that's kinda the textbook purpose of a hold switch on a relay ... keeping the relay powered when the triggering event may be brief (drop target, stand up target, sling, pop bumper, etc).

per what daveh said above, the 500 point relay is initially powered via the circuit highlighted in green, and the relay should stay powered via the red highlight until score motor 10B switch opens.
replay hold switch (resized).jpg

take a look at the schem for how the 100 pt relay is powered via score motor 2C thru the 500pt relay switch, and note on the timing chart how 5 pulses are sent thru before 10B opens.

you'll see hold switches all over the place. When they don't work, sometimes it's obvious, sometimes the behavior is erratic (e.g. sometimes you get a chime, sometimes not), and occasionally it doesn't matter because the activating event is long enough.

if it's not the tilt relay switch, there's two more switches in the power wire path marked below.

if you want to see if power is the issue, jumper red wire 10 to black wire 80 and see if the problem goes away. If it does, then it's one of the three switches or the plug connection.

red wire 10 is on the bonus unit step-up coil, and black wire 80 is on the bonus unit reset coil, so that's an easy place to jumper them together.

cf_power (resized).jpg

problem is, the flippers and pop bumpers are also connected to wire 10, so if they work, then at least part of wire 10 is ok.

wire 10 daisy chains all over the place, and if one of the double wire connections is bad, everything downstream of that point will not work. You'll probably want to go around to everything you can find with a wire 10 and wire 30 connection and tug on the wires to see if one wire segment is breaking/broken off. See https://pinside.com/pinball/forum/topic/1975-bally-wizard-bonus-unit-issue for example.

check solenoids especially since the shock of the plunger slamming into the coil stop is what breaks the wire at the solder point.

do the easy check first ... manually operate the 100pt relay ... the active player score reel should power as soon as the switch with the yellow and yellow/brown wires on it closes.

repeatedly activating the 100pt relay should step the score reel every time. If not, triple check the switch on the 100pt relay with the yellow and yellow/brown wires. The most common issue is the switch is out of adjustment and/or the contacts need filing due to the heavy use/arcing that switch sees.

if manually toggling the 100pt relay consistently increments the active player 100pt score reel, then what you want to know is if there's a problem powering the 100pt relay.

easy thing is hold a playfield switch closed that scores 100 points and look at the 100 point relay.

if activating a 100pt scoring switch doesn't consistently power the 100pt relay, put a jumper wire from the red wire on the 10pt relay coil to the red wire on the 100pt relay coil ... does that help?

if jumpering the red doesn't help and the 100pt relay powering is flaky, take a good look at the coil wire attachment to the solder lugs and the solder joints on the coil.

if necessary, swap the 10pt and 100pt relay coils and see if the problem moves.

@drain1's comment is important.

the way it works is:

- a 100pt event happens and powers the 100pt relay, which should then power the 100pt reel solenoid
- the 100pt relay has a switch with wires 90 and 93-4 that closes when relay is powered. That's the hold switch that keeps the relay powered
- when the reel solenoid plunger pulls in, the EOS switch on the score reel opens, the circuit to 93-4 is broken and the 100pt relay loses power
- the 100pt reel coil loses power and the reel increments

so when the 100pt relay is stuck on, one or more of:
- an activating switch is stuck closed
- the score reel didn't power due a switch on the 100pt relay not closing
- the score reel coil is also stuck on because the EOS switch didn't open and release everything.

when you have an issue, look at both the point relay and the associated reel to see if both are stuck on or just one.

if the hold switch on the point relay doesn't work, the symptom is usually the game missing scoring, underscoring or an associated bell/chime is intermittent ... but you have to notice it since it often works right - depends on how long the activating switch was closed.