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Repair Log… (16/5/11) - MPU3 – MOD4 - STUCK SWITCH


The board runs fine except that when you get a hold the 2nd or 3rd reel holds when it shouldn’t. It’s odd because the hold button still flashes until you press start, then the hold button stops flashing and goes constant and the reel is held.


When you get a nudge, all is well until you collect then the 2nd or 3rd reel will nudge once, unless you press start in which case that reel continues to nudge. If you hold down cancel the reel will spin and the hold is cancelled, but only if cancel is held in. This time I reflow solder onto the orange and black plugs. No change.


Next check all diodes (there are 40 of them), all meter good. (Later MOD6 boards have diode packs. See picture to right).  It can’t be the switch or wiring because it’s fine with another board. Must be something else?


Next I trace back the wire from the centre hold button. I follow it back from the pin on the orange plug, through the diode bank to SW1. Then I get stuck, I can’t find the hold enabling IC. To me the whole schematic doesn’t make sense. SW1 has to be closed to make continuity to IC11, otherwise the circuit seems open (well to me anyway) and yet IC22 seems to control the holds?


My money is on IC22, but after a quick chat to Magik he says change IC23 first, it’s a cheap part and takes 30 seconds to change at least then it’s eliminated and I have them in stock. (Hold enable line comes from IC23).


IC23 is an LS7408.


Change IC23, drum roll...


Fault is gone! - See it run, click here


If it’s any consolation, I would have done IC22 first, then 23. Nearly got right! :)


2 down, 3 more to go...