NeXTStation raising exception on boot

Started by m18e, April 27, 2019, 04:16:33 am

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m18e

Hi Andreas!

When all the RAM is out it doesn't boot; there's a diagnostic LED on the motherboard that flashes. Nothing comes up on screen.

The slab has 72-pin SIMMs; each SIMM is a 16-chip 32MB piece. I've tried a number of combinations of them, both in two and four SIMM configurations and nothing seems to work.

Appreciate the input, every little hint helps.

Andreas

Quote from: m18e on July 11, 2019, 03:05:50 amThe slab has 72-pin SIMMs; each SIMM is a 16-chip 32MB piece.

to be clear - you have a turbo slab with 4 slots for RAM instead 8 slots?

krazycat

Quote from: Andreas on July 11, 2019, 05:36:23 amto be clear - you have a turbo slab with 4 slots for RAM instead 8 slots?
as far as I can tell, the turbo mobos only have 4 x 72-pin ram slots...







m18e

Quote from: Andreas on July 11, 2019, 05:36:23 amto be clear - you have a turbo slab with 4 slots for RAM instead 8 slots?

Correct.

barcher174

It seems I have a mono turbo station with the same problem. Recapped, drive swaps, RAM swaps, and cable swaps with no success. I'm convinced there's a board level fault. I'll let you know what I find. The SCSI portion is identical between the color and mono systems.

m18e

Good to hear! (Well, not really :) ) Hopefully we can get these problems licked; I'm really missing playing around on my slab!

barcher174

Mine is working. I think in the end it's a different error though. I had an exception #4 which I confused with your error #3. For me the issue was a shorted pin on the ASIC controlling the SCSI controller which was only visible under a scope. I bent it back and reflowed with success. Here's what I would try in your case:

There's an exceptions section of the 68040 user guide:
https://www.nxp.com/docs/en/reference-manual/MC68040UM.pdf
You want Table 8-1. Exception Vector Assignments

From this we see that the vector number and code match to the table. So it looks like NeXT used this table directly which is good news going forward. The description says:

8.2.2 Address Error Exception
An address error exception occurs when the processor attempts to prefetch an instruction
from an odd address. This includes the case of a conditional branch instruction with an
odd branch offset that is not taken. A prefetch bus cycle is not executed, and the
processor begins exception processing after the currently executing instructions have
completed. If the completion of these instructions generates another exception, the
address error exception is deferred, and the new exception is serviced. After exception
processing for the address error exception commences, the sequence is the same as an
access fault exception, except that the vector number is 3 and the vector offset in the
stack frame refers to the address error vector. The stack frame is generated containing
the address of the instruction that caused the address error and the address itself (A0 is
cleared). If an address error occurs during the exception processing for a bus error,
address error, or reset, a double bus fault occurs.

Looking at the schematic here:
http://www.nextcomputers.org/NeXTfiles/Docs/Hardware/Schematics/Turbo_slab/CSlab33-04.pdf

The 7F543s look like a good place to start. Check for any really hot ones.


m18e

Two of the 74F543's (U14, U23) do seem to be running hotter than the others. Not hot enough to scald but definitely uncomfortable to touch.

Hardware's not my forte; what else should I be looking at as a consequence of the hotter than normal latches? Broken solder joints?

One thing I did notice when I had the motherboard out tonight is that there's something (capacitor?) soldered across a couple CPU pins on the backside of the motherboard... here's a picture: https://i.imgur.com/Z7MUgzb.jpg

Is that normal?

(I straightened out the pins in the top left so they wouldn't come close to touching)

barcher174

The resistor is normal. That blob of solder next to it is not. You should clean that up. Is your Cpu soldered down or socketed?  Those possibly shorted pins are concerning.

m18e

I don't think that blob between the pins was solder, it seemed to be a bit of flux or something. It cleaned up well, but doesn't seem to have helped.

The CPU is socketed. I've never removed it; though once this ordeal is over I might pop the heatsink off to re-apply thermal paste :)

barcher174

If you have an extra you may want to try swapping out the cpu.