Macintosh 128k w/ 4MB & SCSI expansion - flaky

[polpo]

I was recently given a Macintosh 128k by an old friend and it’s definitely not a normal one. The first clue it was special was a DB25 connector on the back that looked like a SCSI connector. Opening up revealed that it has a Mac Rescue expansion board with SCSI, populated with 4MB of RAM. The expansion board is plugged into a socket that has been soldered onto the sides of the Mac’s original 68000 CPU, and then the expansion has its own 68000. It looks like the ROMs have been upgraded as well, probably to support the extra RAM and SCSI.

Unfortunately the machine is pretty flaky - I’ve only gotten it to boot once. Otherwise it starts booting with a Happy Mac in the middle of a black screen and stops after reading the floppy drive for a while.

Any hints for how to troubleshoot this? What would a happy Mac on a black background mean? I’m willing to try prying the expansion off to see if it’s the culprit, but the way it’s attached worries me that I’ll do damage if I try to remove it.

 

[Stephen]

maybe try booting with no expansion RAM and the. Half of the RAM - and switch the sticks for the other half. You may need to change jumper settings to support none / half ram.

edit: I’m on mobile, about to head out so my message is a bit short -

 

[mg.man]

Wow... how random! I recently won an eBay auction for some original documentation for a Dove MacSnap. I don't have one those (yet?), but for $10, I thought it'd be nice to have the manual - just in case. To my surprise, folded and inside the MacSnap manual, I found the installation guide for a Mac Rescue, which, until then, I'd never heard of! I've attached a scan of what I have here.

I don't think it'll help with your 'flakiness'... but at least you know what the switches do. It does look like it's set up correctly for 4M. And, from the documentation, it looks like it originally used a Killy Clip. My guess is the Killy Clip failed (crumbled?) over time and at some point was replaced with the soldered-on header arrangement you now have.

If you can't get the Mac Rescue to work reliably, I'd be happy to take it off your hands...

Good luck!

 

[polpo]

Wow! What luck to find the Mac Rescue manual! Thank you so much for scanning and uploading it. It’s good to know that it is configured correctly for the 4MB that it has onboard.

So I went ahead and took out the SIMMs and while I didn’t see any corrosion, I cleaned the contacts with a fiberglass brush and applied some contact cleaner to the sockets. And guess what, it now boots perfectly reliably, at least from floppy! So all it took was some light cleaning.

My next challenge is figuring out why the SCSI isn’t working. There are no signs of life from a BlueSCSI, even if I power it externally.

 

[mg.man]

What luck to find the Mac Rescue manual!

And how random that you found one. I only picked up the MacSnap stuff a month or two ago. Glad it helped and glad you got it running.

 

[Patrick]

So is it also an accelerator ? (is that why it has its own 68000 cpu?)

 

[lilliputian]

So is it also an accelerator ? (is that why it has its own 68000 cpu?)

It appears to be an 8Mhz 68HC000, which, from what I've read, is more power-efficient, but not any faster.

 

[polpo]

As for why is has its own 68000 instead of using the original, my theory after looking closely at the board is that it allows this board to more cleanly intercept the address and data lines to allow for expanded RAM. Other RAM expansions that I've seen for the 128k involve varying levels of modification to the original board, while this one just snaps on. I'm curious how it effectively disables the Mac's original 68000.

 

[mg.man]

Other RAM expansions that I've seen for the 128k involve varying levels of modification to the original board

...and to confirm that - here's some pics of my 2.0Mb "128k"...

 

[polpo]

So I’m pretty sure the NCR 5380 SCSI chip on this Mac Rescue expansion is bad so I’ll be replacing it. Since I have to take the Mac Rescue off of its socket anyway, it’s interesting to verify that this is really a cleanly reversible expansion. With it removed, the Mac fired right up with 128K of ram. It’s probably been been 30 years since the original 68000 had been used on this machine instead of the one on the Mac Rescue! The date codes on its chips point to it being made in early-mid 1991.

 

[lilliputian]

Fortunately there's someone currently on ebay selling them for a reasonable price, just ordered one for myself, though I haven't installed it yet.

 

[Willj]

I just got a 512K with the MacSnap and SCSI adapter in it. It powers on and all and I see blueSCSI has power as the red light is on. It does not boot from it. I have not looked into this at all so putting it out there see what others know or can say.

 

[retr01]

Hello @polpo!

I noticed in the picture that you posted of your Mac Rescue upgrade board from Computer Care that the DIP switches are as follows:

1 - OFF 2 - ON 3 - ON

​

Joe of Joe's Computer Museum discussed that back in 2015. Although the Mac Rescue may have been a bit different on his Mac 512k, he learned that when the Mac Rescue board has 4 MB of RAM, all three DIP switches should be on.

​

Have you tried that?

 

[Fizzbinn]

Wow... how random! I recently won an eBay auction for some original documentation for a Dove MacSnap. I don't have one those (yet?), but for $10, I thought it'd be nice to have the manual - just in case. To my surprise, folded and inside the MacSnap manual, I found the installation guide for a Mac Rescue, which, until then, I'd never heard of! I've attached a scan of what I have here.

I don't think it'll help with your 'flakiness'... but at least you know what the switches do. It does look like it's set up correctly for 4M. And, from the documentation, it looks like it originally used a Killy Clip. My guess is the Killy Clip failed (crumbled?) over time and at some point was replaced with the soldered-on header arrangement you now have.

If you can't get the Mac Rescue to work reliably, I'd be happy to take it off your hands...

Good luck!

Can you share scans the Dove MacSnap docs? I actually won one a while back on eBay and intend to install it in my 512k but haven't gotten around to it yet. Would love to see what the original docs were like!