I am having a problem with some chips I got recently. I bought a bunch of OTP M27c020's. The first of these chips worked fine, now the next batch of them don't seem to work at all. I am wiring up the same configuration for two boards and one works and one doesn't. Does anyone have any insight about what types of problems there might be.
I know I am wiring it correctly and don't know what to do!
http://imgur.com/a/6Cch4
If you have a dumper of some sort dump the roms and compare the dump to what it should be. Any patterns or misplaced byte sequences you find are the keys to figuring out where the error is.
Does your programmer verify after writing the ROM? Can you read the ROM after programming it and compare the output to what you gave as the import ROM image? Then you could at least find out if your error is in the wiring or programming.
I use a topmax programmer II and it does verify after the chip is done being programmed.
I don't have a way to read the chip after I program it, or at least know of a way.
I did two boards and both boards are wired the same correct way and one outputs sound and the other only a grey screen. I am so lost!
If you're getting sound out of one, then the PRG may be OK on that one, but the CHR is probably screwed up.
Well if the ROMs were verified then they should be good.
The only things left are the board and it's components, and your wiring.
Are you making the exact same game as the last one?
Like I said, aside from looking everything over closely with a fine tooth comb, the best tool to find the issue you've got is a dumper. If you provided dumps both good and bad I may be able to provide some insight.
Aside from that the only thing you can really do is replace components/boards etc until you narrow down the issue. If it's just grey screen no sound it should be PRG related vice CHR side of the house.
Just a thought, how do you desolder the old ROMs? Depending on your method you could cause a lot of issues here. Overheating with a torch or too much prying with a screwdrive can cause lots off issues with board and component damage.
One thought would be to remove the ROMs and replace the old ones to confirm it for yourself the board is still good. I know it sounds like a PITA but with a cheapo $25 desoldering iron from radioshack it's really not that bad.
What I do it snip off the chip and use a desoldering iron to remove the legs. Then I add the chips and do the rest of the mod.
For one cart, I get a grey screen and nothing else. For a different cart, I get a blue screen and sound, but nothing more.