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
