7 Sep
2014
7 Sep
'14
1:38 p.m.
On 2014-Sep-07, at 10:59 AM, Mattis Lind wrote:
Related to the issue on corrosion on the pins I can
not see any such
problem in this HP9810 I am working with. There is very little black oxide
on the TI chips with silver plated pins and nothing on the other pins. So
it is not obvious to me that it has been exposed to a bad climate. But
still, chips have failed. The PDP-11/04 dated around 1976 and the HP9830,
same vintage, I worked with earlier had only a few failed chips in the
entire machine.
Now the count is 6 NS chips and one Signetics chip. A very annoying N8885
chip has failed. A quad input NOR which unfortunately doesn't have the same
pinout as the 7402. Maybe it is similar to the 7436 which is supposed be
the same as 7402 but different pin config. Didn't find the data sheet
though.
I remember that solitary N8885A when reverse-engineering the 9830, and checking for a 7400
equivalent.
The 7436 seems to be pretty much non-existent. There's an obscure reference here:
http://www.datasheetarchive.com/dl/Scans-005/Scans-00104143.pdf
The strange thing is there are half-a-dozen 7402s in the machine.
Looking at the specs they may have used the 8885 because it's slightly faster than the
7402 and it doesn't look like a 74H02 was being made, but to a cursory examination I
don't see where the few nS benefit would be needed in the circuit.
Oh well .. if you use a 74LS02 or 74S02 at least it's only two pins that have to be
swapped.