[volt-nuts] 7081 won't boot
David C. Partridge
david.partridge at perdrix.co.uk
Wed Sep 18 05:08:36 EDT 2013
Lst Friday I tried to power up one of my 7081s to do some work.
Problem was it wouldn't boot up. The problem looked to be a fault on the earthy processor board (rubbish on the display scrolling off to the left and flashing front panel LEDS). Swapping the board from another meter did indeed confirm the problem was on the earthy processor or "Brain" board.
I soon established that there was no good $4C00 select signal going to the logic board with the result that the processor was continually being reset.
Remembering my other 7081 with the same symptoms was bad ROMs, and seeing that the only ROM select signals were going to $E000 which is the boot ROM (IC412) and to $6000 which is IC430, I used my two remaining 2764s to re-burn and replace those. I also did a full verify and check sum check on all the ROMS. No luck.
My next thought was that it could be a problem with the 74LS138 decoders and replaced them (with much cussing) with new ones in sockets but that didn't help either.
I then put back the two original ROMs and one by one did a swap test on the others (erasing and burning as I went), again no luck.
The problem could still be bad ROMs of course as these are all old parts.
I couldn't swap ROM sets with the other meter as that one uses 2564s.
My only other observation is that the $4000 signal on pin 13 of IC416 is somewhat strange in that it dips from 5V to about 2.5V for a short time and then pops back up rather than being a solid 5V to 0V signal - almost as if it were a timing artefact:
--- --- instead of --- ---
I thought this was a dodgy LS138, but swapping LS138s did no good.
I also tried swapping the processor and DMA chips between the boards, but that changed nothing.
I feel like I've been banging my head on a brick wall for the last five days, as I'm no further forward at all.
So I think it is time to ask for suggestions ...
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