15 Jan
2014
15 Jan
'14
7:51 a.m.
On Wed, 15 Jan 2014, tony duell wrote:
Is that connected to what ends up being pin 1 on J2? That would certainly
explain why it measured +12V, then.
This is stumping me. What exactly is the 555 being used for? If I can
figure out what it's doing, I can debug it further and solve my problems.
I assume this is the one shown on (pdf) page 26 in the printset on bitsavers.
From what I can see, it's a monostable (R22 and C19 are the timing components) to
delay the power-OK signal
(produce by the comparators in U2). It appears that when the +5V, +12V and -12V lines are
all high enough, the
trigger input (pin 2) of that 555 goes high. A little later the output (pin 3) goes low,
causing 'PLB4 DC OK H' to go
high and taking the CPU out of the reset state (see page 43 of the printset).
I would start by looking at the trigger input of that
555. If it's low, then one of the comparators thinks that one
of the supply lines is too low. Is it?
I'd need to check that tomorrow. I'm not sure which would be the trigger
input looking at the board itself.
Could the CPU being stuck in reset explain why only the number "4" is
displayed on the display and sometimes changes position? (Along with a
couple other lights)
-tony
--
Cory Smelosky
http://gewt.net Personal stuff
http://gimme-sympathy.org Projects