Quoted from creepykenny:
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
Nothing generates the low signal. You can't have control over a semiconductor until the voltage is up and stable. Unless there is something in there like a reset/supervisor IC that specifically blocks the output path, the power up is going to create an unknown state until it is stable enough to exert control.
They just didn't think it was a big enough deal way back when - it's relatively harmless because it's so short. There could probably be some kind of adapter made to go onto the solenoid grounds that has a separate power on just like was done for the AMP on the WPC era machines.
How much of a thunk are you getting - it should be VERY short.
The main purpose of blanking is to actually blank the displays during display refresh (that's why it's called 'blanking'.....) - as a nice side note, it's also supposed to safe the lamp and solenoid matrix in the case of interrupts being missed (Generated by 555/556 timer on system 3-7) - in practice, it does not always do this. There is not a separate watchdog circuit that would be the correct way of doing this.
You could have an error on a machine where the software is not running in its proper state, but the interrupts will still plug away happily. For instance, good OS rom but bad game rom. Wms doesn't checksum at bootup, only when you press test button.