You need to clean/rebuild the score reels. On these, the amount of pressure from the contacts can affect whether or not the score reels will index to zero.

Since you need to clean the score reels, you likely need to clean the stepper units, especially the pivots. Make sure you review some videos for step by step hints. There's a special threaded bolt (Screw head and shoulder) with a nut behind it that will often be broken during disassembly. PBR has those.

Sounds like the runout switches are opening too early on the hundreds and thousands score reels.

Check the gap and clean. The runout switch should be the one in the middle between the zero and 9s switches. The runout switch should be closed always except for when the reel is at zero.

In addition to the Runout switches Sea_Wolf mentioned it could also be switches on the Z1/1st & 2nd Player Reset relay:
Spirit of 76 score reel reset (resized).jpg
/Mark

If youve already compared the 3 switch positions 1-8,9 and 0 to a working reel and believe they are adjusted properly, id pull out one of the problem reels during reset and apply some light pressure to the closed switches to and see if it advance to zero. If that works, try and clean contacts/adjust the switch leafs. If that did not work, id try and clean the switches on z1.

If the contacts are touching, but do not react until you apply light pressue, try cleaning the contacts first. I use a metal revlon nail file for the contacts. If that still is not enough, try some slight adjustment of the stationary switch leafs to ensure a slighly tighter switch closure.

My Spirit of 76 reset animation might be of assistance.
You can pause or rewind any part of interest:
https://rumble.com/vtu4f3-1971-1976-gottlieb-pinball-4-player-reset-sequence-animation.html

The LB/Double Bonus relay is a trip relay in the Sequence Bank mounted under the playfield at the very back. It controls whether you get a single or double bonus. It has four normally closed switches. One of them may be stuck open or dirty.

There are wing nuts on either end of the trip relay bank that let you swing the trip relay bank open when they're loosened. Just be sure to fully reseat the trip relay bank and tighten the wing nuts when you put it back or you can cause other problems.

Please get wing nuts. If it’s floating free it will do odd things.

I have a So76 and forgot to put the wing nuts back once. Weird things will happen.

If you are receiving double bonus and neither J nor LB are actuated, check out these two switches on the attached schematic. If either one is misadjusted and open, I'd think that would result in "double bonus-like" behavior. You'd get 2 bonus subtracts and 5000 points per motor cycle.
S76 double bonus error (resized).jpg

As MarkG and Garrett stated above - the torque applied to the relay bank, if it is not firmly seated and the wing nuts tightened securely, might be the cause of the switch on LB being out of adjustment.

LB is a relay, see MarkG post above, are your relays in that bank labelled?

Mounted to the underside of the playfield itself, not on the motor board.

Should be able to identify it from the wire colors.

LB relay (resized).jpg

Actually, it looks like it's the first relay on the right (on the sample game), but verify with wire colors

https://www.ipdb.org/showpic.pl?id=2293&picno=74931&zoom=1

Is the relay bank rotated fully back into its operating position and wing nuts tight?

Have you checked the Ball 5 bulb?

Does the "Double Bonus" light come on for the fifth ball?

Did you clean and adjust the switch on J?

The sequence bank relays are shown in order on the schematic. LB is on one end with an A-15359 coil.

pasted_image (resized).png

Ball 5 and Double Bonus light are the same wire... and Ball 5 is closest to the source (the Player Unit), trace the wire (blue) and check the ball 5 lamp socket.

Use a DMM on the blue wire on the socket, what voltage is it at?

Could you do an experiment for the bonus issue? On ball 1, drop 3 targets (one at a time), scoring 3000 bonus (on the "Bonus Value" playfield lights) then let the ball drain... exactly how much does the score increase as a result of the 3000 bonus?

Great, that is consistent with the error in the bonus path (5000 on first motor turn with 2 bonus subtracts and 2000 on the second with the last bonus subtract). Do you have jumpers? Can you short from point "A" to point "B" on the attached schematic (ignore the red arrows) and repeat the test?

S76 double bonus error (resized).jpg

Short one of the switches, do the test again, then short the other one and retest - that will tell you which of the two needs adjustment.

Consider the wire between the two switches (BL-YEL) as point "C". For the first test jumper between A and C and for the second test jumper between C and B.

The last test you ran determined that either the LB switch *or* J switch is not closed and, as a result, is causing your double bonus problem. Now, we're trying to determine *exactly* which switch is causing the problem, so we'll conduct two more tests. The first test will be to jumper (short) from Point A to Point C to determine if LB is causing the problem. Then run a second test with the jumper from Point C to Point B to see if J is the issue.

Report the results back and then we'll determine the next step.

Subtract bonus jumpers (resized).jpg

According to the schematic, that BL-YEL is connected to both the LB switch and the J switch. You can connect it to either place.

The other end of the J switch has a GR+RED wire - to find a color of a wire, you need to follow that wire on the schematic until you see a color.

You should also start reading some background material that might answer many of your questions (reading schematics, wire colors, types of switches, cleaning and adjusting switches, etc.). Here's a good reference:
http://www.pinrepair.com/em/index1.htm

Are you saying that your J switch does not have that color wire, or you don’t see it on the schematic?

What color wires are on that J switch on your machine?

Can you mark up the schematic, or send a picture of what you are jumpering? I'm not understanding statements like "But if I do the yellow blue switch on 3c on the score motor" - that is not on the schematic that I've included.

1) Does the Player unit solenoid try to advance?
2) What does the Advance Player Unit relay do when this happens?

There's a relay in the machine labeled "P". That's the Advance Player Unit relay.

What is that "P" relay doing when the game stays on ball 1 player 1?