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Threads: 26
Joined: Aug 2005
Unfortunately, the fact that the fan is running does _not_ mean that the power supply is working. The fan runs off 120V
(obtained from the primary of the mains transformer) in these machines.
Start by checkling the PSU output voltages. To get inside, first unplug the mains plug. Then undo the 2 screws on the back and
lift off the cover. Then loosen the 2 similar-sized screws just behind the keyboard/display assembly inside.
Slide the keyboard forward to free the front edge and lift it
up carefully. There are 3 cables linking the keyboard assembly to the rest of the machine -- a 20 wire ribbon cable to the tape drive, a 40 wire ribbon cable (I/O bus) and a power connector (many coloured wires).
You can run the machine without the keyboard assembly connected for testing the PSU. The voltages can all be found on the power connector that you unplugged from the keyboard
The wire colours there (at least in my machine) are : Blue +20V (unregulated) ; Yellow +12V ; Orange +7.5V ; Red +5V ; Black Ground ; Brown Ground ; White/Orange -5V ; White -20V unregulated.
All voltages are with respect to the ground wire, the metalwork over the power supply board and round the I/O edge connectors at the back is also connected to ground.
There is also a -12V supply, White/Yellow wire, on the CPU board and tape controller board power connectors. Check all the supply voltages with a
voltmeter, and if possible use a 'scope to look for excessive ripple.
If it's not a PSU problem, unplug all the ribbon cables in the unit, clean the connectors (use propan-2-ol) and reconnect them. There are 3 such cables :
From the rear edge connector on the CPU board to the lower RAM board (extra timing signals for the MMU, etc) ; from the ROM backplane at the front to the front CPU board edge connector and then to both memory boards -- Memory bus.
And from the tape controller to the I/O backplane to the header plug at the back of the CPU board, and then to the keyboard/display -- I/O bus.
AFAIK, HP never provided official schematics for this machine, but I have repaired mine a few times so I might be able to help.