I have a Volcano and spent two Summers fixing all the issues.

What you describe is generic enough it could be almost anything. However, what does the MPU look like? Original? Was there any battery damage, and was it mitigated properly? Post some photos and please be more descriptive with the issue.

So what exactly does it do? What do the displays show? Any sounds? What exactly happens when the button is pressed to start a game?

What about the other two diagnostic tests? 19 (Displays) and 20 (Memory)?

Are we assuming that you checked the actual switches and verified them? Sounds like something majorly wrong with the switch matrix circuit on the MPU. Could also be bad pins in the connectors from the switch matrix to the MPU.

Looks mean nothing. These old Gottliebs are notorious for weak and loose connections. Even the cleanest looking un-broken pins can be a problem. On mine, the 5-volt power cable to the MPU was completely flaking out thanks to worn out pins; no amount of cleaning or bending could make these into a reliable connection; they had to be re-pinned.

Do you have the manual and schematics?

Do you own a logic probe and know how to use it?

Did you get it figured out?

I'm confused ... it's like you and I are continuing a conversation that we never started. Are you the OP but using a different Pinside account?

What's the context of just assuming U4 is bad? Have you gone through diagnostics? Do you have schematics? Have you followed any troubleshooting from the Pinwiki? What's the history of the machine? Original MPU? Battery damage? etc? etc? If you're not the OP, it might be better to start your own thread rather than hijacking one from somebody else.

You are describing attract mode working after all! And if you have attract mode and displays, the game has fully booted. If you want help, please be careful, accurate, and consistent with posting information.

Do you have schematics?

"Game won't start" could be something wrong with the MPU or it could be something wrong on the game preventing it from starting.

Start with Pinwiki. Read through it... it should not take too long.

https://www.pinwiki.com/wiki/index.php/Gottlieb_System_80#MPU_boot_issues

As far as the switch matrix, it's not just U4... what about Z11, Z12, Z13, Z14, and Z15? You've tested those chips as good already? If somebody working on the playfield accidentally sends high voltage back to the MPU with a screwdriver, etc, it's going to hit and blow out those logic gates before it hits U4. It could go through to U4, but if Z11 through Z15 are good, then U4 is probably ok too.

What about diagnostic test 20?

Depends on what you're testing. If you're on a square wave, then it's going to flicker high/low at the same time. The various MPU tests as described in the Pinwiki will tell you more about how to use a logic probe. From what I recall, you're going to see a lot of flickering on your logic probe in the switch matrix circuit because that's how it works.

IMO, look at Z11-Z15 first and you can test those using a DMM...

https://www.pinwiki.com/wiki/index.php/General#Testing_an_integrated_circuit

As far as Z11, Z12, Z13, Z14, and Z15, as stated previously, I would use a DMM to test the logic gates...

https://www.pinwiki.com/wiki/index.php/General#Testing_an_integrated_circuit

The game boots and goes into attract mode so the MPU appears to be working. Before digging too far into the logic signals, I'd take care of the obvious. Since System 80 games are notorious for loose and worn out connector pins, this is really where I would start. Visually inspect the MPU connectors where the switches' wires come back into the game, clean the board's fingers with a pencil eraser, and re-pin the connectors.

Also, have you tested the diodes in the switch matrix?

The originals are "punch-down" style connectors. You can just replace them with crimp-on pins. The original plastic housings can accept both styles no problem and I replaced mine with crimp-on. It's a little more work when two wires need to connect to one point, but IMO worthwhile.

Single Sided Edge Connectors (KK 396, KK series 2478, KK 0.156”)
Molex 08-52-0072 (18-24 AWG)
Molex 08-52-0071 (same pin as above but packaged on reel strips)

https://www.molex.com/molex/products/part-detail/crimp_terminals/0008520072

These can easily be found on Amazon, eBay, and the various electrical suppliers.

BTW, your MPU is in outstanding physical condition. I don't see one bit of evidence of past battery corrosion.

I notice that your MPU/Driver harness is original. I would inspect it carefully for bent and broken pins. You can also add a second power and ground to this harness to make those connections more bullet proof - it puts a redundant power and ground pin on the other side of the boards effectively pinching the board and doubling the connection points. The Molex pins for this one are obsolete and unobtaineum. If you find any, it may cost $50 to $75 to re-pin this one connector. Docent Electronics sells a repair kit made from NOS pins for $45 and it includes the two extra wires. There is also a pinball guy on eBay selling reproduction pins that cost about half as much.

Screen Shot 2023-04-06 at 3.08.20 PM (resized).png

Double Sided Edge Connector (KK series 4366, KK 0.156”) - MPU/Driver (Qty 66)
Molex 08-03-0304 (18-24 AWG) - OBSOLETE - Docent Electronics kit of 33 replacement wires/pins
Molex 08-03-0303 (same pin as above but packaged on reel strips)
Check eBay for the clone/reproduction replacements (equipment99.com)

https://www.molex.com/molex/products/part-detail/crimp_terminals/0008030304

I can vouch for them. Mailed them a money order and they shipped immediately upon receipt.

In my experience, re-pinning the specific ground wire connections (when re-pinning the whole thing) and doubling up on the power and ground wires in the MPU/Driver harness takes care of the ground issues.

The "ground mods" are a "can't hurt" shotgun approach that does not fix any root issues... just IMO. I did them originally, got a false sense of security, they solved nothing, still had a ton of the same issues to find/fix, then removed them after everything else was resolved/working. Have not had a problem since.

Thanks for the update. FYI, if you have access to the Gottlieb OnTarget newsletters, there's one about Volcano ball save. Volume 2, Issue 7, July/Aug 1981.

I recall mine had a bad solenoid, but the typical issue has something to do with the EOS switch.

This is an old thread about a specific issue with Volcano that was already marked "solved". So probably not the right place for a new Haunted House question. I suggest that you start a new topic so a lot more people will see it... you can even tag the topic with Haunted House.