I recently picked up a Performa 640CD while visiting my sister in Colorado. Pictures showed the Mac side booting fine, and the price was good, and it had the DOS compatible markings on the front!
Was looking for a partner to go with my 6115 with a DOS card, so I picked it up, removed some of the plastic pieces, stowed it in my carry-on bag, and headed back home.
When I got home, I was excited to try it so I made a couple images for my external BlueSCSI (didn't realize the internal drive was an IDE drive), and booted it up. I tried various MacOS versions, various versions of the PC Compatibility software, but every time when I switched to PC, I got a black screen and no boot chime. From reading various posts on 68kmla, reddit, and elsewhere, it sounded like it was most likely due to a bad CPU. So I ordered a new Cyrix Cx486 DX2/66 off eBay. While waiting for that, I took apart the machine, glued some plastic bits back on after they shattered during disassembly, and gave it a good cleaning. I also replaced the electrolytic on the logic board.
Today, the new CPU arrived and I put it on the board, gave it a fresh blob of thermal past, attached the heat sink, and booted it up. On booting and pulling up the PC Setup control panel, I realized I forgot to add the memory back to the board. I was worried this would cause a problem because I had read somewhere (I cannot find it now) that one of the Houdini cards wouldn't work without dedicated memory. But, I was already booted so I thought I'd try as I hadn't gotten anything to work so far. So I set it to use 8MB of the Mac's memory and hit the "Start PC" button. And it booted! The new CPU worked!
But, I figured dedicated memory is better, so I shut it all down and pulled the logic board out, added the dedicated SIMM to the DOS card and started it up again. When I want to switch to the PC side, as you likely have guessed, it would no longer boot. I tried with a couple different SIMMs I had, and achieved the same result each time. So, after swapping the CPU, it is very likely my problem was never the CPU and was the memory.
So, lesson here is if your DOS compatibility on your old Mac isn't working, it may be the CPU, but it could also be the RAM. I'm going to try to find a SIMM that is known to work with it, and I may see if the memory from my 6115 is compatible, and find the same one for this card.
But I now have a working 640CD, and a spare Cyrix 486 CPU.
Was looking for a partner to go with my 6115 with a DOS card, so I picked it up, removed some of the plastic pieces, stowed it in my carry-on bag, and headed back home.
When I got home, I was excited to try it so I made a couple images for my external BlueSCSI (didn't realize the internal drive was an IDE drive), and booted it up. I tried various MacOS versions, various versions of the PC Compatibility software, but every time when I switched to PC, I got a black screen and no boot chime. From reading various posts on 68kmla, reddit, and elsewhere, it sounded like it was most likely due to a bad CPU. So I ordered a new Cyrix Cx486 DX2/66 off eBay. While waiting for that, I took apart the machine, glued some plastic bits back on after they shattered during disassembly, and gave it a good cleaning. I also replaced the electrolytic on the logic board.
Today, the new CPU arrived and I put it on the board, gave it a fresh blob of thermal past, attached the heat sink, and booted it up. On booting and pulling up the PC Setup control panel, I realized I forgot to add the memory back to the board. I was worried this would cause a problem because I had read somewhere (I cannot find it now) that one of the Houdini cards wouldn't work without dedicated memory. But, I was already booted so I thought I'd try as I hadn't gotten anything to work so far. So I set it to use 8MB of the Mac's memory and hit the "Start PC" button. And it booted! The new CPU worked!
But, I figured dedicated memory is better, so I shut it all down and pulled the logic board out, added the dedicated SIMM to the DOS card and started it up again. When I want to switch to the PC side, as you likely have guessed, it would no longer boot. I tried with a couple different SIMMs I had, and achieved the same result each time. So, after swapping the CPU, it is very likely my problem was never the CPU and was the memory.
So, lesson here is if your DOS compatibility on your old Mac isn't working, it may be the CPU, but it could also be the RAM. I'm going to try to find a SIMM that is known to work with it, and I may see if the memory from my 6115 is compatible, and find the same one for this card.
But I now have a working 640CD, and a spare Cyrix 486 CPU.
