Parasitic Engineering Altair Clock Fix Kit...

[Eric Smith]

On Sat, Feb 22, 2020 at 1:37 AM Mike Douglas via cctalk <
cctalk at classiccmp.org> wrote:

> I scanned my Parasitic clock mod documents and put them at the link below.
>
> https://deramp.com/downloads/altair/hardware/altair_8800_computer/Parasitic%20Clock%20Mod.pdf
>

Thanks for that!

Does anyone have a datasheet for the "94618" dual non-retriggerable
monostable that Parasitic used to replace the 74123? At first I thought the
P/N was likely to be Fairchild, but I couldn't find either a 94618, nor a
dual non-retriggerable with similar pinout from Fairchild. 94618 doesn't
show up in the IC Master editiions of that era, either, nor can I find it
from any of the usual internet sources. A photo of the chip from the actual
Parasitic clock mod might help identify the vendor.

The 74221 and 74LS221 are dual non-retriggerable monostables with the same
pinout as the 74123/74LS123, but require the reverse capacitor polarity (if
using polarized capacitors), and require different RC values than the
74LS123. With appropriate RC values it might make a suitable clock fix if
the 94618 can't be found. It's possible that it might use the same RC
values as the 94618, but without the datasheet for the 94618 it's hard to
tell.

Parasitic mentioned that the 94618 has temperature -compensation and
schmitt trigger inputs, and the 74221/74S221 do have both of those
features. The 74LS221 doesn't have as much input hysteresis for the schmitt
trigger inputs, so the 74221 might be preferred.