HP98780 monitor graphics PCB

Tony Duell 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 
the pillars. 

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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