fixing up my busted M82 demo unit

davetheshrewdavetheshrew
in Help Wanted!
Hi guys this is mostly just a log so I can keep track on my repairs and to get advice if anyone has any.



So far:



PSU was fried so replaced with a arcade psu, soldered live/ground to underside of the board, first 3 pins closest to the cartridge slot to ground, next 4 pins to live 5v.



It now powers on, black screen , av doesnt even register.



Removed PPU and cpu from UK NES and replaced.



Now I have grey screen





Next steps:



aquire the m82 rom and split to chr/prg and replace the eproms





Possible faults:



Looks like the psu didnt protect the mobo and fried the PPU at least, possibly the eproms also. Everything looks clean, no burns, caps look good.

Comments

  • retrofixesretrofixes
    I knew that PSU would be a grumpy old bastard.

    Sounds like you know what your doing.



    Just idea here. With the UK NES you can make that into a chip tester.

    Place IC sockets on the board. Then you can test and confirm the chips pulled from the M82.

    In-fact I would place sockets anytime you remove a chip from the M82. Will make future repairs easier and less stress on the motherboard.



    With the tester board you'll be certain the M82 PPU is dead and other chips that are shared.
  • davetheshrewdavetheshrew
    Thanks for the reply mate really appreciate it. Yes Ive tested the PPU and its dead totally. I havent re burnt the m82 chr/prg yet because Ive ran out of eproms that size but have a couple on order. Im going to pull them in a minute and do a read on them and see if the dumps jive well with the rom I found on the net.



    Im actually really quite worried that the asic is fried also but have no means to test here, if so then Im in real trouble. reason I think this is because I get no lights whatsoever on the front panel, voltage is going down the line to it but no response, its possible that simply the lcd is dead and if so thats an easy fix but I cant be sure.



    Really glad I got it from black screen no response to grey screen, it means Im on the right track. Im not letting this slide, it will work again.
  • davetheshrewdavetheshrew
    Update: I cant read the prg or chr chips, one is standard amd 27c64 and the other is 27c128, neither can be read or blank checked, chip id is ff x ff on one (maybe blank) and the other was 00 x 80 ( no idea but wouldnt go further). I assume there is zero protection or pin swapping on these as they are bog standard eproms. Does anyone have any info? There is a dump online of the m82 rom so someone has succesfully done it. Im starting to think these two chips have been zapped by a surge or something as the ppu was dead so its likely these are dead aswell since they are linked, unless someone knows better
  • AaronEAaronE
    AM27C64 should be Chip ID 01 15

    If the other chip is also AMD AM27C128 it should be 01 16
  • davetheshrewdavetheshrew

    Originally posted by: AaronE



    AM27C64 should be Chip ID 01 15

    If the other chip is also AMD AM27C128 it should be 01 16





    yeap I confirmed this with one 27c64 I have lying around, brilliant info thankyou very much indeed. These chips are screwed , Im actually happy about that because I know what to do now



    Another thing Ive done. The lcd panel is borked, the two sn74ls75n  chips get roasting hot within seconds, I get no display on the octo display at all but votage goes through it, so Ive ordered new chips and a new display for it, lets see what happens.



    Ive also ordered 2 new sram chips, 8 x sn74als245an chips, 2 x sn74ls47n , 2 x sn74ls75n and corresponding sockets for all of them. This whole mobo will be socketed, everything replaceable withj brand new chips. Big job but these units need full restoration, go big or go home.
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