Faulty Dragon32

[sixxie]

Whats the chance this could be a faulty VDG?

Seems unlikely as it's generating a picture - seemingly responding to varying data... But that doesn't rule it out completely. We seem to have gone over everything else that could possibly affect it!

[robcfg]

I know it's a long shot, but could it be that the roms are damaged and fail to initialize the vdg or memory in a proper way?

[sixxie]

The ability to type blind and adjust motor / invoke sound suggests RAM organisation (and indeed RAM) is fine. If the VDG weren't configured correctly you'd still get a stable picture, just an incorrect one.

The constant changing implies bad data to the VDG, either due to its connection to the data bus (but the latch and presumably connections to the latch are fine), or because memory isn't being addressed properly. The SAM is otherwise addressing RAM fine (and has been substituted anyway), so that really does only leave the VDG to SAM signalling, which is DA0, and !HS.

Oh, !FS & !MS need to be connected together on the VDG to make that work. One more thing to try I guess - after that I'm completely out of ideas!

[Alastair]

Could it be a short rather than a faulty component?

[acsi]

Alrighty my day just got better.

The replacement MC6847P arrived today. Popped it in and now she is running again.

Murphy's law states it will that the faulty chip will always be the last one you replace.

[sixxie]

Well colour me surprised

Maybe something to do with the data in lines on the faulty one.

Guess anything that can go wrong will... Glad you sorted it!

[zephyr]

Good to see you got it working again!

There are some HM4864P 64K RAM chips for sale at only 0.99p each (+
£1.00 postage) if you are interested in the easy upgrade to 64K of RAM.