14-03-2017, 11:15 AM
It's not the most obvious circuit. It doesn't help that it's across multiple PDFs that I don't fancy trying to stitch together.
What's the deal with the switches with 3 contacts? Are all 3 shorted when the switch is pressed? I'm guessing that must be the case - it looks like you have to hold either "ALARM ADJ" or "TIME ADJ" for the buttons in the circle to work?
I can't see why the collector of Q801 is at 15V when the base and emitter is at 14.5V. So that suggests that it is lifted up there by "A". I guess a 'scope on the collector might reveal a pulse or similar. Obviously the pin could be both an input and an output, depending on how it's scanning the keyboard. Wonder why they didn't go for simple X-Y scanning? 34 switches, 6 times 6, so 12 pins (earth and +V not needed), and conventional SPST switches. But I guess the IC designer had some other idea...
What's the deal with the switches with 3 contacts? Are all 3 shorted when the switch is pressed? I'm guessing that must be the case - it looks like you have to hold either "ALARM ADJ" or "TIME ADJ" for the buttons in the circle to work?
I can't see why the collector of Q801 is at 15V when the base and emitter is at 14.5V. So that suggests that it is lifted up there by "A". I guess a 'scope on the collector might reveal a pulse or similar. Obviously the pin could be both an input and an output, depending on how it's scanning the keyboard. Wonder why they didn't go for simple X-Y scanning? 34 switches, 6 times 6, so 12 pins (earth and +V not needed), and conventional SPST switches. But I guess the IC designer had some other idea...
