Low output voltage is usually bad 140v zener diode in my experience.

1N5275A, 0.5w 140v zener is plenty fine. It was pretty rare they fail, but it does happen. Not sure why.

I am using 1n5382 on my replacement board. I think most 140v zener diodes are obsolete now. That part was actually the cheapest one I could find, ~ $0.04ea. It is bit overkill being a 5w part and the tolerance range seems high, but works fine in this circuit. I am not sure if the drill holes and spacing on the original board is big enough though.

Not exactly you symptoms, but close.

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Power transistors under western brands out of china usually big time pass. There are OK Chinese transistors but they usually have a unique part number and official looking datasheet, ie from LCSC and not ebay/aliexpress.

If you are having trouble with finding the original can transistors MJE3439 can be used if you get the pin out correct. TIP50 is close enough too. You don't really even need a heat sink on either of them based on temperature readings I have done, but a small TO220 can be mounted to them with an m3 screw and nut.

https://www.arrow.com/en/products/mje3439g/on-semiconductor

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Nice Quench! It is easier to understand the way you have it drawn.

yes that should be fine. i measured the temp of MJE3439 used in this circuit just using a large area of PCB foil to heat sink and neither one is over 40c so just about any heatsink is fine and even naked does not come close to maximum temp.

The transistors seem to give up the ghost for no good explainable reason at times. I have had at least one go short Vin = Vout while running in my home. The only thing wrong was transistors and a couple resistors burn presumably after the fact. I am sure you can have human error accidents too. The display PCBs will get covered in dust, ash, and whatever. At some point maybe the HV can arc over. I have not had the HV circuit get blown out in any of my replacement boards that I know of. Small sample size and short length of time.

This can happen too. High voltage is a bitch.
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That burnt display i pictured came in a dead, sitting in a warehouse for probably a long time, game. The SDB was missing but I assume the HV section got smoked too.

I have seen the MPU board 1K2 series resistors burnt to a crisp too. I guess when the digit driver goes short C to E it can put high voltage through the connector that goes back to the PIA chip with that 1K2 resistor in series.

When plugging in the display live HV is at Pin 1 and ground is at 13. Some interesting situations happen when the ground is still floating?

My replacement checks those boxes as long as you count a fuse as current limiting the HV output =D.

FWIW stern's later MPU200 game with the HV shutdown circuit does not seem to work, at least not in all cases, as the sdb board HV section will get damaged on overcurrent. Fuse probably should be added back in for those games.

Yes. That's it. The shutdown circuit is nice idea but the fuse should not have been deleted.

Based on all the boards i have amassed seems more typical of the stern sdb without the fuse to be burnt up when the HV section has a problem.

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leaking too much DC if the ESR is good?

PIA probably not happy about sinking high voltage either. I can't remember if the PIA was bad when I have encountered this, but likely to at least kill one port.