I've got a US Apple IIe Rev B. 64K MB that is DOA, was given to me. Gradually I've been troubleshooting why it doesn't initialize. The PSU is good, power light turns on, voltages are clean and proper values on all filter caps, but no beep on power-up, just a brief flash of about 2" of lines at bottom of Apple Monitor /// then darkness. I'm testing bare motherboard, nothing in slots.
1. On power-up, pin 40 on CPU stays low, it does not toggle low from high and back to high.
2. All clock signals are present and correct frequency at clock timing circuits, they swing ~5v p-p roughly, but they are 2.5v above and 2.5v below ground. (this has to be a major clue).
3. The 74LS125 tests good
4. I'm preparing to test the 74S02 next, which has no clock on pin 5 but a good clock on pin 6.
These off-centered clock signals do appear at pins 14, 15, and 16 of the HAL.
Anyone run into this before?