Macintosh II Schematics

[MaCJaX]

Hey all !! First post here. Was wondering if someone can send me or point me in the right direction for getting Macintosh II logic board schematics. Not the Mac Classic II or LC but the original Macintosh II board schematics part number 820-0163-B. I have a board that is MINT condition and I am dying to get it working as it wont boot. No chime of death, nothing on power up. ICs at location UG and UH get pretty hot and it seems like I am lacking a clock on most pins. I really need a schematic to troubleshoot more but google searches turn up empty.

Thanks !!!

MaCJaX

 

[chiaki]

Maybe this can help you?

View attachment Macintosh_II_Schematic_(bomarc).pdf
 

Explanation of the Power logic generally:
https://www.downtowndougbrown.com/2015/03/explanation-of-the-macintosh-iiiix-power-onoff-circuit/

Somewhere i should have a copy of the PSU schematics too, if you need this?

 

[Bolle]

Macintosh_II_Schematic_(bomarc).pdf

Nope, that's the newer revision of the Mac II logicboard.

As far as I know a schematic for the older revision is not available.

ICs at location UG and UH get pretty hot

That's normal.

 

[MaCJaX]

Thanks guys !! I actually have both revisions of the boards. They are pretty similar so this schematic should help. I really appreciate it guys ! As far as UG and UH getting hot, they get so hot I can't even touch them. I gonna fire up my working board to compare it with. 

Thanks again !!

 

[Bolle]

Everything is pretty much the same except that the later revision boards use the NuCHIP integrated Nubus controller while the early boards have all the Nubus logic implemented in discrete PALs.

General big box Mac II troubleshooting starts with replacing caps (shouldn't be an issue for the early Mac II boards, they should not have leaky SMD electrolytics installed on them)

Then see if you can jump start the board - again powerup circuit seems to be working enough so the power supply at least starts up in your case.

Head on and check the voltage rails for 5V and 12V. The Sony soundchips monitor the 5V line and keep the machine in reset when that's too low. Next check the state of the reset line... notice a pop from the speaker or headphones when hitting the reset switch?

Once resets are verified to work check if the CPU clock signal is present. If that's there check for activity on the address and data lines right after resetting the machine.

This should give a few hints already what might be wrong with it.