PDP-9 Restoration Blog Starting 2019

Go to the earlier restoration blog.


Moved to the new Lab space.

3/2/19 Hours: 40412.4

We powered up the PDP-9 for the first time in the new lab space. Bit-9 is stuck on in all of the registers so that is the first thing to fix.


We made a longer power cord out of 12-3 SOO cord.

We measured the power consumption at 11A @ 120VAC, so it will $0.30/hour to run the CPU.

We swapped the B131 modules in slots A21 and A23, these are the Adder modules for bits 8 & 9.

The stuck bit moved to bit 8, so we know that the B131 that came out of slot A23 is defective.

We don't have a spare, so we will need to repair it.


The A BUS input on pin S was at -.011V (logic 0) and the other boards were at -3.46 (logic 1).

The SUM output on pin V was at -.3.53V (logic 1) and the other boards were at -0.11 (logic 0).

We measured the input and output voltages on the misbehaving B131 FlipChip, and on the two working neighbors.

Right away we could see that the output voltage was not correct on the broken B131, which explains the stuck bit-9.

The Adder circuitry in the PDP-9 is very fast (for its time), very complicated, and mostly analog.

The internal circuitry has four possible current/voltage levels depending on the state of the four inputs.

We made a benchtop test setup for the B131 so we could manipulate the input signals and measure the output signals.

Then we compared the behavior of a good B131 to the bad B131.

After a few hours of pondering the schematic to see how it works we were able to devise tests that would show if each of the individual transistors were working correctly.

Eventually we determined that Q4, a 2N3669 transistor was not working at all.

We replaced it with a New Old Stock part, and now the B131 is working again, and the stuck bit-9 is gone.

Alex is making an LT Spice model of this FlipChip so we can better understand how it works.

Click on the image of the B131 Adder for a larger view.

When running at the slowest possible speed the PDP-9 will run two JMP instructions.

Otherwise the behavior is inconsistent, and the Deposit & Examine doesn't always work on the first try.

This sounds like timing issues, so we need to go through the procedures in the System Adjustment manual.


We did some more debugging on Sunday to determine what instructions work and found that the JMP instruction goes to the wrong address.

After some digging we found a stuck bit-17 in the AC register.

We swapped the B213 module on slot C39 that holds that AC register bit, the B169 in slot A38 that gates the output to the A BUS, and the B169 in slot B38 that gates the input, but did not see any changes.

We have spare B213 and B169 modules so this should be an easy fix.


Well, the fix was not so easy.

After several hours of boards swaps and debugging we eventually found a bad B213 FlipChip in slot C39.

This FlipChip provides bits 16 & 17 in the AC register and was the cause of the stuck bit 17.

After this repair the system ran MAINDEC-9A-D1AA-D PDP-9 Basic Memory Checkerboard Test for a few minutes and then halted at location 223.

This is the correct behavior when it detects a memory error.

The DEC-9A-H0AA-D PDP-9 System Adjustment Manual describes a procedure to adjust:

    • Clamp and Slice voltages in the Core Memory
    • MA Setup, Stagger Delay, Strobe Delay, Pause Delay, and the Write Delay timing in the Core Memory
    • Current Start Delay, Start Delay, Width Delay, Loop Delay, Strobe Start Delay, CLR Position Delay, and Current Delay in the Control Memory
    • And check other system timing and pulse width values

We will run through the System Adjustment procedure And then try it again.

We will also run the MAINDEC-9A-D01A-D_Instruction_Test_Part_I_Nov67 and MAINDEC-9A-D02A-D_Instruction_Test_Part_II_Nov67 to make sure that all of the instructions are working.


We fiddled with the system a little before we made all of the adjustments.

We ran MAINDEC-9A-D1AA-D PDP-9 Basic Memory Checkerboard Test for a few minutes and then halted at location 223.

C(AC) = 000000 = Memory Address where the error occurred

Press CONTINUE C(AC) = 200000 = Correct data word

Press CONTINUE C(AC) = 740044 = Incorrect data word

Press CONTINUE C(AC) = 000000 = Pattern control word, should be a 000000 or a 777777

Hopefully if will behave better after we adjust everything.


We decided to run MAINDEC-9A-D01A Instruction Test Part 1 to make sure that the processor is fully functional.

The diagnostic failed immediately at address 000575, which is E83.

After reading through the source code we restarted the diagnostic at 000551 and single stepped the processor.

This test clears the AC and Link, compliments the Link, and rotates the Link bit left through all of the AC bits.

It failed when it tried to rotate the bit from bit-12 to bit-11.

We swapped the B169 modules between slots B22 and B26 and the error moved to bit 9/10.

We put the B169 from slot B26 back in B22 and put a replacement B169 in slot B26.

Instruction Test Part 1 now runs OK.

In the video below we configured the PDP-9 in Single Instruction mode, locked Continue on, enabled Repeat, and set the repeat speed to the minimum.

This causes the machine to run a few instructions per second so you can watch what it is doing.

The upper register on the right shows the contents of the memory that is being accessed and usually contains the instruction being executed.

The lower display is set to show the AC register, part of the machine being tested.

We were able to successfully run other diagnostics, but not at full speed.


