If it's the original masked rom (no window/sticker) just replace it with a burned eprom and change the jumpers. Most people just change the jumpers on their -35 bally boards to take 2x2732's in u2 and u6.

If you peel the sticker off and it has a little window on it, it's just an eprom and can be replaced with another eprom, which lots of people can burn.

I'm saying that a lot of people just convert all their boards of that era to use the same 2x2732 eproms - the one in u2 replaces both u1 and u2, and the one in u6 replaces both u5 and u6. If you really wanted to you could have eproms in u1, u2, and u6 on your machine. What the first flash not happening is telling you is that one of the rom chips (whether it be a masked rom or an eprom.... the software can't tell) is failing its checksum test. It might be the u6 chip or it could be either of the u1 or u2 chips. It tries to test all 3.... in all probability u6 might be ok since that's where the bootup code actually resides to turn OFF the led when you turn the machine on.

This is also why you should just either convert to 2x2732 and order 2 replacement eproms that way or order all 3 if you want to stay original (assuming all 3 are already eproms... this way you don't have to change any jumpers on your board)