MERCS B-Board #1: No video. Actually a fix on the C-board, simply one of the inductors had a broken leg. Reattached, works fine.

MERCS B-Board #2: No video. Also a fix on the C-board, this time by reflowing the solder on the big CPS B-12 chip.

Renegade bootleg: Not much sound. It would scream at me when I first turned it on, then nothing. The audio CPU would work at startup for a moment then go silent, albeit without /HALT or /RESET. Checking the audio cpu ROM, it gave me no matches on the mame database. I pulled a few romsets and found that by inspecting other ones.. (to be continued, forget which board I was working on..)

Cook Race: No audio. The 6502 that controls the audio seemed dead and hot to the touch. Replaced with a spare, audio returns! One of the sound effects was super loud compared to the others. The sound effects all get to the final amplifier by way of a simple resistor-based mixer, and one of the resistors was 47k instead of 4.7k, giving that sound effect a much larger amplitude. Judging from the flux around this resistor, it was replaced at some point with this incorrect value.

Nibbler: Wrong colors and missing half the sound effects. Made an adapter to read the color PROMs and they are "Pioneer Balloon" PROMs, so someone did a conversion and didn't quite finish! For the missing sound effects, I started checking the schematics and quickly found that one of the '161s (IC58) wasn't outputting the carry bit, which is needed.

Spiders: What a neat game. This should be a classic. Anyway, this game only had sound issues. I spent a few days looking at the terribly scanned schematics and combing through the MAME drivers (which were super helpful). Basically, after verifying everything should be working, I happened to notice that one of the main analog switches (CD4066) had a floating GND pin. I actually still don't know where the break is, but it wasn't connected to the rest of the GND net. I soldered a little jumper to a nearby ground and all the beautiful analog sound returned. Seriously, the sound on this thing is all generated from 555 timers.

Galaxian (Midway): Missing the stars and the explosion noise when the ships get hit. This article on Marco's Arcade gave me some quick tips on fixing this. Neatly (word?) enough, the starfield and explosion noise are both generated by a pseudo-random generator consisting of some '164 shift registers. I checked the '164s and one was bad. Replacing the '164 brought back the starfield and sound but the stars appeared to be streaking beyond the border. Marco's article also says the '366 gate at 2B controls this, and while that chip seemed to be OK, one of the enable pins was stuck low. Traced back one chip to a '10 (3-input positive NAND) which was borked. Replaced it and we're good. 

Mania Challenge (bootleg): Distorted audio, replaced one of the 4558 audio op amps, fixed.

Nebulous Bee (Galaga bootleg): Booted but no sprites. 3 of the PROMs were disintegrating, replacing those brought back the sprites and got sound going. All the music had the wrong pitches, replaced a few Fujitsu TTLs on top and got it back to normal. The culprit was a '157 at F8. After that, everything was working except the tractor beam, which would flash white. This is on the "4L" ROM. Unfortunately the video board is littered with Fujitsu TTLs, so I started replacing things around that ROM. The Galaga proper schematics are useful if you can get your bearings since the layout is completely different.

NOTE TO SELF: Never try to shotgun a board by pulling like 10 chips at a time. If you do this, and you put the chips back, and your board now appears dead, it's going to take you a LONG time to figure out the mistake you made.

 Still working on this final issue with the tractor beam.

Police 911/2: Apparently this game is was a kit offered to upgrade Police 911, which makes sense since it's such a unique cabinet with its body sensors. These boards will often have a dead timekeeper, which is a M48T58Y-70PC1 from ST Microelectronics. This is a bit of SRAM with a battery and realtime clock. When turning on a Police 911 that has been converted to 911/2, a dead timekeeper will show "CONVERSION FAILED", whereas a missing timekeeper will show "RTC MEMORY FAIL". I removed the original timekeeper and put in a socket. Since there is no dump of the Police 911/2 NVRAM contents, I attempted to program the new timekeeper with the NVRAM contents of Police 911 (the first one) from the MAME romset, using my GQ-4X EPROM burner. There is a note that pin 26 must be shorted to pin 28 during writing. I did this, and while it would program properly, the verification would fail, so I'm not actually sure if this part of the process worked. Nevertheless, I put the timekeeper back in the socket and booted the game (with Police 911/2 security dongle and CF card in place). The game booted, this time I got a "CONVERSION SUCCESS". Next it showed "RTC MEMORY FAIL" (I think) but this is to be expected. It sent me to the test mode where I exited, and (barring some warnings about sensors missing), the board booted properly.