Console Powers on, but does not display

Hello guys,

I had bought an N64 a few days ago that was described as broke. Judging by the picture that was displayed in the listing, I thought the seller might have been ignorant of the fact that it won't work without a jumper pak since there wasn't one in it in the picture. So I bought it because it was only $5 and I thought I could repair it pretty easily, as I am the console repairer at a small game shop here in town. I got it and imediately tried it out using a working Majora's Mask cartridge and the wires, expansion pak, and controller from my working N64, but, just as the description said, it powered on but did not display. I checked my cables just to see if they were the problem, but my working n64 was playing the game just fine on the same cables.

I thought that it might have just been dirty, so I went to the store and bought some compressed air and Dawn. I got my game bits out, took it apart and cleaned it thoroughly, letting the water dry for 24 hours before reassembling it. When I reassembled it, it still didn't work. I am out of ideas of what I can do. I am wondering if anyone else had this problem and fixed it. I really don't want to spend $20 on a new mother board for it. I want to fix it for my little cousin who has never played a game in his life. Thanks in advance.