What is your 5v rail reading with all displays running?

probably a wire on 1 of the display connector since it works in player 1 . make sure power is off and check for loose wire

Agree with Steamfitter. Everything else is good I think a loose 10s digit enable wire. On the older games before IDC connectors the wires are double crimped and daisy chained to the next display. Since the 1st player works start there at pin 5 and see if the wire crimped is loose.

They're right. Disregard what I said. I was thinking you were losing the digit after adding more displays...what I was thinking didn't really make sense.

You should be able to turn the game off, and check continuity from pin 5, display 1, to pin 5 display 2. If you cant find the disconnect, you can always run a jumper between those and see if that clears the problem.

So if jumping 10 digit enable from display 1 to another display with weak '10' had no effect... maybe that 10 enable has a weak connection before it even gets to the displays? But if that was it, why is display 1 always good even if you have them all plugged in? Unless it's combo of weak connection before it gets to the displays + another weak connection(s) at the connectors. Oh well I tried. Maybe try jumping 10 enable to display with weak digit, from wherever 10 enable line actually originates at the board.

Funny how when there's a weak digit anywhere, the first display is always okay whether it's 1, 2 or 3.

before you changed to the current displays, what did you have?

Add some dummy load to the 5v and see if gets worse? Try jumping 5v directly from the ps board to one of the dim displays and see if improves? But if was the 5v, why would it only affect that one digit...

i feel for ya, this isn't of any help, but did have a similarish (that's a word) problem where when HV displays were in, the pin was perfect, but upon adding LED displays the mpu went into reboot.

checked the 5V which was at 5.08V.

at 2-3 LED displays installed they were perfect, add one more and reboot happened.

kept the HV displays in.

different brand manufacturer.

Try switching PIAs 1 and 2 (U10 and U11) on the MPU and retest with at least 3 LED displays.

He's got an Altek mpu, don't think that would apply, the chipset is totally different.

The Alltek has two 6821 PIAs (just like the Bally board), but at U2 and U6. You can try swapping them.

So do these aftermarket LED displays typically draw a whole bunch more 5v than an HV display does? Or just a little, or about the same?

That's the thing about these LED displays. With all 5, they draw about 1 more amp than the plasmas. The existing LM323 5v regulator is spec'd at 3A max, and already providing about 2A to the game with plasma displays, so is pushed to its max of 3A with LEDs installed. If you're replacing the entire set, I always thought LEDs should have their own, dedicated 5v PS.

So your options are:

1. Leave as is (which may work, or may be problematic, stressing your regulator)

2. Replace the LM323 with a higher current device (impossible to find, but you may be able to use one of these https://www.ezsbc.com/product-category/voltage-regulators-2/)

3. Bypass the game 5v PS and add your own (an old PC PS would have more than enough capacity)

4. Put the displays on their own 5v 2A (minimum) PS

While Pinside was down, the O.P posted his problem on other forums and a test determined the issue is with the 10's digit drive signal from the MPU board - he was asked to swap the PIAs but is yet to respond:

https://www.aussiearcade.com/forum/pinball/pinball-technical-and-repair-questions/2201757-bally-night-rider-ss-10s-digit-display-issue-need-help?p=2201933#post2201933

This is also likely a combinational issue with the Pinscore displays potentially putting more load on the digit drive signals.

Since only the 10's digit gets dimmer as more displays are added it says this isn't a 5V supply problem, otherwise all digits would get dimmer or worse the game continually crashing.

I was kinda betting it was still a 10 digit drive board/connection problem + now seeing that these displays apparently can suck up a lot more 5v maybe it is only showing up because of the latter. And one would think a display with 40-year-newer devices would go easier on those drive signals, not harder (?)

But there are LED displays that say "Very low current usage, no worries for your 5v power regulator." So I'm only bettin' a nickel.

Schematics or pictures of the Pinscore circuit board would help
Their web site is just coming up empty.

A cached version of Pinscores web site states their displays are using efficient LED blocks that only consume 10mA per segment.

https://webcache.googleusercontent.com/search?q=cache:8kKm3Nf91_IJ:https://www.pinscore.com/technology/9/41-why-pinscore-leds-are-more-efficient

By my calculation displaying an '8' in every digit of each display will draw roughly 0.4 - 0.5 Amps. (7 segments in a digit * 10mA * 5 displays + some overhead). Only one digit at each display is active at any time - they're just refreshed fast enough in sequence that our eyes perceive them as static.

Yeah, I see what you mean. If they only use about .5 amps for all five, that's not bad at all.

I multiplied the 6 digits in, and got over 2 amps, but that would not account for the multiplexing.