The Restoration of the PDP-8/I Minicomputer (PAGE 13)
Now here's something interesting :-)
On the previous page, I had mentioned that the MQA and SCA instructions on the EAE (KE8I) don't work.
I replaced the M617 at D22 hoping that it was a defective signal on the MQ ENABLE and SC ENABLE
lines -- the output of the gate was low.
My father was with me and he made the bold suggestion of measuring the resistance to ground, to see
if something was holding the line low (like a short), or if it was just a bad gate.
Lo and behold!
The two outputs were shorted to ground via a blue wire-wrap wire!
It looks like whoever installed the EAE (KE8I) on this unit did not clip all of the correct wires!
So I manually clipped F39C2-F39F1 and F39F2-F40C2, and voila, MQA and SCA now work.
I'm waiting for the list of "clip these blue wires" before proceeding to diagnose the other problems
in the hopes that they will be fixed by clipping the appropriate blue wires...
The curious thing about this is that I was told by the original owner that it came with the EAE (i.e., he didn't install it), and
he got the PDP-8/I from an auction which was held by the company that used the 8/I -- so one assumes
that the EAE had been in use (and presumably working) -- I guess they didn't run a thorough diagnostic
of the EAE after it was installed. Weird, huh? Weirder still, is the fact that someone went to all the trouble
and expense to load up an EAE into the machine and then never got it working. I'm sure there's a story there
somewhere. If someone from project SCORPIO remembers this machine, please drop me a line! :-)
For reference, here is the "list of blue wires to snip":
- F39F1 to GND. MQ enable
- F39F2 to GND. SC enable
- F24C1 to +3 volts EAE NO SHIFT ENABLE
- F31D2 to +3 volts EAE MEM ENABLE
- F33P2 to GND B EAE ON
- F28R1 to +3 volts EAE ON (0)
- F23H2 to +3 volts EAE E SET
- E28L1 to GND EAE L DISABLE
Turns out none of the wires were snipped on my 8/I!
With these wires snipped, the EAE (KE8I) appears to be fully functional!
That just leaves the following to fix:
- TTY input
- figure out what's wrong with the intermittent nature of the core, and fix the second bank of 4k.
After that, it'll be on to the PDP-12!