Long story short, I had a customer's Bally MPU -35 machine blow a transistor. Normally this is because a blown coil (and then you're stuck ordering that anyways), but this was with a coil that was fine and new, but the transistor and possibly other components blew because of a backwards diode on the new coil (or backwards wiring, however you want to look at it) that he had missed when hooking up the new wires.
Due to certain reasons, this machine had to be fixed soon and correctly. With Radio Shacks shutting down again all over, I was unable to find a proper replacement part locally like I had in the past (and I prefer/need the original specification of TIP102's vs. TIP120's, which Radio Shack seems to only carry locally) and had none left on hand, and time was of the essence. It was very possible the diode and resistor were bad too. "S***!"
[Please note that as boards can be swapped between machines you should really order the correct new parts if you have the time, this is just for "emergencies", but you could totally redo the small coin lockout circuit too by all means if you ever wanted to/needed to or receive one of the bastard boards I caused...]
[This is a bit of an advanced technique and you need fairly/quite good soldering skills]
What I did without reference and with on-hand knowledge (and to be honest "hoped for the best"):
- I knew that as this was in a home situation and they *WERE NOT USING QUARTERS*, that their coin lockout coil on the coin door was no longer needed, and in most home situations these are already disconnected by cutting the wires off of the coil.
- I knew that the coin lockout coil likely had a place in the long lines of transistors on the driver board (it did)
- I found a list online of all of the transistor assignments for each early Bally title. This list was not finished yet and thus wasn't on the site at all. Crap.
- Looking through all of the other pages that WERE there, the coin lockout coil, which is commonly placed in nearly every pinball machine ever, was always controlled by, and I quote, transistor Q19 (and its associated surrounding hardware) It is common in each machine and thankfully it seemed to share the same ID across all of the machines of this era, makes sense
- I cut Transistor Q19 off while leaving as much of the legs as I could (you should desolder FOR SURE if you have the tools with you) and installed it into Q10, the spot for the transistor that had blown.
- I cut the diode for Q19 off of the board (no desoldering gun with me), being very careful to get as much as the legs as I could
- I decided to skip the resistor (*after removing it from Q19*) as I had no reason to believe it was bad and it was harder to get at than the diode and transistor were, and with one shot it was too risky
- I cut the bad diode off of Q10 (I forget the specific reference of the diode itself) and installed the working diode from Q19 to Q10 by soldering the legs of the good cut diode to the stumps/pads of the bad diode, now also cut off
- To be thorough, I also cut off the paper capacitor surrounding Q19 as with the rest of the parts gone it effectively went nowhere (but still was in-line to active voltage on one end) and was a waste
- Made sure my solder connections were thorough and reached to the bottom of the board in places where I couldn't utilize the through-holes due to short leads (risky - but had to do what I had to do, and if it gets a good connection, it'll stay)
- Back at their place, REWIRED THE COIL PROPERLY FIRST(!) to avoid blowing the components and fuse out again the moment I turn the machine on
- Cut the leads off of the coin lockout coil (one was already disconnected, told you), and just decided to remove it fully to avoid confusion - even though the circuit was disabled make sure the leads won't short against the door, each other, or anything else - incase another board is ever installed
- Put the board back in, reinstalled all connectors
- Powered the game on, got a good boot and nothing locked on
- Started the game, it started (ball kicked out properly)
- Tested the pop bumper (which was the coil/transistor position at hand), it fired correctly as triggered for the first time in a while, no fuses blown
- Walk out of there calm and collected like I had full control the entire time and totally do the happy dance when I get out of sight...
