Progress (PDP-8/L repair)

From: Ethan Dicks <>
Date: Tue Jan 14 18:57:00 2003

--- Charles <> wrote:
> I'm still looking for an M220 board since one of the six won't
> carry from its LSB to MSB. Probably just needs a 7482 adder; I
> have one on order.

All I have (not installed in working machines) is short stack of
defective M220 modules. If you have an extender card, you should
be able to trace the fault pretty quickly with a logic probe or
an oscilloscope.

Without an extender card, it's a lot harder.
> This weekend, after quite a bit of signal chasing, I found that I
> had put the variable delay line in the wrong slot some time ago
> while cleaning connectors/mice nests!

I've done stuff like that.

> When I replaced it in the
> correct slot there was still no STROBE output. I then found that
> the 7400 output buffer was internally shorted to Vcc (probably
> from the mis-slotting). Replaced the 7400. Now it cycles.

I wouldn't be so sure that it was hosed by that. M-series modules
use the 4 pins for power and 32 pins for signals. It's possible
that you accidentally hooked two outputs together and the 7400
lost, but it's also possible that it was previously hosed.

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Received on Tue Jan 14 2003 - 18:57:00 GMT

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