Power looks o.k. at those points, but are you measuring just at the connectors? (In other words, is the power really getting from the connector IJ17 to the MPU?) Check the +5VDC test pint on the MPU, and with the game off ohm out the ground from the board. Perhaps those might reveal that the connector IJ17 needs repinned.

The +5VDC test point is located near the right of the MPU sound ROMs and the ground test point is located near the left side of the battery holder. Put your positive (red) lead on +5VDC test point and negative (black) lead on the ground test point for a more accurate indication of the power on the board. To check the integrity of the ground you can simply measure continuity from the MPU ground test point to the power supply board's ground plane (the copper around where it is is attached with a screw) and to the back panel metal.

Not sure what you are saying. The ground planes are these metal areas that the arrow is pointing two - which have the board mounting screws that go through them, and should be all making good contact. Regardless, you are measuring 4.8VDC at the MPU test points, so I don't know why this person told you that you have a power supply issue. It looks like you are getting satisfactory power, so now we need to find out why your game is not booting.

What are the indicator LEDs doing when you power up? (7 segment display on System 11, or the three status LEDs if System 11A)

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You're getting power. First a question, is there any sign at all of corrosion around the battery terminals or components in that area? If you don't know what you are looking for post a photo.

The next steps would be found in this section of the repair docs:

"3f. When thing don't work: Fixing a Dead CPU"
http://techniek.flipperwinkel.nl/wms11/index2.htm#dead

Oy veh. Yeah, somebody has been tinkering there. Also noticed a connector a bit melted like someone bumped it with a soldering iron, and U22 looks like it has a pin problem as well. I would extract those two and try to boot the board without the U21 and U22 ROMs first, at least to see if I could get some flashes on the diagnostics LED. At minimum those pins need to be straightened, and who knows if the sound ROMs are any good or not. Battery area looks o.k. but there could be corrosion around some of the PIAs, hard to see but I pointed them out on the pics.

b088ecafa03175df8d0acdecb9fd025302d59438_(resized).JPG

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For ICs never go by the label that is on them (in the case of ROM chips), you install them based on where pin number one is - in the case of this board it is marked with a white dot. The DIP ICs have a semi-circular cut out in the end pin 1 (and the last pin 16, 32, depending on the number of pins) is on or they have a dot indented into the case near pin one - you must match this up to the markings on the board . In your picture in post #10 the sound ROMs are installed backwards but the game ROMs are inserted correctly. I don't have any personal experience with these exact ICs being installed backwards but there is certainly a chance that the sound ROMs are now shot - if you did indeed install the game ROMs backwards and turn on the game you may have destroyed them (if they were good to begin with) now as well.

Have you tried the test below from this link: http://techniek.flipperwinkel.nl/wms11/index2.htm#dead

System 11a, 11b, 11c Diagnostic Flashes.
These games do not have a 7 segment display. Instead there is a LED which indicates the memory chip test being preformed. To start the memory chip test, turn the game on or press the bottom CPU diagnostic switch button SW2 on the left side of the CPU board. Quickly count the CPU board LED flashes of the middle (diagnostic) LED, and compare them to the follow chart.

Flash Display Message Explanation
1 U25 RAM Failure U25 RAM could not be used. No other tests are performed; the game is locked here until the problem is fixed.
2 Memory Protect Failure This message means: (A) the coin door may be shut; (B) the Memory Protect switch may be stuck in the ON position; (C) the Memory Protect logic is protecting the audit memory; (D) a U25 RAM failure is occurring. Note this test assumes the coin door is OPEN since this test is initiated ONLY by pressing the CPU diagnostic switch (SW2).
3 U51 PIA Failure 6821 PIA at U51 has failed. *
4 U38 PIA Failure 6821 PIA at U38 has failed. *
5 U41 PIA Failure 6821 PIA at U41 has failed. *
6 U42 PIA Failure 6821 PIA at U42 has failed. *
7 U54 PIA Failure 6821 PIA at U54 has failed. *
8 U10 PIA Failure 6821 PIA at U10 has failed. *
9 IRQ Failure IRQ has malfunctioned. It may be missing or too slow or too fast.
10 U27 ROM Failure The ROM (or EPROM) at U27 did not pass its internal checksum. This could be a ROM failure, or its associated components and connections may be causing this problem. U26 ROM test is skipped.
11 U26 ROM Failure The ROM (or EPROM) at U26 did not pass its internal checksum.
* Alternatively, the PIA's associated connections or components are causing the PIA to appear bad.

He wasn't getting any flashes, bobukat. Just the 5V indicator LED.

I didn't see where he specifically said he tried pressing the CPU diag switch, that's why I asked.

BTW - is it just me or have there been a LOT of HS problem posts lately?

Ah, I see what you are saying. It's supposed to run diags on boot or with the diag switch. I get the impression this is a dodgy board with a lot of problems.