23 Jan
2017
23 Jan
'17
3:38 p.m.
On 23/01/2017 21:38, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:
I've measured resistance from all points to all points, there's 9:
74LS373 (output, new socket checked for solder bridges, new chip)
4x 2764 EPROM (new chips, no new sockets yet)
MC3242A (new chip, no new socket yet, this one had bad verdigris)
74LS139
74LS21
74LS365
and it's 0.6ohm max.
I picked A7 because it and its multiplexed D7 are special cases and go to
quite a few locations. D0-D6's input to the teletext chip from the CPU is
via a 74LS240, D7's input is based on ALE/A6/A7 and a few other things
involving clocks that aren't yet present (lack of SOD and IO/M) so I'm
fairly sure the teletext chip isn't getting a full byte.
Both my analysers are cheap chinese clones, I suspect I need something
beefier.
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
I normally trace connections using my DMM, but of
course you have to know
what will fool it (i.e. low resistance components). I find I have to desolder
relays, switches, inductors, transformers, low value resistors, etc before
starting to trace the circuit.
There's been a lot of that here too.
should go high
at the same time as long as it's part of the same trace, so
the LS373 to MC3242A to pin 3 on all four ROMs should pulse together and
they don't. If there's a short they should all get it.
Oh... The trace isn't open-circuit, is it? Check it with the
multimeter. Given the
corrosion damage to the tape drive I could well believe PCB problems of a
similar nature.