back into the cab
the good news is that the slight "wobble" of the monitor is gone, the bad news is that the distortion at the top is not
time to try the potentiometers! i expanded the picture vertically which made the picture already better. then i realized that by moving the picture further to the right with the horizontal centering potentiometer, the distortion at the top got better and better. moving the picture to the very right of the screen almost makes the distortion disappear entirely. however, i don't want to have 3 centimeters of nothingness on the left and the picture itself "glueing" on the right of the screen, so i moved it back a little bit. now it's almost centered and still looking much better than before. additionally, i've just found in the manual that there is a horizontal raster position adjustment (= a jumper) by which you can move the screen to the left and the right. i'll try that the next time i'm up at my mum's place, for now this is what the screen looks like at the moment
then i turned to the next problem, the missing +12v. grabbed one of the splice connectors i received this morning and a yellow wire for the switching power supply connection and set everything up
one push with the pliers (as suggested by andré) and the connection is super tight!
hooked the new wires up to the switcher ....
... and 12 volts we have!
connected the coin door to the wiring harness, turned the machine back on and immediately heard a hum from the speaker - so far, so good. walked around the machine and saw one coin insert light on, the other one off (note that i had replaced both of the lights with new ones). tried to add a credit with the microswitch: nothing! had a look at the non-working coin insert light and the moment i touched it the entire system went down
. disconnected the coin door from the harness, power everything back up - still working (puh). then i tried to add a credit the way that bela had suggested by touching the ground pin and the coin mechs pin simultaneously on the harness connector, but nothing happened at all, no credits were added to the game!
i forgot to measure whether the +12v are reaching the end of the connector (but why should they not), i'll have to do that later!
any suggestions?
