[sdiy] Siel DK80 strange puzzle...

[Bob Weigel]

I know how you all like to hear one of these every once in a while.  
lol.  It's such a great group for getting usually good suggestions on 
how to proceed on 'defies all reason' type issues.

I'm getting a reset every so often.  It's initiated I see by the fact 
that there is no DAC enable signal.  It just sits there.  When it sits 
there it allows an oscillator to run that sends the reset pulse ever so 
often.

The 74LS138 decoder that is supposed to be doing that I've pulled and 
tested. It seems ok in my chip checker.  I see the address lines ...the 
first one only moves when the reset happens. The others have constant 
signal on them and those lines come straight from the cpu.  They aren't 
shared by any other chip on that particular three address lines it 
appears.  The pull ups resistors all measure ok from 5V.  I see no leak 
particular to the one on ground either.  What in tarnation..I tried 
swapping a different cpu..nothing.  The EPROMS verify (now. Initially 
one got stuck in backward by someone due to the sockets being placed in 
backward on these!  If you ever work on one watch for that.  All the 
chips go the same direction...

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