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Phoenix: The Blanking Signal

The day after I thought I had fixed my Phoenix Pinball’s problems and had a working machine, that solenoid fuse blew again.

This time, however, I could work around it by removing the fuse before I powered on the game, and only inserting it after powering it on.

Not having a clue as to how Williams pinballs work, I begin doing testing and a lot of internet research. I figured the fuse was blowing because all solenoids would lock on simultaneously at power-on. I remember reading at various places saying that running all the data & address lines over an ‘intraboard connector’ was a big design mistake by williams. I somehow got it into my head that another ‘design’ mistake was using a PIO that would actuate all the solenoids before the CPU gained control of them. I was looking in all the wrong places — I kept thinking the CPU was taking longer to boot than it should, allowing time for the fuse to blow. Could it be the crystal? The obsoleted & impossible to find clock & reset chip? The broken capacitor I found on the CPU board? Nope — all those things check out OK (after replacing the cap, of course).

Then it hits me — I recall reading about this “Blanking” signal. Not sure what exactly it does, I do find out it goes over the intraboard connector at pin 37. Whip out the DMM for a quick check … +5v on CPU board — check. +2v merely 2mm away on the driver board — WTF?

It turns out that even though the intraboard connector pins look like they were making contact, they weren’t.  I was able to get the signal to pass thru by shoving a paper-clip in the connector.

Anyway, here’s the logic behind this mysterious “blanking” signal… There is a 555 (actually 556) timer that acts as a watchdog timer for the CPU.  If the CPU stops ‘feeding’ this watchdog timer by sending consistant pulses to it, the timer expires and it is supposed to pull the blanking signal low.  This blanking signal goes into an AND gate with every solenoid cpu-driven PIO output before hitting the transistors to switch them on.  Basically, if blanking is low, no solenid can be actuated by the CPU.  It is used to prevent the solenoids from actuating before the CPU boots and to disable them if the CPU locks up.  Now what did we learn in basic digital circuits class?  That an “open” is commonly seen by logic gates as a “high”?  And a high-blanking signal means all the solenoids are under CPU control — but the CPU hasn’t actually booted yet.

Its clever in its own way — On one hand it does good to prevent burnt up coils from a CPU malfunction, on the other hand it becomes one of the single most important signals sent thru that inter-board connector…

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