NMS8280 vwry strange basic math division or multiplication error (defect ??? ) --> OVERFLOW

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By marcoexo

Expert (83)

Аватар пользователя marcoexo

22-12-2017, 13:07

Ok, it turned out to be the defective Basic mask rom, replaced with 32K EPROM and now it's OK, all tests involving basic calculation now pass correctly including #10 monitor test that hanged with 'overflow' on circle drawing...

Just one alert for everyone who attempts to repair the '8280 beast'... In the attempt of replacing the eprom I had to remove the 15 screws holding the metal protection and video board... once replaced the eprom minutes of panic... 'Black screen!'... I checked cables and set up the scope to check signals on board, before revealing that I inadvertently pushed the small switch at the center of the board, which disables the rom for testing purposes Shocked!

Thanks everybody for helping solve this new repair ! B-)

By l_oliveira

Hero (534)

Аватар пользователя l_oliveira

22-12-2017, 13:12

Your ROM have "bit rot". Sometimes silicon devices rot inside and silicon die connections get undone. May cause it to completely stop functioning or happen to get that kind of glitch.

Your ROM chip seems to have bits D7, D6 and D5 zeroed out for the area which the bytes mismatched.

By gdx

Enlighted (6221)

Аватар пользователя gdx

22-12-2017, 15:04

Congrats on the repair!

By marcoexo

Expert (83)

Аватар пользователя marcoexo

22-12-2017, 18:53

Thanks, and happy 1987 to everybody here ;-)

(Retro-Wishes from Italy Wink )

By hamlet

Scribe (4106)

Аватар пользователя hamlet

22-12-2017, 21:53


By Wierzbowsky

Guardian (3608)

Аватар пользователя Wierzbowsky

23-12-2017, 15:22

Not a good sign that masked EPROMs started to fail. Now we have VDP, EPROM, caps and RAM problems to check when repairing the computers. But good that one more MSX has been brought back to life.

By Grauw

Ascended (10772)

Аватар пользователя Grauw

23-12-2017, 15:43

Yeah, I’ve read here about maskrom failures before…

By Louthrax

Prophet (2465)

Аватар пользователя Louthrax

23-12-2017, 16:26

Hi Marcoexco,

So I opened a VG 8250 a few months ago to fix the audio, and it did not restart after that (black screen). I've not checked yet but I suspect I also pushed accidentally the ROM disabling switch (was not aware of that thing)! Will give that a try, probably just after XMas.

Also, I've also already seen several arcade PCBs with corrupted maskROMs (but never on MSX yet...).

By marcoexo

Expert (83)

Аватар пользователя marcoexo

24-12-2017, 01:44

Yes, this is my second time of a masked rom failure, first one was a video characters rom from an Apple II, but I believe this would be a secondary and solvable problem, primary problem is the failure of a gate array, which is unfortunately an unrecoverable matter :O
Edit : Louthrax , yes I think that the switch is an unbelievable design mistake, it's so easy to have it pushed that I wouldn't be suprised if black screen problems could arise by the simple pression provided by a shake during transport

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