have you checked the coin microswitches on the door?

try unplugging and reconnecting the door plug

Are you sure the 2 coins per credit ever worked?

The ROM at U6 for Space Invaders (version 720-37) was changed to support a couple of different coin/credit settings. A side effect is that dip switches 1 and 9 are ignored meaning the 2 coin settings are no longer valid.

If you really want 2 coins/credit, you need to replace U6 with version 720-35 ROM.

See here for more details:
https://pinside.com/pinball/forum/topic/oliver-rom-combine-trouble#post-3650011

BTW, a peculiarity with Space Invaders is that it only has "50" point tens scores. It means 8 out of 10 game over match numbers will never be achievable.
In the ROM at U1 if you change the following using a hex editor, the sling shots will be changed from scoring 50 points to 30 points:

U1 Address:
0000: change 51 to 53
01BE: change 05 to 03

Post a picture of the ROMs. Lets see what version you're running first.

I just had another look at the 720-37 U6 code. DIP switches 1 and 9 are confirmed to be ignored. The only way it can kind of work as you mentioned is if you had credit/coin switches set as per the option in the red box shown below. See the *** description.
All other 2 coin DIP switch settings are treated as 1 coin settings.
SI_Credits-Coins.jpg

Thanks for this!

Sorry, they're no help.

Did you by any chance shuffle the coin chutes around? Each chute is controlled by a separate set of switches. Verify your switches are set for 1/2* and note which coin chute you are using for this setting.

Now THAT would bug me tons more than the coinage issue, in home use anyway. A 2% match percentage, what a royal gip! Sort of falls under the category of an oversight rather than a outright bug.

Did you look at the U6 ROM to confirm which version it is?
If it was version -37 did you set the DIP switches as I mentioned above?

Same here!

Although it is a bug when you think about it, the odds of match are still 1:10.

I did something similar on RGP for SI but I changed the score to 10 instead of 30. I like the 30 point idea much better as it's closer to what was intended with a similar result.

Oh yeah you're right, like if an EM match unit was stuck on the same number it would still be 10% although that would be the match unit being stuck, not the scores only landing on certain numbers like SI does. So it's a bug but has no real effect as far as the player is concerned. Doesn't 'bug' me anymore.

720-37 U6 ROM.
The only way it ran 1 credit / 2 coins was per the DIP switch settings I listed in red in post #5 above.
But as per the note, the 4th coin buys another 2 credits not 1
If you want pure 1 credit / 2 coins you need to downgrade U6 to 720-35

Link?

These games all use the -35 U6 ROM. Download from IPDB.

#Nitro-Ground-Shaker
Silverball Mania
Harlem Globetrotters On Tour
Dolly Parton
Future Spa
Rolling Stones
Mystic
Viking
Hotdoggin'

If it's a black 9316B PROM like yours shown above, then jumpers stay the same.
If it's a 2716 EPROM, then you need to modify the jumpers.
If you're not sure, post a picture of it.

U6 is the operating system code - there's nothing in it specific to Mystic.
U1/U2 is the game specific code where the Mystic personality/rules are.
So if you download the Mystic set from IPDB, just program the U6 code to a 2716 EPROM or 2816 EEPROM and change the jumpers accordingly.

It sounds like you have a MPU board from a Mystic with Mystic ROMs that you want to convert to Space Invaders - right?
If yes, leave the -35 ROM at U6 as is.
You will need to replace both Mystic U1 and U2 with Space Invaders U1 and U2 each in 2716 EPROMs. You could if you wanted merge Space Invaders U1 and U2 files into a single 2732 EPROM fitted at U2 and jumper accordingly - the U1 file goes into the lower half of the 2732, the U2 file goes into the upper half of the 2732.

In that case, I'm confused. Why are you programming U1/U2 when you need to swap U6?

Get one of the Viking sets from here, it also uses the 720-35 ROM at U6 and his are 9316 chips:

ebay.com link: itm