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600XL Memory Test Issue

hardware memory 600XL rom

28 replies to this topic

#26 shoestring OFFLINE  

shoestring

    Dragonstomper

  • 546 posts
  • Location:Sydney

Posted Tue Dec 25, 2018 6:50 PM

If you install the 4464s as is, the machine will still see the 16kb.

Check the 138 demux as well, I don’t have a schem infront of me right now but I believe that handles the output enables to the roms.

#27 asommer OFFLINE  

asommer

    Space Invader

  • Topic Starter
  • 16 posts
  • Livin' the Dream!
  • Location:Boone, NC

Posted Tue Dec 25, 2018 10:21 PM

I switched out the memory and the same ROM test failure occurred,  I've ordered some 74LS138 replacements, so I'll report back when they get here.

 

Thanks again



#28 Nezgar ONLINE  

Nezgar

    Stargunner

  • 1,577 posts
  • Location:Regina SK Canada

Posted Wed Dec 26, 2018 1:35 PM

I have the 64k upgrade kit, would you recommend doing that?


You can swap in the 2 new RAM chips from your 64K kit to test, they will just function as the same 16KB RAM until you complete the rest of the upgrade. Good for a test.

#29 asommer OFFLINE  

asommer

    Space Invader

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  • Livin' the Dream!
  • Location:Boone, NC

Posted Tue Jan 15, 2019 9:24 AM

I've replaced the 138 demux with a new 74LS138 and have the same issue with the ROM test.  When the machine works (usually when it's cold and only for a few minutes) the CE pin on the ROM has 0.9 volts (low) and when the ROM test happens the voltage is 1.5v.  The same can be seen on the OS pin of the MMU. 

 

Not sure what to try next?

 

Thinking to get a new MMU from Best Electronics cause they're pretty inexpensive and either way it may be nice to have a spare.  Also, the traces between the MMU and ROM look good and test fine for continuity.







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