Very possible game was working. Battery corrosion re-reared it's ugly head. Sorry board repair guys, but I replace with new if you want long term reliability....

Replace with Alltek boards.that is what I put in my EK

Looks like some battery corrosion was badly repaired. At least they put a new battery on.

No 5th flash means one of the PIAs is having a problem (http://www.oocities.org/timessquare/arcade/7800/flash.html), which usually isn't related to the corrosion though. Try swapping U10 and U11 and see if anything changes. Even badly corroded boards can sometimes be quite reliable, though it's better to clean it up or replace in the long run. If it was sold as fully working, and no mention of the corrosion was made, I'd probably complain.

Allteks or https://nvram.weebly.com are a good replacement. Most board repair people won't touch corroded boards.

You’ll likely have to replace the connectors on the J4 plug before plugging in that new Altek. The corrosion easily climbs up the header pins and on to the crimped on connectors, causing all sorts of reliability issues.

Was the game sold "As Is"?

I would suggest repinning all the connectors on the MPU, SBD, and rectifier board. It is inexpensive and eliminates a common failure point on these older Bally games. If you want to repin the lamp board and displays, go right ahead. They don't seem to need replacement nearly as often as the MPU, SDB and rectifier board connectors. Don't forget to address the header pins, too.

If you're getting a new MPU to use in one specific game, I recommend getting this: https://nvram.weebly.com/new-pcbs.html
It saves a bit of money over the Alltek.

I keep an Alltek mpu around for troubleshooting the original MPU. If that's a lost cause due to corrosion, I buy from weebly.

I picked up an EK and the GI worked, but the MPU did not boot past 4 flashes and the displays stay dark.
Checked all of the voltages, clipped some of the obvious shorted solenoids like the Knocker.
Finally read that four flashes means U11 isn't booting. Pulled the U11, held it up to the pinball gods, plugged it back in and the MPU Booted!
Displays all work.
Sometimes it's just that simple.

Now to address the other issues.
1. Feature Lights on pf do not come on, checked the T1 test on the solenoid board and 5V+- is good
2. Changed one of the feature light bulbs to see if they might have all blown out, nope, still nothing
3. Solenoids not firing except for chimes. Pressing the flipper button, the flipper makes a noise, but no flip (low power or just shorted?)
4. I need to go through all of the solenoid diodes to see if they are shorted first
5. I'm almost out of tequila

Figured it out:
1. Feature Light were all blown because the connector at J1 on the power supply wasn't on correctly and 43V blew them all
2. #1 fixed the solenoids too (duh!)
3. Changed the BR1 on the power supply and the feature lights are amazing
4. Tequila has been replenished

Take a good look at the SDB and J4 on the MPU.

Check local ads for Tequila on sale, see if Uber eats will deliver for you

That MPU clearly has seen better days. Unless you like posting for help every once and a while I would replace the MPU with either a Alltek or another new style MPU.

I believe the best way to go is to replace it, it's just to nasty. You will need to inspect the connectors for corrosion and repair those too. I like Allteks, but NVRAM are good, I like all that guys stuff. The nice thing about Allteks is you can check out other games right away when you get them. In the mean time you can look for a bargain -17 on eBay. I have a working one that has corrosion but not nearly as bad as the one pictured here. I'd sell it to ya' for $65 + actual shipping, that would save you a few bucks but there is a lot of value in having an Alltek around.