No address/data bus activity. Is there a dead CPU in my Mac II?

[davidg5678]

Hello all,

I have a tricky Macintosh II that I am trying to repair. This machine has been recapped, and the PSU's voltage rails are good.

The issue is that there appear to be no signs of life on the address or data busses when I probe them with my oscilloscope…

In an attempt to troubleshoot this issue, I have pulled the video card, RAM, co-processor, and even the four ROM chips. No matter what I do, when I probe the bus pins on the 68020, it seems they are all either high or tri-state.

I have verified that the /RESET line on the sound chips is pulled high when the computer is on, and pressing the reset button makes this line go low. I have also verified that there is a ~16MHz clock signal going into the CPU.

I noticed that the 74LS651 NuBus transceiver chips were all getting extremely hot when this board was powered up. I tried pulling all of these chips, but after probing the bottom of the 68020, there is still no bus activity.

I suspect that the 68020 CPU itself might be a dead chip, or that there is a short circuit on the bus somewhere causing these issues. Maybe the GLU or UG13 could be dead and making the CPU seem dead? If there is some kind of short on the address or data bus to a voltage rail, I don’t know what the procedure is for finding it; any pointers would be appreciated!

If I understand correctly, I believe CPU is probably entering some sort of /HALT state when it can’t see the ROM. Probably pulling all of the 74LS651 chips is not helping with this, but admittedly, this is where my understanding of the machine’s architecture starts to get fuzzy.

This Mac II has me stumped! If any of you have ideas for further troubleshooting, I’d be very grateful! Thanks!

 

[Argyle Gargoyle]

Those bus drivers getting hot is a clue.You should look for a short on the lines that the hot 651’s were trying to drive. You could use a DMM to check for continuity to ground on those bus outputs as a first check, although they wouldn’t necessarily have to be shorted to ground. When you power the board up next, run your hands over all the chips to find any that are too hot (take off your ring first if you wear one).

 

[Addicted]

I had an LC II recently that did not seem to be accessing its ROMs. I studied the 68030 bus protocol and was able to identify a single line between the `030 and the bus ( /AS ) that was open, a broken trace right at the CPU. Repairing that solved everything.

I am far from an expert on this, and there are others here who are .. and I have no experience with the II, but some experience with the IIcx. I'll venture that if you probe the various bus control signals from the `020 at the GLU on your system and compare the findings to the bus protocol, perhaps you can rule the CPU in or out as the cause?