Firing up the pdt11

Chris Zach cz at
Fri Oct 16 19:32:09 CDT 2020

Assuming people are still interested I'm working on other, lesser issues 
now that the little thing works.

First thought: The interconnections between the CPU board and the IO 
board/memory boards are weird. Basically it's a plate, insulator, board, 
very odd spacer that provides interconnection, bottom board, insulator, 
plate, and the bottom hinge that holds the cpu board above the floppy 
controller board.

These interconnections are very sensitive to torque. Make sure all 
screws are snugged down and all the bolts on the bottom (6) secure the 
screws to the bottom hinges. Otherwise torque will cause the 
interconnection to be intermittent and you'll go crazy trying to 

Another odd issue: When I have the top CPU board vertical everything 
works fine. When I lower it, at about a 20 degree angle all hell breaks 
loose. The floppies no longer work, and the system drops into ODT. At 
first I thought it was the ribbon cable but that looks fine. Then I 
thought it was the sockets having poor solder connections so I reflowed 
them on the CPU board and the disk board. Still no. At this point I'm 
thinking it is noise from the CPU board affecting the disk controller board.

There is a short of shield made of a layer of plastic paper, then a 
layer of foil, then another layer of plastic paper that goes between the 
boards. However it's not grounded, and I think that's part of the 
problem. Any idea why there would be so much noise, and how to reduce 
it? All of the ground wires are installed and there are a lot of them in 
this thing.


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