So my Mystery Castle is currently suffering from a 5v dropout issue. Just to get you up to speed the CPU and DMD controller are both NOS replacements. I've completely rebuilt the 5v regulator section of the power supply including the header pins/connectors on the transformer input and outputs of the power supply.

When I have all boards connected I'm measuring 5.18v on the power supply, within about 20 minutes that will steadily decline to about 4.7v and below at which point the game crashes.

With the CPU and Display Controller connected (sound board removed) the fault disappears and the +5v stays at a constant level.

With the CPU and Sound Board connected (DMD Controller removed) the fault also disappears and the +5v stays at a constant level (but slightly lower maybe around 5.14v)

It's only when the sound board and DMD controller are connected to the 5v line together does the voltage slowly plummet until the machine crashes. I've swapped out the DMD controller with a spare to the same results. I'm at my wits end on this one, if anybody has any ideas please let me know so we can get Mystery Castle back up and running reliably.

Thanks, James

Only the heatsink for the 5v regulator itself which I replaced alongside every other component in the 5v circuit

Again this only happens when both the sound board and DMD driver are connected together. The fault disappears with either board running on its own

So far I've replaced all capacitors in the 5v regulator section comprising of C2 C5 C7 and C9

I was wondering about going this route but it would be a last resort as generally I don't like adding additional hardware etc. and would prefer it running properly off it's original boards. But I might not be left with much choice.

I'll work on sourcing the rest of the caps on the board and see where that takes me

Nope it doesn't use that style. Alvin G use an LM350 regulator