(Topic ID: 233137)

williams system 4 briefly firing all controlled coils at power up


By creepykenny

6 months ago



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  • 19 posts
  • 6 Pinsiders participating
  • Latest reply 5 months ago by Atari_Daze
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    #1 6 months ago

    williams system 4 phoenix machine

    so, dragging this one back from the grave and now have it up and running mostly correctly but still a nagging issue, when the power switch is turned on, all of the controlled coils fire for a split second and then the MPU boots and all is normal - none of them are stuck on. i am not that familiar with the intricacies of a system 4 machine but i don't believe this is normal behavior

    i am assuming it may be a blanking issue of some sort, so monitoring the PIA at position 18 pin 4 there is pulsing activity, pin 19 on same PIA goes from nothing straight to high with no pulsing at power on

    timer 556 pins 9 & 13 are both high no pulsing

    i feel like i am missing the initial low signal that would prevent these coils from firing at power on

    or am i?

    #3 6 months ago

    thanks! i have done that, the blanking signal (high) is present on the driver board - monitoring pin 37 of the interconnect with a logic probe shows on power up signal transitions from zero/nil straight to a steady high - no low signal is present on the driver board

    the coils are firing before the high signal appears - while there is no signal being sent from the MPU, so am i missing a constant low on the blanking line that should be present when power is turned on before MPU boots?
    i feel like i am, and with out the low signal the coils are firing .. since if i understand how it works a low signal on the blanking line prevents the coils from firing?

    i could easily be wrong tho

    #7 6 months ago

    i do have a spare 556 so a swap is not a problem and thats what i will probably do just to see if it is at fault, without pulling it and putting it in a tester i don't know how to verify its out puts when its used on this board so theres that.

    as for the power up state ... well this is what i am really asking, is there a (or is there supposed to be a) low signal present upon power up on the blanking line and what generates it? the 556? it would have to be instant low to effect blanking the coils before the mpu boots and changes its state to high

    maybe its the nature of the system 3-6, haven't seen other posts about this "issue" but my googlefoo kinds sucks

    #9 6 months ago
    Quoted from zacaj:

    By my understanding part of the point of the blanking is to kill the outputs until the CPU boots, but I don't have a board handy. None of the sys3-7 games I've owned ever made a thunk though

    this is my understanding as well, otherwise whats the blanking point? .... will pull the 556, socket its replacement and post results

    #14 6 months ago

    OK, originally every controlled coil (8 total i think) briefly fired at power up every time i turned the machine on, initial plan was to pull the 556 -test and replace if bad, thought it over alot & honestly not convinced thats the actual fault but it could well be, in lieu of pulling the timer i/c i spent yesterday going thru all of the drop target assemblies cleaning those little pcbs & applying some nice new decals provided by the owner to the targets, once done i turned the machine on same as always and now none of the controlled coils fired like before - totally silent

    grab logic probe and apply it to pin 37 on the driver interconnect, power machine on and all coils fire same as before (signal transitions from nothing to high same as before) , power on with out the logic probe on pin 37 and coils are silent again

    all grounding screws are present on both mpu & driver boards

    need to probe the 7408 inputs but starting to think broken solder, damaged trace, bad connection or a flaky i/c somewheres although everything works correctly once we get past the mpu boot blanking going high

    #16 6 months ago

    OK, sounds like good advice, i have everything on hand except the capacitor and i may have one of them and just not know it yet will have to dig around and see

    thanks!

    2 weeks later
    #18 5 months ago

    i finally replaced the dual timer 556 & the 4403, did not have 1uf tant on hand so i did not replace C-31 just yet, but so far so good - none of the controlled coils are firing during initial power on and this is after many, many tries

    prolly pick up some of those tants on my next digikey order and replace it some times later

    so i'd say i am almost done or mostly done with it

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