Sharp X68000 VRAM Repair

My X68000 (CZ-600C) was failing VRAM checks for Text RAM, and there wasn’t much info available about that online, so I’m writing some up here.

TL;DR information that is good to know:

• On CZ-600C, IC1-IC16 is Text VRAM. IC17-IC32 is Graphics VRAM. All 32 of these RAM chips are on the Graphics sub-board.
• The RAM IC’s used are MB81461-12 (32 of them).
• Addressing for these can be found on the X68000 schematic. This will help narrow down which one is faulty, if you know the bad adresses.
• Donor RAM IC’s can be found in Mega Drive units (two per unit).

Debugging the issue

Running MEMTEST (philly 1993) showed this error for TEXT MEMORY.

For more indepth understanding of the errors I ran Memtest68k which showed the range 00e725c – 00e725fc being bad, with Bits in Error Mask: 0000f000.

Looking at the technical guide for X68000 available here , this means that the error is in the last page of Text RAM, and the problematic bits are the highest 4 adresses. Looking at the schematic available on that page, this looked to be either IC16 or IC32 (turns out later it was IC16).

Addressing for Text and Graphics VRAM is slightly different, but the lower numbered IC’s map to the lower address ranges (as expected), and the data bus numbers for the pins in the schematic can help narrow down where the issues is.

To verify if it was IC16, I desoldered pin1 and pin2 on it, and wiggled them a bit while running memory checking. This produced more errors in the page of the already faulty range, which verified that it was indeed this IC that had issues. The main reason for doing this verification was that I was not sure if the first or last 16 RAM chips were for the Text part of VRAM.

After this, I was sure which part was bad.

Fixing the issue

Fixing the issue at this point was pretty straight forward, since it just meant swapping the bad part. Sourcing these are a bit awkward, but there are some available at aliexpress. I didn’t feel like waiting though, so I opened up my Mega Drive and grabbed the video RAM from that, since it uses the same IC’s. I’ll replace that later with the aliexpress ones, whenever they show up.

For desoldering these, you NEED a proper desoldering station/gun. I use a HAKKO FR301-22, but you can use cheaper ones as well. Overall, extracting this type of package is more annoying that DIP packages, but it worked out fine, with not messed up pads.

Next step is removing graphics sub-board and RAM chip from the X68000. Then just solder the Mega Drive RAM in the place of the bad RAM.

And there we go, memory test works now! All is good.