We spend most of the time wiring new AC outlets for the PDP-9, so only a little debug time.

The core memory seems to be working OK even after we fiddled with the voltage and delay settings.

The core memory still needs the final tuning.

We found that the JMP instruction just goes to the next sequential address instead of to the address in the instruction.

That will be our next debuging project.

We also noticed that one of the G882 FlipChips is missing from the TC02 DECtape controller.

We will need to find a replacement before we can try reading/writing DECtapes.

A very generous Anders Sandahl sent us a G882 from his collection.


We did a lot of studying of the circuitry that makes up the Control Memory, known as microcode on a modern computer.

We determined the correct sequence of events, and made a plan to check the Control Memory sequence and why it wasn't executing Control Word 74 for the JMP instruction.

Click on the image for a larger view.

After a lot of 'scope work we determined that we could actually see the signals go active in the Read Only Program memory and could verify which Control Memory words were being executed.

In the 'scope image above the yellow trace is CM CURRENT signal that cause the contents of the Control Memory to be read.

The green trace is the CM STROBE that latches the data from the Control Memory. The signal going positive and negative is actually a DEC standard pulse.

The purple trace CMP7 signal and the blue trace CMG4 are the address lines for the Control Memory that select the Control Word for the JMP instruction.

Oops, you can see in the third sequence where purple goes up and blue goes down that Control Word 74 is being executed.

So much for the out of sequence Control Memory theory.

The last of the four Control Word sequences in the 'scope image is 10, BGN, where it gates the PC into the MB, and waits for the core memory to fetch the next instruction.

The processor will stop at this point if the Single Instruction switch is turned on, which we did.

Now that we knew that Control Word 74 was being executed we verified that all of the bits in the Control Word were being latched.

We quickly found that the PCI bit that latches the new memory address into the program counter was inactive.

That explains why it would not JMP.

We replaced the B213 Jam Flip-Flop module in slot D21 and now the JMP instruction is working again.

Time to go back to adjusting and tuning the core memory and processor, and then on to connecting and debugging the TC02 DECtape controller.


last week we found that one of the Control Memory Flip-Flop FlipChips was defective and replaced it with a spare.

That fixed the PCI (Program Counter In) Control Memory signal, and the JMP instruction.

Unfortunately it looks like we broke many of the other instructions in the process of fixing the JMP instruction.

Each of the B213 modules Flip-Chips, like the one that we replaced, contains two Flip-Flops.

Replacing the B213 in slot D21 fixed the PCI bit, but now it looks like the other half of the FlipChip is broken, and that is affecting the PCO Control Memory signal.

Having a misbehaving PCO (Program Counter Out) signal would explain why many of the other instructions are misbehaving.

We have a few more untested B213 modules we can try, and then it will be time to fix the pile of broken B213 modules we have.

Click on the image of the B213 for a larger view.

We don't have a FlipChip tester that is capable of testing the B/R/S negative logic modules, so we will need to make a bench test setup.

We have a connector module that will hold the FlipChip, and a 3x voltage power supply that we can use to power it.

All we will need for testing is a signal source for a -3V->GND fast pulse for the strobe, and a logic level for the data input.

We even have NOS 2N3009 and 2N3639 transistor for replacements.


We installed the G882 FlipChip Anders Sandahl donated in slot C23 the TC02 DECtape controller.

We borrowed two TU50 DECtape drives from the PDP-8/I. It still has three TU50 drives.

We installed both drives above the TC02 controller in the expansion cabinet and cabled them to the TC02 controller.

We still need to borrow a 779 power supply from the PDP-8/I and also install it in the expansion cabinet.

The 779 power supply will power the TC02 controller and both TU50 DECtape drives.

The bottom controller is the TC-59 for the TU20 1/2" 7-track magnetic tape drive.

The upper controller is the TC02 for the DECtape drives.

The lower of the TU50 DECtape drives is older and has an extruded aluminum frame instead of a cast frame.


We spent the day chasing the JMP instruction not working issue.

We checked the system timing using the procedures in the DEC-9A-H0AA-D PDP-9 System Adjustment Manual.

All of the timing measurements were within specifications.

We checked all of the microcode bits in each microword and found that the PCI flip-flip was not being set in microword 74.

This will prevent the address from the JMP instruction from being transferred to the PC register and prevent the JMP from going to the correct address.

The PCI flip-flip was being set in microword 21, so we knew that the transformer for the PCI signal on the G920 module was OK.

The MBO flip-flop was being set in both Microword 21 and 74, so we knew that the microcode was being executed in the right sequence.

In this 'scope image you can see that the PCI flip-flip was being set in microword 21m and not in microword 74.

It looks like the CMSL13 signal from the G920 transformer is near ground at microword 21, but is near -2V in microword 74.

We think that this is why the PCI flip-flop is not being set.

It will take a lot of investigating to determine why the PCI flip-flop is not being set.


After pondering the Microcode problem for a week we spent an afternoon verifying that the microcode sequence is what we expected.

An instruction starts with Microword 10, then 21, then 12, then it decodes the instruction and with the JMP goes to 74, and back to 10.

The processor will stop after Microword 10 if Single Instruction mode is on.