Gameboys come to me in all kinds of conditions, some are in surprisingly good condition, while others are beyond repair, but every last one of them IS of value, and there is always something to salvage! A totally corroded board may still have a completely restore-able shell and buttons, or maybe even a perfect screen that could be swapped onto a perfect board that has a bad screen.
I do my best to buy Gameboys in wholesale lots, although I frequent many tag/garage sales as well. Once I have amassed ten to fifteen dirty Gameboys that need repair, it is time refurbish them, in mass, in order to be as efficient as one man can be.
First step, test each system and notate any observable issues, then sort by the problem type, as well as the severity of the issue (all issues are graded by; Light, Medium, or Heavy). At this point I am looking for:
Power - Are the battery contacts corroded? If so, how badly? Does the power indicator LED turn on? If so, is the power stable, or do you have to fidget with the batteries? Does the A/C Adapter port work?
Screen - Does the screen work at all? If so, are there any issues, ie; Vertical Lines, Dead Pixels, Horizontal Lines, or dark spots/"bleeding"? Are there surface defects ie; scratches, cracks, or chips? Forget the screen protector, I mean the actual screen. If there are any defects, what is the severity? Scratches, gauges, cracks?
If the Gameboy turns on and has some screen functionality, it is time to test the Contrast and its Potentiometer. Scroll the Contrast wheel until it's completely dark, then until it is completely light. Does its reach 100% black? Does it reach 100% white? Continue to roll the wheel back and forth, both slow and fast. At any point does the contrast flicker or seem unstable? If there are any missing lines (vertical or horizontal), do they seem to turn on or off as you scroll the contrast repeatedly (this will hint at the severity of your screen issues)?
Button functionality, and the sound quality are next to check. So slap in a game that you DON'T CARE ABOUT (all games that are this old should be cared for, I just mean don't use your precious Pokemon Game from when you were a kid) as most of these as-is Gameboys will have dirty, un-cared for cartridge connectors, and you don't want to chance corrupting or losing important save files.
Start up your game of Tetris, or what have you, and put the volume potentiometer on it's highest setting. Do you hear any sound? Is it Quiet, Medium, or Loud? Is the sound clear when the game's music kicks in? Test the volume potentiometer in the same way that you tested the Contrast, turn it all the way up, and all the way down. Do this multiple times both slow, and fast. Take note of how the speaker reacts. Does it increase and decrease in volume consistently, does it sound stable?
Start the game, and try every button multiple times, and in multiple ways. Notate whether or not each button works all the time, sometimes, or not at all.
The advanced modder would now check for link cable functionality, however I will not be discussing the hows and whys here/now, as it completely depends on the game you test with, and any modder who is testing at this level should know how to do so. How ever if you need help here, or at any point, email me or comment, and I will to do my best to help you.
Each Gameboy should now have a functionality rating of good, medium or bad for Power, Screen, Contrast, Sound, and buttons. Each of these categories are important regardless of if you are repairing one, or many Gameboys, and before you begin your process you should always understand all of the issues before you attempt to repair them.
Grading your Gameboy is very easy, however it is extremely important, we are done for now however in part 2 we will discuss cleaning and refurbishing the shells, in part 3 we'll discuss PCB repair, and if we get some traction, part 4 will discuss Modification.