HP 9845 C with 280 option startup failure

I am not sure what that board is (I have my own names for all the PCBs, since I worked it out long before I saw the boardswapper guide). What I call the 'video interface PCB' -- the board in the left hand mainframe cardcage that connects to the monitor is supposed to be the same for both the 98770 colour monitor and 98780 enhanced mono monitor. Certainly the one in my machine sends 15 bits per character up to the monitor (7 ASCII code bits, 1 chracter set select bit, 4 attibute bits, 3 colour bits, the last 3 are not connected on the monitor backplane PCB in my machine). I asusme you have some idea what that board contains. There are 2 80-character (15 bits per char) buffer memories. One is read out to the text display board in the monitor, while the other is loaded from the mainframe's memory. There are 2 address counters on the PCB, one for rerading, the other for writing (it is not one per memory). And of course cotnrol logic. The board connects to an internal bus on the PPU card -- basically the address/data bus of the PPU hybrid -- and therefore talks to memeory via the buffers on the PPU card. 'My' schemaitcs do show pinouts of all the connectors. OK, my names, not HP, but at least it's a start. -tony