Playboy requires system ROM "720-30" at U6, so go with option 4)

By zero flashes do you mean the LED is locked on?

jlbintn had a similar problem with his Callahan Mutha MPU board - anyway might be a common issue with these boards.
https://pinside.com/pinball/forum/topic/fireball-ii-club-members-only/page/10#post-5328538

First of all, what voltage do you measure at TP2 (12 volts test point) on the MPU board?
And what voltages do you measure at each leg of the zener diode (VR1) on the MPU board - should be in the reset area.

One thing to note about the Mutha board is it uses a different CPU (a 6802) vs Bally that used a 6800. A key difference between the CPUs is the external clock circuitry. The 6802 uses a crystal that takes longer on power up for the clock signal to stabilise (compared to Ballys setup) meanwhile the Mutha board still uses Ballys reset circuitry that has a short time to release and let the CPU start.
The specification for the 6802 is that it takes 100 milliseconds on powerup for the crystal to start oscillating to provide the clock signal.
From previous tech support threads measuring Ballys /RESET signal release is about 4 - 10 milliseconds on power up.
In the case of this board, the CPU is coming out of reset *before* the CPU clock signal has started oscillating where best practice is that it should happen *after*. So this is potentially a problem.

What might be worth trying is delaying the /RESET signal release on powerup.

If by chance you have a 22uF capacitor, try connecting it between ground and the base leg of transistor Q1 (base leg is the middle leg) on the MPU board to further delay the reset.

Hope I'm making some sense.

Mutha MPU looks like they have a crystal across p39 and p38 of the CPU chip. I could not get Motorola or STMicro 6802 chips to work reliably with a 2mhz crystal in this setup even with a long reset from a MCP130. 3.58mhz crystals worked and Hitachi CPUs did work fine at 2mhz tho. The moto cpu and 2mhz crystal would just not reliably start oscillating. Power cycling usually got a good boot. Looking at the datasheets Hitachi has slightly different requirements for p38 and p39 and i figure thats the reason Hitachi would work.

Going to a complete external oscillator then then Motorola chips would run at 2mhz divided by four.

If you have a hitachi cpu laying around, try it. HD6802P is the part number.

Sounds like just a connector issue than. If the 5v is lit the 12v most be good at least at the driver board, so i'd expect the problem to be connector pin at MPU J4. Or maybe on the other side of that 12v wire.

maybe it is the male pin then?

I am assuming the 12v LED is just across 12v and ground with a series resistor. It being out would mean no 12v. No 12v board likely stuck in reset.

Maybe that is a bad assumption.

You mean when it doesn't boot, you don't even get the initial power-on flicker on the green test LED?

If u9 is a MC6802, EF6802, F6802 or S6802 try changing it to a HD6802. If crystal start up is the problem the hitachi HD6802 CPU should work better.

If you don't have easy access to a hd6802p to try you can check to see if the cyrstal start up really is the problem. With a scope or maybe even logic probe or DMM on frequency test you can watch the "E" pin #34 of the 6802. As soon as the reset lets go "E" pin should pulse around 0.5mhz. If it is slow to start up or way out of time on the failed boots that is probably the issue.

Haven't seen a schematic for that MPU, but looking at pictures it appears the crystal setup is exactly what I tried and found to be unreliable at times with motorola CPUs so I can't expect that board to work any better.

If it turns out to be a crystal startup problem and you cant get a hd6802 you could also try changing the load capacitors, the CPU, and/or the crystal itself to see if you get more reliable boot. What value are the two capacitors in the pierce oscillator circuit near the crystal? You can try a range of about 27pF to 50pF. Fine tuning the load capacitors here may help. NPO/COG type capacitors are normally whats recommended to be used. The cheap ceramic caps drift big time with heat.

My blank boards are manufactured in China. I have an order tied up hoping would get shipped before Chinese New Year and now the virus delay on top of that. Numerous items are going to go out of stock soon =(.

the design is changed slightly. I don't have enough parts for the older style but I have 3 blank boards left on hand.

Yeah, just try swapping in the HD6802P and see if it boots up every time. Nothing else needs changed

The Hitachi part is much more forgiving about starting up the crystal at 2mhz than Motorola and other brands of 6802 in my XP