19 Jul
2004
19 Jul
'04
10:40 p.m.
In message <1090317397.28481.21.camel(a)weka.localdomain>
Jules Richardson <julesrichardsonuk(a)yahoo.co.uk> wrote:
Of course, it's
likely to be something simpler I expect...
Get an oscilloscope and a logic probe
out then :)
One trick that should work on LS type logic chips is to bend the pins in so
you end up with a chip that will sit on top of another one and make
reasonable contact with the chip below. Bend the output pins up, then hook up
a scope. Use one channel to check the chip that's in circuit, use another to
check the known-good chip on top. Solder the chips together if you want
(probably a good idea) but DO NOT connect the output pins together.
Obviously, if the logic differs, then the chip on the board is suspect.
I'd also be tempted to desolder the video RAM, fit some turned-pin sockets to
the board (Augat sockets are fairly cheap if you buy them by the tube,
they're also pretty good quality - Farnell sell them), then replace the RAM
chips one by one (or two by two, whatever it takes to get a full byte). I
don't know how the video generation in the Dragon works, but I'd check the
video RAM, main RAM and character generator ROM first, then move onto the
logic circuitry.
Later.
--
Phil. | Acorn Risc PC600 Mk3, SA202, 64MB, 6GB,
philpem(a)dsl.pipex.com | ViewFinder, 10BaseT Ethernet, 2-slice,
http://www.philpem.dsl.pipex.com/ | 48xCD, ARCINv6c IDE, SCSI
... "Bother", said Pooh, as the read/write heads flew across the room.