Eventually i got it out, turns out you have to undo some screws around the frame and 3 screws at the back of the CRT module.
And finally its out, and AHA here is the "graphics card" on the left side of the module!
Its quite a neat assembly actually, turns out this whole thing is a standard part by the name of "HP 1345A Digital Display". Service manual here:
http://vectorvgapro.com/PDFs/HP/HP%201345A%20OSM%201985.pdfIt is basically a CRT module that runs from 5V and +/- 15V while having a 16bit parallel interface. Capable of drawing vector graphics and vector text (No raster images tho)
The neat thing about it is that if you just power it up without connecting the data cable it goes into a self test mode and displays a test pattern. Once again its clear that it draws lines just fine, yet text is a mess of "a" characters again
So pouring over the nice service manual of it shows well how it works. It uses a special vector drawing chip that outputs 8bit data to a pair of X and Y DACs (Off to the right) for steering the beam. That self test turns out is done by a 8bit MCU sitting on the bus, and it seams to be the only job for it! A whole MCU just for a self test feature!
So after poking my scope around i noticed that the outputs of the character ROM chip(Shown in red) are always 0 while the address lines are all moving about. Reseating it in the socket does nothing, it gets power and all, so i conclude that this chip is dead.
And here is the top side of this vector graphics board. Red arrow is the dead character ROM.
To further confirm it i tried to unplug the chip and run it, indeed i got garbage where the "a" characters used to be, and if i tie one of the lines on it to 5V i get different characters in place of the "a". So yes this thing is toast.
I have tried to find ROM dumps online but so far i could not get anything. So i looked for alternatives. One option is getting a new display module, as of right now these can be found on ebay for about 300 EUR. Since that's almost 3 times as much as i payed for this whole thing that's not so great. Another option is to buy a NewScope5 LCD retrofit kit:
http://www.simmconnlabs.com/1401/2501.html But that costs 400 USD. At least that gives me a modern LCD with color but still thats a bit steep.
Does anyone know where to find a ROM image of this chip?
As a last resort i did get the idea to try and guess the contents of the ROM trough brute force. Its only a 512 byte ROM and only half of it is actually used up. The input to it should be ASCII values (The char set is in the service manual), i just don't know what the output should be. Essentially this ROM looks up the address of a character drawing in the larger 2KB ROM, while also generating some control signals for the vector chip. Maybe a CPLD could be used as a ROM emulator to quickly change contents while running. Once i have the correct ROM image from that i can then burn it into a actual memory chip and replace it. Sounds like a lot of work still...