Seeking expert advice for WMS S7 I want to buy

As long as it boots and plays, you are in pretty good shape.

As a first SS, a system 7 is a pretty good place to start. All of the sub-systems (GI, switch matrix, coil drivers, and light matrix) concepts pretty much carry across all Williams machines up through System 11.

As system 7's go, that one looks pretty clean an unmolested. If it already plays and runs good, you're well ahead of the curve. Other than the battery holder, which is a good thing, doesn't look like anything was changed or modified. Very original looking.

-Hans

Remote battery holder installed, that's good. Caps on power supply and sound board look original and will want replacing. MPU hasn't been rebuilt (original interconnect and rom sockets), so doing that can't hurt but may not be needed. I don't see anything that would worry me.

A new mpu is a few hundred, but you shouldn't need one

Caps are like $10-20 total. Connectors and sockets another $20 maybe. I wouldn't bother with the connectors until it becomes an issue.

All good advice so far. Two other things I would do while you are in the backbox is add fuses to the bridge rectifiers and swap out the 100v display diodes with 91v.

This thread is invaluable:

https://pinside.com/pinball/forum/topic/vids-guide-to-bulletproofing-williams-system-6/page/2

One more potential issue would be the resistors for the lamp column drive in the lower right corner of the driver board. Looks like one has been replaced already. After 40 years of heat-cycling, those are usually in pretty rough shape.

Head comes off. It's about the same size as an EM head.

Cab is potentially a bit taller due to the second playfield.

When you reassemble it at home make sure you double check all connectors' wiring colors on each side - there's 2 square connectors that can be plugged into each other incorrectly and if this happens, you will blow every 5 volt component on the mpu and driver board. Not a cheap fix.

They didn't pay a lot of attention at the factory to the colors of the harness plugs, some were black, and some were white, sometimes black goes to black and sometimes black goes to white. Always check the wire colors going into the connector when reassembling.

I watched someone "helping" out at a show set up someone's Black Knight that was a bargain at $800, but as soon as the connectors were put in incorrectly, that $800 black knight just became a $1200 black knight instead. Every single chip on the mpu board was fried.

[This is one reason I never let other people set up my games... then I have noone to blame but myself!]