HP98780 monitor graphics PCB
ard at p850ug1.demon.co.uk
Thu Jul 28 12:25:35 CDT 2005
Does anyone here have an HP98780 (enhanced mono monitor for the HP9845B)
easily to hand, and if so, could they do a little electrical test on one
of the PCBs :
Remove the top cover (4 screws), the screening cover under it (4 more
screws). the 2 board hold-down strips (3 screws each), and the 2 screws
holding the heatsink pillars from the rightmost board to the right side
panel (directions given with the screen towards you). Pull this board
out. When putting it back, put a little silicone grease on the ends of
In my monitor it's a 98780-66552 Rev B, I am particularly interested if
you have that version...
Anyway, the test : Check for continuity between pin 7 of U53 (this is a
74LS157 mux chip) and pin 2 of U55 (a 4116 or 4816 DRAM).
The reason is that on my board, these 2 pins are not conencted, although
the other outputs of that mux -- and of 3 other similar mux chips -- do
go to the DIn pins of the RAMs. The 'missing' one would be bit 6 (of a 16
bit word), and I can't for the life of me think of a good reason why it
should be different. I wonder if I have a cracked trace in the board
somewhere (this is a multi-layer board so I can't check by eye).
Thanks in advance
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