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....
![Evil :evil:](http://www.dragonslairfans.com/smfor/Smileys/default/icon_evil.gif)
WTF
![Huh? ???](http://www.dragonslairfans.com/smfor/Smileys/default/huh.gif)
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 !!!
![Grin ;D](http://www.dragonslairfans.com/smfor/Smileys/default/grin.gif)