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Macintosh II Nubus Sadness

Hello,

I posted this in the Lounge yesterday but today realized this is probably a much better location. I apologize for duplicate posting.

I've got a Mac II that I'm having trouble with. It was working fine when I put it in storage in about 1990. I recently had it re-capped and had new batteries installed. I also had all the plastic simm sockets replaced with ones with metal clips. The power supply has been re-capped and tested. I've probably got $500 in it at this point.

The machine seems to boot fine, but nubus cards do not work. That means there is no video. I'm no expert, but I think that makes the computer kind of hard to use.

I've sent it to a really competent troubleshooter. This is what he had to say:

"
I think I've made some progress, but hit another roadblock.

I've been comparing the signals on the Nubus slots to my Mac II cx. On the IIcx (with the video card installed) there's activity on all 32 address/data lines as you would expect. On yours, there's absolutely no activity while the computer is running. This tells me it's not detecting the video card.

Just after reset on the IIcx, there's a burst of data on all 32 address/data lines, even with no video card installed. I assume this is when it's polling the bus to see if there are any installed cards.

On yours, I see the same thing, except on AD21, 21,22, and 23. Those are just flat line. This seems very suspicious, but not certain without another identical board to compare to.

I've ruled out the chips that drive the address/data lines by swapping the suspect ones into other locations. The next step up the line gets more complicated.

The schematic I have is for the board 820-0228-A, which is a revised motherboard. Your Mac II has 820-0163-03. Unfortunately, the circuit that drives the Nubus slots is very different.

It looks like I'll need the correct schematic to make any more progress. I haven't found a schematic for this board. Even with one, there's a high likelihood I'll run into a custom Apple chip that I can't conclusively test, or find a replacement for.
"

I'm wondering if anyone out there knows of schematics for the newer version of the II, or if anyone has any tips or suggestions.

Thanks in advance for any leads.

Signed,
Sad Mac Owner
 

LaPorta

Well-known member
Was there any cap/battery leakage on the board? It is usually certain that some caps leaked past where they were placed and may have interrupted nearby data lines. Let's see if there is any other schematic that we could come up with...
 
Was there any cap/battery leakage on the board? It is usually certain that some caps leaked past where they were placed and may have interrupted nearby data lines. Let's see if there is any other schematic that we could come up with...
Hello well-known member,

Thanks for the reply. There was no noticeable leakage prior to re-capping. I dug through the archives and found a thread where someone had a bad trace near C7. I emailed it to the guy who is working on the board. No response yet. I really hope I can find schematics for him.

OH CRUD, I misread his email. He has schematics for the NEWER version but my board is the OLDER version. So schematics for the older version are needed. Sorry for my lack of attention to detail.

Thanks Again.
 
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LaPorta

Well-known member
Yes, it would appear that your board is indeed older by the number. The widely-available BOMARC schematics are for the later board. It is possible someone else here will know of a source for the earlier revision. Either way, I would think that on the underside of the board, the first thing to do would be to buzz each of those lines that are not initialized out from the NuBus slots to wherever they head to. If all of those are intact, then it could be issues between the chips that control those lines.

Do you have photos of your board? Being able to see what you are working with is one of the most helpful things for those who are experienced with this to work with on this forum.
 
Yes, it would appear that your board is indeed older by the number. The widely-available BOMARC schematics are for the later board. It is possible someone else here will know of a source for the earlier revision. Either way, I would think that on the underside of the board, the first thing to do would be to buzz each of those lines that are not initialized out from the NuBus slots to wherever they head to. If all of those are intact, then it could be issues between the chips that control those lines.

Do you have photos of your board? Being able to see what you are working with is one of the most helpful things for those who are experienced with this to work with on this forum.

Thank you for your help. I appreciate it very much. I will ask for pictures and post them here. I'm in IL and the troubleshooter is in TX.
 

demik

Well-known member
Do you have a picture of your board ?

First Mac II didn't have a NuChip, it was a series of PALs. Check the chip where the lines 21-23 go. IT may be a simple 74 buffer which will be easy to replace.
 

beachycove

Well-known member
It’s packed away in storage so no pics, alas. I recently completed a cross-country move and and dealing with having to sot through more important crap for the present! Getting a round tuit involves getting through the more pressing bits and pieces first. We want a house free of boxes!

I am, however, grateful for the advice, and will follow it up when that tuit comes around.
 
Here are photos of my sad mac:DSC_1968.jpg
 

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Melkhior

Well-known member
@happymacunhappymac Indeed as @demik mentioned this seem to be an original, PAL-based Mac II and not the NuChip-based revision that Bomarc did the schematic for (the replacement circuit is zoomed on in photo 1971). The schematics and content of the (NuChip-replaced) circuit on your board would be very useful to know, as it could help recreate NuBus controller circuitry with modern components. And there's probably not many of those boards left. Hopefully, the content of the PAL is recoverable - and if not it also probably means the machine is dead unless a set of PALs from another one can be duplicated.
Thinking out loud, probably way off the marks; if there's never anything on some of the AD lines but the others work, that means either
* both the A and D drivers are dead on the line [or A dead and the D driver is never enabled possible if only a probe is issued ]-> change or swap parts (seem to have been tested already)
* there some issues with the trace between drivers and slots -> continuity testing between the outputs of the 74ALS651 and the slots (but which 74ALS651 ? if apple kept the position UE7 would be U64 on your board, but the routing is completely different, on the newer board most traces are on the top layer); each AD line from the slots should reach 2 drivers (one for A, one for D) ; also -> check that those traces are not grounded
* some of the control logic is broken -> logic analyzer to see if the initial probe make sense NuBus-wise (but it would affect all lines from one or more drivers rather than a subset, so it's likely not that)
Maybe some of the board re-creators can offer more informed suggestions?
 

bdurbrow

Well-known member
Just thinking aloud...

Generally speaking, it should be possible, working from first-principles, with the documentation that's available online (such as the above linked document, the 68020 datasheets, etc) to design a work-alike module to replace both the PALs and the NuChip... it would, of course, be a Project with a capital P, but suitable CPLDs and instant-on FPGAs are available (er, well, they were before the global chip shortage struck... they're often-as-not on backorder now).

:unsure:
 

Trash80toHP_Mini

NIGHT STALKER
I think the first chapter outlines Macintosh II NuBus hardware as well as anything I remember seeing offhand. Not sure, but highly expect that a schematic for the first edition Macintosh II never walked off campus. There might be enough in DCaDftMIIaSE for someone to noodle out what's going on.

No expert here, but the first thing I'd check would be to trace/test continuity on the six discrete NuBus Slot ID lines and the rest of the signals bulleted for that section of the system? That'll give him a feel for the setup, if he wants. If those traces seem like they should be in working order it'd be time to track down where those three address lines are borken.

You could also jump right into the addressing issue. Does activity on those lines drop off on the system bus side or the NuBus side of the transceivers? That'd be my guess and hope at where the problem lies. Are the transceiver ICs stock parts as ISTR?

MacII-NuBus-pre-NuChip-00.JPG

I've always wanted the first rev Macintosh II board documented in that tome just to get a feel for the way NuBus was first implemented. With board and that book in hand, a working system for comparison he just might noodle out the schematic for that subsystem.

@nickpunt thanks for the cc on this one, it saved me a bit of drudgery by making this a good deed. ;) I wanted to check that PDF to see if the first rev II had a forerunner of ASC as it does of NuChip, but right there it sits. Does anyone know if ASC is custom Apple silicon or their named version a standard available part?
 
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