The 5th flash was incomplete, so yes.

Check for solder bridges on the socket pins. Check to make sure the legs of the PIA chips are clean and not tarnished (you can clean with a pink rubber eraser). Swap the PIAs around to see if there is a change in behavior between the chips. If yes, the chip could be bad (and if you have a neoloch tester, you could test it). If there's no change in behavior, check for continuity issues from the chip's legs through the socket and onto the PCB.

http://pinwiki.com/wiki/index.php?title=Bally/Stern#Fourth_and_Fifth_flashes

The LED is connected to U11. U11 has failed the internal register tests leaving the LED stuck on.
This internal test can fail when one of the Port A pins (PA0 - PA7 on pins 2 - 9 respectively) has an external short circuit - it usually happens in one of the bypass capacitors on one of the signals near a pin header.

Do a resistance check on those U11 pins with respect to ground looking for any with much lower resistance. If you find any with lower resistance than the others then unsolder the respective bypass capacitor on that signal to see if it's resistive (shorted). You can boot the board without that capacitor.

MPU-Bally35_U11-PortA.jpg

If during PIA test the diagnostic LED flash stays illuminated that is a fail for that test. Treat it as if the flash was missing. Usually flash stay on = defective PIA chip itself.

Theory of operation describes each flash.
http://arcarc.xmission.com/Pinball/PDF%20Pinball%20Misc/Bally%20Theory%20of%20Operation.pdf

You're still losing some solder pads (or was that from someones previous work?) - I think we might have discussed this before but care to tell us your method of desoldering/removing the IC sockets and your equipment?

Ok, I think the key issue here might be that you're having trouble melting the old solder which seems more common with the Bally MPU boards and it's causing you to put excessive pressure and extended heat on the pads. The white spots you see on the fibreglass PCB next to the pads on that second picture above are indicative of this.

After pulling the pins, I add a touch of solder to the pads so solder flows and then desolder the holes.

Maybe next time instead of using my time expensive method of pulling the socket cover/pins one at a time, just add a touch of solder to each of the socket pins so you get the solder flowing then desolder like you would previously had and *gently* remove the socket. Note, when desoldering don't put pressure on the pads, just bring the desoldering gun back a touch so all its contact is against the pin/solder. Swirl the gun around the pin till you feel the pin has become loose in the molten solder then pull the trigger. Again, don't swirl with the gun against the pad.
If then the socket doesn't want to come out, carefully inspect each pin by moving them with something pointy. The pins that move freely are desoldered, the pins that are still stiff need to be redone - add a touch of solder and desolder again.
When all the pins are loose the socket will easily come out (I know easier said than done..)

I tend to have my desoldering gun set at high temperature but everyone has a different opinion about this.

I add solder often before 'sucking' out old components. Getting a good flow with fresh solder makes the removal easier. Even when you pull the leg or component and there is some solder remaining, add some new and then suck it clean.

I run my temp at 650. I do chage it occasionally, but not often. I also have a traditional iron and have no clue what temp the iron runs. But experience tells me when I'm too long (hot) or too short (cold). I also find tips important. Small pointy tips for board work and slightly larger for thicker holes. Medium when soldering wires. And so on. The time it takes to heat with the tips also makes a difference.

Indeed it does. The other thing is keeping the desoldering gun capture cylinder and filters regularly clean.

And that's likely because the old solder is playing difficult to melt. Adding new solder beforehand will help a lot.

Ouch, that's cold! So I shouldn't be complaining about the 60°F we had today..

Rubbing the desoldering gun tip against the board is a good way to make the pad fall off.

Something I see people do when using a desoldering gun is they pull the trigger too early. You gotta let the solder melt all the way through the hole to the top side before pulling the trigger or else you get a half desoldered hole.

Also tip... If your hole did not desolder 100% clean on the first try add more solder. It will not desolder clean until you do so.

Another tip... On bally board desoldering the black open frame sockets with a single leaf (RN sockets i think they are) like to pull tight and the leaf sticks to the component side of the MPU. When desoldering those sockets I use the gun tip to carefully push/hammer out the contact away from the component side which stops them from wanting to stick to the component side. Then they just fall out or you can pick them off with your fingers with no force. I actually find those RN sockets harder to desolder than the close from orange/brown or AUGAT ones because of that effect.

I always use the pull the socket frame technique at this point, and I'm the guilty one of heating the pin from the top and pulling it out (quickly, obviously) with my fingers.

Afterwards I use desoldering braid to clear the holes, since my Hakko 808 needs to be rebuilt or something - it just stopped working effectively. I always add solder, and I'm really strange vs. other people in that I like to solder/desolder around 780-860 or so. More heat for less time. I haven't lifted a pad off in years.

Heh, I heat from the bottom and pull out with my fingernails The pins that are originally soldered with their leads bent over and don't want to come straight out make it interesting on the fingertips!

I currently set my desoldering gun to around 680°F

I do 750f usually. I like big tips too =D. With the higher temp and big tip when you have a bunch of solder joints lined up in a row like an IC socket you can just drag the iron across the pins and as long as you keep adding the right amount of solder surface tension fills in the solder where it needs to go. It goes a lot quicker that way.

Thankfully I desolder very rarely anymore unless I make a big mistake. I have grown to hate doing desoldering from the time I spent fixing originals.

I am guilty of heating a hole and banging the board on the desk to get the solder to fly out if I just need to clear one hole. A blast of air will do it too.

I use machined IC sockets and solder the top side with my pointy tip iron. Sometimes I'll bend the offcut of a thin component lead into an oval shape and solder it to the leg so it looks like a solder pad.
When the ground trace along the bottom of those MPU boards has been eaten away by corrosion, I use copper tape across the corroded trace and tin solder it. You can't tell it's there.

C65 must not be the problem, but because you found an odd resistance reading there I'd say good chance the problem is on that PIA port. I just checked a board I have out and I get 7.75K resistance across ground to every digit drive port. Here is the kicker... when I short two digit drive PIA ports together i then get about 4.7K resistance across ground. I think you are going to find that U11 P6 is shorted to something else which is causing the PIA power on self test to fail.

470pF = 0.47nF = 471 cap code. 0.49nF reading is close enough.

Good catch Bob. I think you found the issue..