Oh...I also had to add on a fuse holder and fuse in one of the wires. All done.
As a precaution I measured the coil for resistance. It was about 18 Ohms, sounded OK to me.
There is a damping diode in parallel across the coil but you can't measure that because of the low resistance of the coil which "spoils" any diode-check attempts with a DVM (you will get a short/beep in both directions). So I figured it would be OK because -ususally- diodes go short when they die, in that case I would have measured something very close to zero ohms.....
So had it hooked up and powered on: flash, that was the fuse....
Mmmmm.....OK but why ? Let's measure what comes out of the transistor on the PCB: argh....a constant 30V...that's not good, the coil would burn if there hadn't been a fuse !
SO: the transistor must be dead. Luckily there are 4 identical transistors on the board and only the knocker and coin counter are usually used. The other two are for optional coin-lock out and an extra coin counter.
So I compared them with DVM in diode check and sure enough all three others showed other values than the one that triggers the knocker.
So I removed the bad one and moved one of the others to that spot.
Let's try again.
Power up, one trigger of the knocker and FLASH there goes the 2nd fuse....
WTF
How can this be....damn....
mmmm-mmmm-mmmm. OK the only thing I KNEW I hadn't tested was that damping diode because it's soldered so tightly over the coil terminals....but now I HAVE to. So I cut it loose instead of unsoldering it and measure it:
OPEN in both directions ! OK I KNOW semiconductors can fail in two ways but I've actually ALWAYS seen diodes gone bad in short !
Conclusion: the damping diode could not do it's work when the trigger was fired. What this diode does is prevent a "back current" into what steers the coil. So this is what happened and blew the fuse.
OK....replace the diode, it's a standard 1N4004 or so....and try again.
Because I only had fast 1A fuses and it actually needs to be a slow-blow I decide to go for a bit higher value this time because maybe this is also causing the fuse to blow.
Try it again.....nothing......but this time the sound dies too after a few seconds. So I look at the 30V indicator diode -> no light !
This time the 30V section fuse was blown....argggh..
OK so I check the transistor again and sure enough...the failing diode had blown the transistor (again).....so I move the last "spare" one, put in a 1A fast fuse again, replace the 30V fuse on the PSU board and try again.....
Holy crap....it works.....and it works. ......and it works.....sweet victory !!!