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?

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

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

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

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

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!

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