9 Jul
2011
9 Jul
'11
1:13 p.m.
Looks like the +12V startup and -12V (actually -15)
are fine. I'm
Have you checked these with a 'scope? Excessive ripple on a supply line
could cause exactly the fault you are seeing.
Hong one one more second.. When you checked the signals on the backplane,
what, if any,boartds did you have in the amchine? The point being that
the AC L and DC L signals are open-drain (essentially the same as open
collectror) signals and need a pull-up somwehre. With the PSU running
correctly, the otuptu transistorw will be turned off, so if there's
nothing elase connected to them they will flat, and a 'scope will see
mains pickup...
pretty sure the fault is on the power monitor board.
Op-amp E1 (LM339) has +5V @ the + input. The - input I believe is AC
E1 is a quad comparatior IC. It's a bit like an op-amp, but with open
collector output.
derived from the HF signals on the main board (pin J
on the sub board).
This signal seems to be approx 32KHz and is 14V p-p at E1 pin 8 and 16V
p-p at pin 6. The curious thing (I'm learning as I go!) is that I'd
That sounds right.
expect the outputs (pins 1 & 14) to oscillate
between -15 and +12 but
they're both stuck at -15. I tried replacing the LM339 (E1) and get the
Without anything to pull them up, there will be no change at the output
of the comparator. The pull-ups are actually R15 and R18. This threw me
for a bit, remember that E1 runs between the +12V and -15V rails, so
resistor from the output to ground is actually a pull-up wrt the -15V rail.
same result. These signals feed the trigger inputs to
the 555's and it
is these that seem to drive the DC L and AC L signals. My understanding
is the 555's are a one-shot that should be re-triggered regularly but
since the output of the 339 isn't oscillating this isn't working. The
coupling between the 339 and 555 is too complex to get my little brain
around so I can't be sure this is how it's supposed to work mind you!
Waht is the output (pin 3) of each 555 doing? Rememebr this wil lalso
switch between -15V and 0V (llok at the power conenctions to the 555, it
runs from the -15V rail again.
Next step is to fix/replace the scope.
Rhat sort of 'scope is it?
BTW - if I tie the DC OK and PWR OK signals to +5
would you expect the
system to boot? or is the timing of these signals coming up important
somewhere?
Tying the signals hard to +5V is likely to fdamge the output FETs Q2 and
Q3 on the power monitor PCB
I think at least one ofthese dsignals is used as a reset input to the CPU
so it really should only go high after the PSU rails are stable and correct.
-tony