Atari 520 STFM - Issue 4x Multi TOS Upgrade

Started by DigitalSkunk, 13-04-2025, 21:59:14

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DigitalSkunk

Hey guys, I am probably overlooking something silly, but I am a bit stuck.

  • I got Petari's awesome TOS Improved 2 versions, and Atari's original TOS 1.04, 2.06
  • On 2x 27c4001 EEPROMs
  • A17 and A18 (Pin 30, 31) are TOS toggles. This seems to be working. Switch off has both pins at 5v. I used 33ohm resistors I had laying around, is this high enough for protection?
  • With both switches grounded, A17 and A18 low, it successfully boots TOS 1.04 original.

I can't get the other three versions to boot, and am not sure what to check next.

I have a strange revision of 520 STFM, ROMs are under the PSU, 2 chip version. So, to be less cramped and reduce the chance of ROMs, GAL shorting to the bottom of the PSU shield, I actually installed the GAL on top of the 68k, and ran a fairly long CE1 line from GAL 15 pin to Pin 22 of new ROMs.

  • Pin 24 on new ROMs are pulled up from the socket, connected to GND on mono. (OE)
  • Under pin 24, I ran A16 to Pin 2 of new ROMs.
  • GAL pin 15 CE1 is running to Pin 22 of new ROMs. This pin is inserted in the socket still.
  • When I unsoldered the CE blob jumper on the board, TOS 1.04 original quit working until I soldered it back on 1M pad.
  • A16 and A17 pads are still soldered on 1M like stock.

I hope this is enough info to start. Thanks in advance for nay help anyone can offer. I am rather confused at this point. lol
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