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. If someone has a motherboard out in the open a test to see if grounding pin 2 of UJ18 starts things up would be interesting. <ftp://ftp.macnauchtan.com/Circuits/macIIfx_startup.GIF> I also note that the collector of Q3 also drives pin 87 of the NuBus. That makes it possible for a failing or marginal NuBus card to interfere with startup. If C9, 47 µF, were not there or had a seriously lower capacitance the rear panel startup might not work but the keyboard switch should be fine. When the rear switch is activated, and 5 volt power is on, pin 1 of UI18 is pulled low. That turns Q2 on and pulls the power connector pin 15 down, The machine shuts down. The flip-flop action of the cross coupled gates in U18 forces the power supply to the off state until its filter capacitors discharge. That way the startup side of the rear switch is kept non-functional. C61 and C62 are radio frequency noise suppression devices and are unlikely to be a problem. Note that UJ18 derives its operating power only from the lithium cells while UI18 gets its power from the normal power supply. UC12 handles the ADB bus. R66 biases the data line high so that keyboard and mouse need only ground the data line for signalling. R65 and Q1 allow UC12 to send data back to the keyboard. Those parts have nothing to do with startup. I can think of much better, and simpler, ways to do it all. It makes me wonder if all IIFXs are the same. And no. I don't know what a "Microdarad" is. -- --> From the U S of A, the only socialist country that refuses to admit it. <-- -- ----- You received this message because you are a member of the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/vintage-macs Support for older Macs: http://lowendmac.com/services/
