9810 question and offer

[Tony Duell]

(02-16-2014 02:58 PM)Katie Wasserman Wrote:  Tony,

Yeah I forgot about the bottom board. On the schematic included in the DVD set they show these "high voltage buffer" IC's U1 and U2 with part number 1820-0668 (aka 7407 non-inverting hex buffer) and U3 and U4 as 1820-0471 (aka 7406 inverting hex buffer). The schematic is dated 1972, after the release of the 9810, so likely they changed the board design in later models.

I don't have the DVD, or anything to read it, so I am not sure what that schematic shows. But my schematic (available on the Australian museum site) shows the TTL
outputs of the shift registers being buffered by '06 and '07 ICs, then feeding the 2N2907 transistors.

Since this side of the printhead element matrix needs to be pulled to +24V to turn the element on, I don't see how an open-collector TTL buffer could do it on its own.