PDP-12 Restoration at the RICM

Michael Thompson michael.99.thompson at gmail.com
Sat May 23 19:59:57 CDT 2015

We spent Friday and Saturday debugging the PDP-12. We replaced a bad SN7400
driver chip and three bad bulbs in the front panel. We can now trust what
we see on the front panel for debugging information.

We tried some of the PDP-8 and LINC instructions and noticed that some of
the bits in the Instruction Register were stuck on. We swapped the two M216
(six flip-flops in three SN7474 ICs) flip-chips that make up the IR and the
stuck bits moved. We replaced the broken M216 with a spare, and now all of
the IR bits work correctly. With a working IR, we found that lots of the
PDP-8 instructions, and many of the LINC instructions now work. We can turn
the relays on and off and make noises through the speaker.

During other DEC restorations we have replaced LOTs of SN7474 ICs. We
pulled all of the M216 flip-chips and ran them in Warren's tester. We found
and replaced another bad M216, the one in slot E8 that controls the core
memory states. Now core memory works!

We went through the troubleshooting guide in Maintenance Volume-II. It has
a procedure for doing a quick test of core memory that revealed a problem
in the upper addresses. From looking at the prints it had to be one of two
G221 flip-chips. We swapped in a spare and found that the one in slot C09
was bad. Now all of the first 4k of memory works.

There is a problem with any PDP-8 instruction that has an address in the
lower 9 bits. All 12 bits of the instruction are used, so it makes a mess.
Debugging that issue will be the next project.

Michael Thompson

More information about the cctech mailing list