Measurements from power connector P9
pin 1 - 9.47V
pin 2 - +16v
pin 3 - +16v
pin 4 through 7 GND
pin 8 AC LO 4.8v (actual 3.53v)
pin 9 DC LO 4.8v (actual 3.53v)
pin 10 AC LO 4.8v (actual 3.53)
pin 11 line clk
pin 12 DC LO 4.8v (actual 3.53)
Note the system actually uses AC LO from pin 8, and DC LO from pin 12. 8 is in the middle of the third column, 12 is at the top of the 4th column.
3 6 9 12
2 5 8 11
1 4 7 10
conclusion #1 - repair the H742 Power Control Board. AC LO and DC LOW should be closer to +4.8v
The AC LO line goes from pin 8 of the P9 connector to the top right pin on J18. The DC LO line goes from power connector P9 pin 12 to the bottom left most pin.
- - - AC
DC - - GND
AC LO - 1.72 (should be 4.8)
DC LO - 3.52 (should be 4.8)
Conclusion #2, something is pulling AC LO down further, in addition to a fault in the H742
With the M7235 removed, the voltage on J18 pin 8, the AC LO, returned to 3.52.
Conclusion #3, there is a possible fault on the M7235 card pulling AC LO down further. But because the reading is exactly half with the card installed I think this is by design, not a fault.
Checking the manuals...
NEXT: remove and service the h742a power control board. I was told on a DEC user group list to suspect the caps on the AC LO line.