"Gareth Magennis" wrote in message
...


"Arfa Daily" wrote in message
...



No I don't have a schematic, but it is a small PCB. The _WR line also
goes to 2 DRAM chips, but nowhere else as far as I can tell.

Yes, the CPU could be faulty, the RAM could be faulty, the EPROM could
be faulty, and a few other things too.
This is a very old board.
(Apart from the EPROM, and now the chip I replaced, none of these chips
are socketed)

I will go back tomorrow and see if I can see anything else on the scope
that might look a little suspect, and what, if anything, is actually
happening - I didn't get much further than noticing the _WR was
definitely screwy.

Cheers,


Not supposed to be a pull-up on that /WR line maybe ? Little sm resistor
gone open ? Worth hanging a 1k up to rail in case some internal pull-up
on one of the chips has gone open ?

Arfa



I tried a pullup on _WR when first investigating. Although it produced a
nice zero, there was no activity.

I have since found another duff line, which was only producing 3v - I
quickly tried a pullup on that with the pin off the board and in the air,
and it suddenly shot up from its 3v and stayed at full value swings, but
with an obviously faulty waveform (large RC curves).

So yes, I suspect it it the output tri-state interfaces/pullups that are
failing on this device.

There are no sm devices on board, this is from the 1970s/1980s!

http://www.youtube.com/watch?v=4lhE1upzemU



Cheers,

Gareth.


Well, managed to get an NEC 8080AFC off the Interweb, socketed it onto the
board, and the AMS is now working again.

Hurrah!


Thanks JW and Arfa,

Gareth.