12 May
2008
12 May
'08
2:04 p.m.
Vincent Slyngstad wrote:
Sourcing them is hard. Maybe a pull from some other board that's been
designated "parts"? I looked at some boards I have (where some idiot
shaved off all the metal from the edge connectors) but no 8235's there.
Same here - I already checked my collection of UNIBUS and OMNIBUS boards
(at least the ones I was willing to scrap) and no 8235s. I think there's a
couple on the M8330 PDP-8/E TG board, but I'm not willing to kill one of
those :-)
Are you looking to repair a TD8E, specifically? Those
look like simple
B/!A
switches, activated by SDRD and SDRC instructions.
Yep, I've got a real TD8E to fix that has two bad 8235s on it. Both chips
have one output that's stuck low and right now I can't even plug the TD8E
into the machine - it hangs the OMNIBUS when you do. The board looks nice
and clean, though, so I'm hoping that's all that's wrong.
I did a little research and the 74xx TTL family has many AND-OR-INVERT
type gates (e.g. 7451, 74S64, etc) but even ignoring pinouts and output
drive capability, none of them are equivalent to the DEC8235. The DEC part
has active low inputs (it's really an "INVERT-AND-OR-INVERT" gate) that
nobody else seems to have, so replacing 'em with off the shelf parts would
take at least another chip.
Does anybody know if the DEC8235 is the same as the National DM8235? I
don't have a source for the DM8235 either, but at least that sounds easier
to find. The chips on my board look like they have National logos on them,
but they're house marked with the DEC part number. They're also stamped
"7419", but that's almost certainly a date code - a real 7419 is an
inverter
of some kind.
Thanks,
Bob