I'd check where the blanking comes into the driver board after the interconnect, make sure it's toggling at power up correctly. If not, trace it back

Interesting, I'll be watching this topic to see what may come of it as I had something similar caused by my own hand but is still a mystery!

https://pinside.com/pinball/forum/topic/can-you-smell-what-atari_daze-is-cookin-or-who-let-the-smoke-out

I'd probably start by just replacing IC23 if you have one handy.

This could very well be a case of the indeterminate nature of inputs not turning on in a known state - (like the Gottlieb system 80 thunk on powerup, or WPC thunk on turnon)

There's always going to be this type of state on power up until the power is stabilized, unless some kind of separate reset timing type circuit is added (as in, no power to the coils until the power is stable.... something along those lines).

FWIW a couple of my system 4-6 games do this as well. I'm kind of used to it.

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

The other point of it is that if the CPU locks up, it'll kill the solenoids and displays before they burn

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.

I tested my Black Knight since I happened to have it open. Pin 37 is low for a good quarter second or more before going high.

Blanking doesn't blank the displays during the refresh on sys3-7. It should stay high during the entirety of normal operation according to the manual.

I put a logic probe on it, there's no pulsing.

I'm thinking Bally world then - I thought the blanking worked the same way on williams. There is a blanking signal going to the display board but obviously, it's just for watchdog and not actually performing the blanking.

For the OP, had a thought - are the board grounded with all the screws present? If your ground was floating slightly on your transistors that could certainly cause some issues.

I'd replace C31, Q5, and IC23 and *hopefully* be done with it. Any/all of those 3 can cause the coils to fire upon boot-up. I had a very similar problem with my Black Knight a while back, but it was activating all the coils *and* blowing the 2.5A slo-blo coil fuse upon boot-up...but it was only doing it intermittently...and if it managed to boot without blowing the fuse, it would work perfectly fine for hours on end. Drove me nuts. Still can't remember how i came across the suggestion to replace those components for the blanking circuit, but that fixed it and has worked fine ever since. They're easy enough parts to replace, and if they don't fix it at least you know you have a good blanking circuit for the foreseeable future.

Here's the relevant portion of Clay's guide that i got the info from:

Likely Blanking Signal Problem Parts.
Remember the likely cause for a low blanking signal (other than the things mentioned above) is a timer chip failure (556 for s36, 555 for s7) at IC23, a bad C31 (1mfd 25v polarized) capacitor, or a bad Q5 transistor (2N4403). The Q5 transistor tests easily with a DMM, and replace cap C31. The timer chip is small and cheap to replace. So start with those thing. In system3 and system4, the 556 timer is also used to generate the IRQ signal, so suspect a bad C31 or Q5 first. On System 6/7, the timer chip (556 for sys6 or 555 for sys7) is only used for blanking. Also on sys6-7 look at the outputs of IC25 (4020 1ms timer) with a scope or logic probe. If the interrupt signal is not being generated at the correct speed, then 555/556 timer IC23 will not generate the required "pulse".

Correct. The Blanking circuit watches for regular pulses from PA2 on PIA1 (Displays). If the displays aren't being updated the blanking circuit assumes that the CPU has crashed and therefore drops the blanking signal to LOW.

Code is not executed on the CPU until after the reset cycle, and once running it will take a few more cycles to fully initialize the PIA chips. It's possible the PIA's may be putting out the wrong signals during this time.

You could pull the GAME2 ROM to prevent the CPU from doing anything at all after the RESET. Power up and keep your finger on the power switch and see if the coils fire longer, or indefinately, or even at all. Of course, if they do stay on, kill the power to prevent overheating and/or possible damage etc.

EDIT: The above is what I learned from System 6 boards. I am assuming this particular aspect of Williams boards also holds true for Systems 3-7 since they are in the same family.

Hmm, I've got some of these, may try it on my OE board for Laser Ball while my potential buyer and I await a shipping quote from Fastenal. It's been a week!

Thanks for the update Kenny.