• Please review our updated Terms and Rules here

Rev 04 Apple II+ - No Clock signal

Something I discovered by accident. I was using my logic probe on B1 and accidentally shorted pins 6 and 7.. and then I got a clock at pin 7 all the way up to the CPU. Still no life otherwise. The clock ran for a while and then shut down. I shorted again, same thing.. runs for several minutes or less and then stops.
This is interesting as B1 pin 6 is the complement of pin 7.

I wouldn't put much into the fact the system didn't start working despite the clock being present. Shorting the two pins together is not how the system was designed to operate and it appears we've identified a problem between B1 pins 5 and 10. This problem could be affecting more than just not having a clock signal on B1 pin 7.
 
Okay my report was incorrect. B1 pins 5 and 10 match, always. Every time you power up it's a different story, but both pins are either high, low or have clock signal. So I'm off track there. What I'm doing now is verifying the links between pins of each IC in the clock circuit connect where they're supposed to. I'm also checking things like power in case one or more chips aren't getting any.
 
Partial success. I'm not sure what changed.. but here's what I did. First, I followed from pin 8 of B2 to every point it went to. Everything checked out, all had the correct signal and were connected. I then started following SOFT 5 from pin 8 of A2, which I think handles the CLR lines. They were low. Since pins 9 and 10 of A2 are tied to ground, the output should be high. To check whether it was the chip or something else pulling the line low, I bent out pin 8. Soon as I did that, I got a double beep - and this!

20220319_163616.jpg

So now I know a few things - the crystal is good, nothing wrong with Phase 0 or Phase 1 clocks other than something was stopping them from operating. I bent pin 8 back in on A2.. the machine went back to zilch. I then removed A2 and plugged in another 74LS00.. machine still works.

I then went to try and investigate why it's crashing on boot. I read a bit on this and tried swapping the 8T97s with the originals - immediately on the first chip we were back to zilch again and it would *not* come back with replacement of the chip that was there. I then fooled around and reseated the CPU, really pressed it into socket (the socket plastic basically has separated at one end from the pins).. and got it back again.

So I think, unfortunately, I'm dealing with potentially multiple issues including socket and/or board issues. The board to my eye doesn't look rusty or anything.. very clean on the both sides.

Anyway, I will keep at it. At least I got something now!
 
Partial success. I'm not sure what changed.. but here's what I did. First, I followed from pin 8 of B2 to every point it went to. Everything checked out, all had the correct signal and were connected. I then started following SOFT 5 from pin 8 of A2, which I think handles the CLR lines. They were low. Since pins 9 and 10 of A2 are tied to ground, the output should be high. To check whether it was the chip or something else pulling the line low, I bent out pin 8. Soon as I did that, I got a double beep - and this!


So now I know a few things - the crystal is good, nothing wrong with Phase 0 or Phase 1 clocks other than something was stopping them from operating. I bent pin 8 back in on A2.. the machine went back to zilch. I then removed A2 and plugged in another 74LS00.. machine still works.

I then went to try and investigate why it's crashing on boot. I read a bit on this and tried swapping the 8T97s with the originals - immediately on the first chip we were back to zilch again and it would *not* come back with replacement of the chip that was there. I then fooled around and reseated the CPU, really pressed it into socket (the socket plastic basically has separated at one end from the pins).. and got it back again.

So I think, unfortunately, I'm dealing with potentially multiple issues including socket and/or board issues. The board to my eye doesn't look rusty or anything.. very clean on the both sides.

Anyway, I will keep at it. At least I got something now!
When you said you replaced A2 with another 74LS00 and the machine still works do you mean that after the replacement you still saw the screen display and the double beep? One thing to keep in mind is merely bending out A2 pin 8 does not ensure the line is high as I do not see any pull-up resistors that would ensure it was so.

What do you mean by "...got it back again" in the sentence about reseating the CPU?
 
Back
Top