BillO wrote:It's really hard to see the timing between the two traces. It would be great if you could turn up the graticule (all the little squares) so we could see more accurately.
In any case, it looks like the time between the fall of the yellow trace and the rise of the blue trace is less than 50ns. It should be 70ns or more. You might be able to adjust this by decreasing the duty cycle of the yellow trace. Try to make it more like 50% rather than 56%. You can do this by decreasing R42. 5.6K might work. And you might try reducing the duty cycle of the blue pulse. Right now you are well over 100ns but only need ~70ns-80ns or so. Try 10K or so for R41. That should give you ~100ns. Maybe Mike has a better suggestion, so wait until he responds.
Also, the voltages seem way too low. They need to be 9V or higher. Yours look like they just over 4V. Not sure what's causing that. Can you make sure IC N is a 7406? Also check to make sure resistors R29, R30, R32 and R33 measure to be 470 ohms? I had to decrease those resistors on mine a bit, but even so I was not off by that much. Also check that you are getting 12V across D1, you might have put a 5.1V Zener in there by mistake.
Edit: the implication of putting a 5.1V in for D1 means you might have put a 12V in for D2 which would mean the -V rail is getting ~-8V (or whatever you set your PS to). Better check that first.
I checked IC-N and it is a 7406. I also checked D1and I'm getting over 12v on D1. I recall checking both D1 and D2 as part of my initial checks after the build and they both got the correct voltages. I'm starting to see what you mean about the low voltage though. I just uploaded another screen shot from the clock test I did on the 8800b CPU board. And the voltages that board is showing are more like what you're saying. Ironically that CPU board doesn't work and my reproduction CPU above seems to work fine. I'm not sure about the resistors. Is there a way they can be tested on the board or do they need to be removed?