I picked up this accelerator in a somewhat roundabout way. Months ago at the Electronics Flea Market in Saratoga, I passed a pickup truck with a couple Macs for sale. Both were facing away from me and neither were in good cosmetic shape, so I didn't think much of it at the time. Later I met a gentleman who bought a Plus from that truck and I noticed a "Brainstorm" sticker on it. I offered to trade his newly-purchased Mac in unknown condition for my known-good Plus, and we exchanged contact info.

Long story short, we swapped logic boards and indeed, he got a working Mac out of the deal. On the other hand, my Plus, though now equipped with a Brainstorm kit, no longer booted.

My Brainstorm Plus chirps out the signature "boot beep" at startup, but then during POST immediately produces a Sad Mac with error code 0101C3. The leading "01" signifies which test failed; in my case a ROM checksum failure, suggesting some kind of ROM corruption. The remaining digits are largely useless, based on the (incorrect) result of the checksum algorithm.

Armed with my rudimentary electronics skills, I cracked open my Plus to investigate. Here is where I noted a few details about how the Brainstorm works.

As I mentioned, the main board of the Brainstorm clips to a socket that straddles the original 68000 CPU. A 56ohm resistor connects the R1 pad of the Brainstorm board to the near lead of the ferrite bead FB1 on the Plus logic board.

Elsewhere on the Mac Plus logic board, the TSM-- a PAL16R4A-- is replaced by its unlabeled Brainstorm equivalent. This chip controls the frequency of DRAM refresh and its timing is coupled to that of the CPU. (I tried running my Plus with this replacement TSM but without the Brainstorm board, and while the Plus did attempt to start, its chime was razzy and the video display was unrecognizably scrambled.)

With the Brainstorm installed, each of the Plus's two ROM chips has a blue... support?... board underneath it sharing pins 1, 14, 15, and 28. I'm not sure what the purpose of these boards is-- none of their pins have any continuity, and removing them has no effect on the Sad Mac I get.

The main Brainstorm board itself consists of four ICs: a Toshiba TMP68HC000P16 CPU, a custom chip next to the 68K labeled "ASX-6" copyright Brainstorm, and on the underside two unlabeled surface-mount chips.

I beeped out the two chips on the underside and I strongly suspect they are ROMs, possibly even EEPROMs, based on the pinout. They seem to match the pinout of the 27C512 chips used by the Mac Plus ROMs, albeit in a SOP-28 package. The first troubleshooting step I'd like to try is to dump the contents of these ROMs, but alas I don't have anything that fits an SOP-28 footprint ( @tubetime ?). Likewise I'd like to dump the Plus ROMs just to verify their expected contents.

@blitter a minipro should be able to handle the dumping process, you'd just need the socket adapter. very interesting project you've got!

@tubetime I don’t suppose you already have one? That maybe you could please bring to T5 this coming Friday? 😉 At least for the DIP-28 chips which shouldn’t need an adapter.