Looking for suggestions!

Picked up a Sorcerer recently, and the last owner said there were major issues with the MPU. It had battery damage that was repaired, and the board was actually salvaged, as the game mostly works other than no music, sound or speech. However, there are some hacks that were done, and some parts were not repopulated as you can see in the pics below. The board is completely missing SW2 (sound check switch) and I'm not sure, but the section that is missing components looks like it has to do with a reset (??). I can't even tell the jumper configuration at W1 and W2. As far as what comes out of the speakers, it is a humming sound that fluctuates with the lights, and moving the volume knob in the cabinet, the humming does fluctuate. The sound also fluctuates when moving the speech/sound mix pot on the sound board. I've looked at numerous threads about the sound issue, but not sure if I need to repopulate and repair the current damage before diving into the sound issue anymore. I've even contacted Dumbass about buying a new System 9 board, but the wait is long. Would love to have one though. He also suggested the Rottendog MPU9211, but I'm not sure who could burn the EPROMS correctly so that the board would work in a Sorcerer.

A few things I've tried (haven't checked any voltages yet, and I don't have an oscillator):
-swapped U4 and U13 memory, game boots and plays
-pulled and re-seated associated sound roms
-pulled the board and checked for continuity on the repaired traces and they show good
-many screws were missing form the MPU and sound board, installed all screws and they are good and tight

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Thanks for pointing this out. I've been referencing the PinWiki quite a bit and missed that section. Didn't even bother looking under the "MPU boot issue" section as mine is booting fine. It's section 5.4.10 if anyone else was wondering

https://www.pinwiki.com/wiki/index.php/Williams_System_9_-_11#An_Alternative_to_Rebuilding_the_Reset_Circuit_.2F_Using_a_Reset_Generator_.28System_9.29

Woa this is getting over my head lol. Last night I looked at the schematic for the pin that is not soldered and I figured it would be pin 2 not pin 13 lol. WTF? the pin is all that way at the end, not in the middle of RAM! Wow these schematics are hard to read! What is an E signal? What is VCC? U11 is a 6808. Swapping the jumper and installing different types of RAM is a solution? How? Sorry for the goofy questions! Much appreciate all of the helpful pointers and comments!

Yes I will probably get one of his boards eventually. I plan to keep this machine, not flip it. It is a project. However, most everything is working well, better than I thought when I got the machine the other day. I've got every switch working, every solenoid, and all of the lights, except for the diagonal string and top string in the backbox that are part of the same circuit. Fixed the drop targets and spinners, working good now. Game works and plays well, scoring bonuses, specials, multi ball. Even the knocker and bell work. It does need some LED's (game is very dark with the incandescents) and some new rubbers. Has the incorrect plastics over the slings. Oh and the backglass is great! Cabinet is ok, fine for me.

Just a quick update. A fellow pinsider was able to get most of the board back to stock configuration and working 100%! Me and guests have been playing the machine quite a bit for the last week and it’s worked great so far. I’ll still probably get a Dumbass board eventually after the playfield restoration is done. Thanks again for all the excellent comments and information.