On Jan 26, 2010, at 8:00 PM, Doug McNutt wrote:
At 18:29 -0800 1/26/10, Nat Hall wrote:
Keep in mind the IIfx won't power on from the "soft" power key on
the keyboard (at least, mine doesn't). I have to use the "hard"
power button on the back.
UJ18 is a quad Schmitt trigger chip. It's wired so that a low
voltage on its pin 2 pulls the base of PNP transistor Q3 to ground.
That turns on Q3 allowing the serial voltage of the two 3.5 volt
lithium cells to deliver positive voltage through two diodes D5
and D6 to power supply pin 15 which turns power on.
The keyboard switch pulls UJ18 pin 2 to about 0,5 volts through a
diode in the keyboard. The rear switch tries to pull pin 2 to
ground by charging C9 which it can do because the +5 volts from the
power supply is not keeping C9 charged through R170 and R5. When
power comes up C9 is charged and the rear switch does nothing to
pin 2. That must be a way to make the rear switch act as both a
start and a stop button.
It's very difficult to see a way that the keyboard switch can fail
to start while the back switch works. A bad diode in the keyboard
or a broken connection between R4, the ADB connector, and the
keyboard return to ground are about all there are.
Interesting, I never knew the keyboard switch was SUPPOSED to turn
the IIfx on. I thought all these years the IIfx was one of the models
that didn't support "keyboard-on."
I'll have to try a different ADB keyboard on my IIfx and see if it
works to turn it on. At least then I will know if it's a keyboard
problem or not.
Nat
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