I just want to inform about things I've done/found out about the Gameboy, GBA or GBM (Gameboy Micro).
If you can sometimes see scanlines on either display when moving the Nintendo DS or DS Lite around try adjusting "VR1" and "VR2" which can be found under the battery cover where the left one is VR1 and the right one is VR2. You need a quite small Phillips/crosshead screwdriver. Don't rotate them around for fun, it could maybe damage the displays of your DS when rotated to the extremes.
So, to get your display adjusted, open up the cover, turn your DS on and start the program/game where you can see the scanlines most easily. Then begin to carefully adjust VR1 or VR2 and look at the display, until you find the setup which don't shows anymore scanlines (waving the DS around a little bit helps sometimes to see it better).
When you have just repaired a DS / exchanged displays between damaged DS's, then you should also look for these VR's and do the best for the displays - when setup to the sweet spot this does possibly also increase the lifetime of the displays (deadpixel probability, etc.).
This was discussed earlier in the gbatemp.net forums - link to the direct thread seems to have moved, sorry
I can confirm this Gameboy Micro Link-Port Schematic - at least for: SO, SI, GND and VDD3 - I tested them to do a Gamecube-to-GBA cable and it worked.
Additional Information: The "Power Supply -" seems directly connected with GND. At least that's what a multimeter sees when the Gameboy is powered off.
Really helpful Technical Data about the Gameboy Advance and Nintendo DS, -- all the registers explained, nice if you want to code some stuff with it :-)