Major Havoc PCB repair attempts.

jkoolpe

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Hey all!

I'm still fussing with this Major Havoc PCB for a friend. It's totally dead and won't even let me use the self test.

Here's what I've tried so far:

1. Swapped in known working chips for all socketed chips (RAM, ROMs, CPUs, AVG, etc.)
2. Tested all sockets
3. Replaced 2 74LS244 chips at 2P, 2Q
4. Replaced 1 74LS161 chip at 5Q
5. Replaced 1 74LS00 chip at 5P

Note: #4 & #5 were shown in the schematics to be part of the alpha-CPU input so that's why I gave them a try. On my scope, the input to the IRQ pin (pin 4 of the alpha-CPU) is dead so I thought I would try these chips.

Also see lots of hi/lo toggling going on for various pins of the CPU and the aforementioned LS244s as I'm assuming that the program just isn't executing.

Tried disabling the watchdog circuit (no change), and the clock seems to be OK.

I'm still trying to learn about this stuff so any other suggestions would be appreciated.

On the plus side, my friend had given me 2 non-working MH boards, and I got the other one working as the self-test led me to a bad RAM. Replaced it, and it came right up again :) . Wish all board repairs could be that easy...

Jon
 
Check the address/data lines and the enable lines on the program ROMs to see if they are activating.

No enable lines = no reading of program ROMs = address decoding issue. ;)

RJ
 
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