PDP-9 Restoration Blog Starting 2021

Go to the earlier restoration blog.


We tried to demonstrate ADSS last week, but it would not boot. After testing with toggled in instructions we found that all of the instructions that we tried worked OK, except for the JMP instruction.

We connected 'scope and the logic analyzer to the microcode address to see if we could see it execute the microcode steps. We were thinking that it was not getting to the microword for the JMP instruction. After lots of fiddling we were never able to get the logic analyzer to decode the negative logic levels and display the correspond logic states. After many hours of fiddling we demonstrated what was wrong to one of our volunteers, and of course it worked. After several tried it booted ADSS. Something is marginal, and we will need to find out what it is to make the machine more reliable. A project for next week.


The PDP-9 booted OK and ran fine for a few hours, but then the TU-55 DECtape drive in the CPU cabinet stopped working. The right hub motor had no torque in any state. I swapped the G850 SCR flipchip for the right motor with one we borrowed from the PDP-8/I and the drive works OK. We will either find a replacement or fix the one we have.


We ran an ADSS demonstration and left the system powered up in case other visitors wanted to see it in operation. After a few hours of power on the DECtapes would not work. Time for some diags to see what is broken.


MAINDEC-9A-D01A-D Instruction Test Part 1 failed at address 25. SKP not working? I toggled in a little program to test the SKP instruction and it works OK. Not loading the paper tape correctly? I loaded MAINDEC-9A-D1AA-D PDP-9 Basic Memory Checkerboard Test and it runs, but the lights don't look right and it doesn't beep periodically. I toggled in a little program to copy the switches to the Status A register in the TC02, and can make it move the tape in both directions. At least much of the CPU, I/O, and TC02 are working.


We ran the built-in diagnostics and noticed that the Memory Buffer display shows bits 8-17 counting, bit 7 is off, and bits 6-0 on. This pattern is reflected in all of the other registers. In step one this diagnostic gates the Adder Register onto the O-bus, turns on the +1 signal to increment the Adder Register, and gates the O-bus into the Memory Buffer register. In step two it gates the Memory Buffer onto the O-bus and gates the O-Bus into the Accumulator, Adder, Program Counter, and MQ Registers, and also gates the console switches onto the O-bus. Turning on the 7 Address switch on the console turns on that bit on all registers except the Memory Buffer register. It looks like the problem is in the Adder or the Memory Buffer. Since we have seen failures in the B213 JAM Flip-Flops before we will look there first.

We swapped the B213 Jam Flip-Flops in slots B16 and B20 that implement Memory Buffer bits 6, 7, 8, and 9. It didn't change the behavior.

We swapped the B169 Inverters in slots B17 and B21 that multiplex lots of inputs to the adder bits 6, 7, 8, and 9. It didn't change the behavior.

We swapped the B131 Adders in slots A17 and A19 that implements bits 6 and 7. The broken bit moved from bit 7 to bit 6, so the broken flipchip is now in slot A17.

We swapped the B131 Adder in slots A19 and A21 that implements bits 7 and 8. The broken bit moved from bit 7 to bit 8, so the broken flipchip is now in slot A21.

So now we know which B131 is broken, the one originally in slot A19. We checked all of the diodes and transistors on the B131 with the diode voltage drop function in a DVM, and they all looked OK. After we put the board back in the PDP-9 it worked OK. Oh well, just a bad connection.

We tried to boot ADSS, but no luck. We tried the run the Instruction Test #1, but it looked like it didn't load correctly. Maybe w