Mac Plus 68020 Accelerator

[chrisrueckert]

I'm looking for the manual and any installation guidance for this Accelerator.

The board replaced the 68000 CPU and was soldered directly onto the logic board.

View attachment IMG_2695.jpeg



It came with a separate power supply:View attachment IMG_2698.jpeg

 

[chrisrueckert]

I managed to power the board up and got this screen. Any thoughts what the issue could be? No startup sounds at all. The analog board is known working and was fully recapped. The Mac works without the accelerator.

 

[ArmorAlley]

Is the RAM on the accelerator or on the motherboard of the Plus?
I wonder if you have mismatched speeds somewhere, either RAM that is too slow, a conflict between bus-speeds etc.

 

[lobust]

Hold up!

The part number and the silkscreen style - is that a factory fitted upgrade??

Edit:

This hints at such a prototype...

Folklore.org: Evolution Of A Classic

folklore.org

 

[croissantking]

Is the RAM on the accelerator or on the motherboard of the Plus?
I wonder if you have mismatched speeds somewhere, either RAM that is too slow, a conflict between bus-speeds etc.

It’s on the motherboard.

Hold up!

The part number and the silkscreen style - is that a factory fitted upgrade??

Yeah, super interesting!

 

[David Cook]

The part number and the silkscreen style - is that a factory fitted upgrade??

Wow! Now @chrisrueckert really does need to get it working. A Big Mac prototype!

was soldered directly onto the logic board.

I see the crystal has also been swapped out for a 16 MHz version.

 

[Bolle]

Have you tried it the way it is shown in your photo above? You might have to keep the original 68000 in the socket on the accelerator for it to work.

 

[zigzagjoe]

Yeah, I think you will need the original CPU installed in that socket for the E clock. However, I think it usually will need some software support also (extension/control panel) to patch the rom for 68020 support. Unless perhaps modified ROMs were supplied too?

 

[eharmon]

More pictures and a ROM dump would be interesting!

 

[chrisrueckert]

Here's the logic boar in full view. I bootet it as follows:
- without the 68000

- with the ROMs from a known working Plus.​

- without the ROMs​

Same results as described earlier for each of the two tests scenarios. I now ordered a 68000 CPU to test the board with it too.
View attachment IMG_2695.jpg

 

[joshc]

So this board came without ROMs? My assumption would be that they had custom ROMs for this.

 

[lobust]

This hints at such a prototype...

Folklore.org: Evolution Of A Classic

folklore.org

Wow! Now @chrisrueckert really does need to get it working. A Big Mac prototype!


I see the crystal has also been swapped out for a 16 MHz version.

I googled and found actual pictures of a Big Mac, and it's definitely not that. I should have checked that before I posted!

Also I think the Plus has a 16Mhz clock anyway.

It's the fact that it's soldered to the logic board and that the part numbers are in-series that leads me to think that this is not an aftermarket upgrade...

I could be wrong though, I don't want to get anyone's hopes up!

The Mac works without the accelerator.

Can you clarify this statement? Do you mean this logic board works with the accelerator removed? As in you de-soldered it and installed a stock 68000, installed stock roms, and it worked?

Or do you mean the rest of the machine works with a different logic board?

 

[chrisrueckert]

Thanks @lobust . The Mac Plus works with a known working (other) Logic Board. Sorry for not writing that clearer in my post earlier.

Also, the 68000 CPU of the stock Plus runs on 8MHz.

 

[zigzagjoe]

That 15.6672mhz crystal is normal as that's the pixel clock used for the CRT. Half of that becomes the ~7.83mhz CPU clock. So that accelerator runs at the the same clockspeed as the original CPU.

 

[Realitystorm]

I've started updating my list of accelerators to include photos (when I could find them). I still haven't found anything that matches your photos. The only accelerator I don't have a photo for is the Ryad Mac Engine GT 128K, 512K, Plus (16MHz, 4MB). https://www.savagetaylor.com/2022/06/07/68k-accelerators-fpus-and-other-cpus/

 

[Realitystorm]

Found pictures of the Ryad boards, and it is not a match. So whatever you have I haven't found it listed anywhere yet.

 

[Realitystorm]

Just realized I had misread this thread, I thought this was a 68000 accelerator running at 16Mhz, now I realize it is a 68020 accelerator. I'll let you know if I find a match as I add photos to that section of my list.

 

[chrisrueckert]

Thanks, that would be fantastic.

 

[Realitystorm]

Thanks, that would be fantastic.

So, I've found photos for a lot of the boards, but so far no match. The one board that I haven't found a photo for, but has a similar description is the Spectra MacAccelerator. It is a 12MHz + FPU accelerator with no memory expansion, BUT it says it has an option for a Big Picture monitor connector, which I don't see on your accelerator. But there may have been two versions, one with the connector and one without.

The other option is you have an accelerator that I haven't found listed anywhere yet.

Spectra MacAccelerator 128K, 512K, 512KE, Plus (12MHz, FPU)
by Spectra Micro Development, Tucson AZ (from MW 1990.02)
No MMU, Connector for the Big Picture monitor from E-machines
Reviews: MW 1987.01, MW 1988.02, MW 1988.08

 

[MOS8_030]

Very cool. That's a pretty early pair of chips on that board.