Up until recently I had serial #4 Muthaboard installed in my Power Play. It too quit after 25 years and has been replaced with B&I board. Not sure why you would want to disable the tunes but yep, there's a switch setting for that.

Check the sling shot coil assembly. They tend to work loose from the playfield and can touch the bare switch wires that cross under.

Bad connection @j4. 4.2v is too low for the board to boot.

Here is a simple test. Plug an ordinary outlet tester into the service outlet. It will tell you right away if there is an issue with ground and polarity.

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I think someone is working on a circuit board LED replacement for BPM similar to the one available for MMPacman?

If J3 is in two pieces it may have overheated and broke in half.

Always match up the keys. If keys are missing beware! Follow Quench's diagrams carefully.

Dimming lights coupled with flipper flipping is a common issue with Bally games. There are a few things to check,

EOS switches should be opening at about 80% of stroke.
Flippers in need of rebuild.
Transformer is optioned for low line, 115v in 120v region.

You cannot eliminate the problem entirely but doing the above can minimize it.

Sorry, I had it backwards as far as the transformer goes. If the transformer is optioned for high line -120v and your line voltage is actually 115vac that would reduce output voltages and cause power issues.

No 7th flash usually means you have no solenoid volts and F4 blew. Replace the fuse and check your flipper EOS switches for proper adjustment.

I was about to say that maybe the game is set to award free balls and he's hitting a scoring threshold and winning two more balls? But you do say the BIP counter is advancing to ball 5. Number of balls is controlled by DIP switches 31 and 32. Perhaps switch 31 is dirty? Have your friend flip those switches back and forth a few times to clean them internally. Be careful on Alltek boards as the diode numbers next to the switches are often confused for the switch number. 31 and 32 should be the top two switches.

A common problem from that era as there was a defect in the die Bally used to stamp out that part of the frame.
The factory fix was to install a thin breaking rod through the side holes in the target frame. This dampens the upward travel of the target bracket.

I use epoxy to hold the wireform in place. Something to try before going to more extreme measures.

Try 'floating' the sound board by disconnecting it from the metal brackets. If that solves the problem secure it with insulating washers and screws so it's not making electrical contact with the grounded frame of the game.

Common causes for weak flippers and dimming lights when flipping,
* Misadjusted or miswired EOS switches - including lower flipper dual-EOS not properly adjusted break/make
* Transformer optioned for high line in a low-line location
* Time for a rebuild.

Book shows its part number A-613-89. However that part seems to be hard to find. You can probably substitute A-613-115 https://www.marcospecialties.com/pinball-parts/A-613-115.

If the other displays are good I would try resoldering the display's connector pins.

Cabinet Speckling was done at Bally early on in the Solid State era. I think Evel was the last game that had it.

First 300 games, SNs 1000-1300, were pilot runs for dealers to test and to gage potential sales. It's not unusual to find non-standard features on these games. I had a production Paragon in my collection for a long time and do not remember it being speckled but it's entirely possible it was done in early pilot runs.

Speckling added another expense and step in the manufacturing process and as it did not translate into increased cashbox earnings it was a prime target for cost cutting which is why I do not recall it showing up in any production games post EK.

The cabinet should sanded down completely to bare wood and as smooth as possible for the Pimps stencils to work properly. The few times I've had stencil fail was because I did not follow this advise.