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!

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.

Galaga (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.