Gauntlet IV Tengen "via hole" fix, success!
Heyo! So I picked up a nice copy of Gauntlet IV recently, aware of the aging issues with Tengen cartridges and that I'd probably be in for a project. Sure enough it was dead, nothing but a black screen despite cleaning the contacts.
I'm an electronics novice and had no clue what "via holes" were for in a PCB until now (connecting a trace or component from one side of the board to the other) but I went ahead and opened it up planning to poke some holes based on a few success stories.
As a newb I wanted to do as little as possible to avoid fucking anything up and didn't feel right just poking everything willy-nilly so I went one by one and tested continuity wherever there were via holes involved (mostly from the cartridge contacts to legs of the ROM chip, and the little doodads with two legs a couple of times). Visually following the board traces, I couldn't see where they went under the ROM chip so I used pics from another dude here who'd done a ROM chip swap on Grindstormer as a reference.
They all tested great except for two, marked in my photos. One hole looked like it was filled with solder or something, and the other looked covered with blue just like all the others. The solder-filled one gave me zero continuity from IC leg to cartridge contact, and the center blue one only made the needle on my multimeter move halfway (whatever that means).
I went ahead and started prodding with my SIM tool, and quickly found it was too large. I annoyingly scratched away some of the blue covering around the surface of the hole close to the center of the board trying to push through, but it was taking too much effort so I switched to one of my wife's safety pins. That punched through the hole easily and I gave it a little wiggle to carve it out a bit. Tested, good continuity!
So I gave it a test on the Genesis; still black screen.
I was unsure about the solder-filled hole that gave zero continuity because I hadn't seen anything like it in pictures before but I punched through it easily with the pin, again gave it a little wiggle, wiped them with ISA 91% and gave it all a good spray with compressed air. It tested good!
So back to the Genesis, fired it up and boom, it works!! Just had to poke a couple holes. If you have one if these Tengen carts, it's worth a shot!
I played some two-player with my 7yo for a while and everything works great. Looking for any potential issues, I did notice some white pixels appearing on black backgrounds during transitions, and the music very faintly sustains the last notes that were playing (with a long decay) when the game is paused. I don't know if either of these things are present in the original game though.
For more info see the console mods page: https://consolemods.org/wiki/Genesis:Repair_High_Failure_Rate_Tengen_Games#cite_note-Via_Repair-1