DEC PDP-12 Restoration

Moving day.

Moving the DEC_PDP-12

The RICM rescue and moving team.



I couldn't resist cleaning up the really pretty front panel before doing anything else. The left side of the panel controls the PDP-8/I processor and the right side controls the LINC processor. The pivot pins on one of the lime green switches are broken. We will need to repair the pivot pins, or find a replacement switch.

Today's mission is to:

  • Reform the capacitors in the power supply (working on it)

  • Reform the capacitors on the core memory controller boards (done)

  • Clean the processor cabinet (done)

  • Clean the I/O expansion cabinet (this can wait a while)

  • Replace the 30A plug with a 20A plug so it will plug into a normal wall socket. (done)

  • Slowly power the power supply on with a Variac to make sure that nothing gets damaged. (done)

Reforming the monster capacitors in the power supply. We also reformed the capacitors on the G624 Resistor Board in the Core Memory subsystem. While the capacitors were reforming we started cleaning. Everything was coated with dust and cobwebs, but otherwise was in very good condition. We cleaned the TU56, VR14, and PC04 and reinstalled them in the processor cabinet. We cleaned the front of the DW8E, RK05, and RX02 and reinstalled them in the expansion cabinet.

The Poly Vinyl Acetate adhesive between the CRT glass and the external implosion shield in the VR12 is failing. This is called screen rot or cataracts. Others have successfully removed the outer protective layer and removed the failed Polyvinyl Acetate adhesive. Some fellow vintage computer enthusiasts suggested looking at a YouTube video on how to remove the outer glass lens and the Polyvinyl Acetate that adheres the outer lens to the CRT. The process involved heating the tube and using a hot air gun to remove the outer shield. Without practice it looked like an easy way to make a mess of the tube. Others suggested using a hot wire to cut through the PVA. That procedure sounded like it had less chance for destroying the tube. We also had a suggestion to soak the front of the tube in water for a few weeks.

We disconnected the wire harnesses from the power supply output. When we powered on the power supply we were pleased to see that the output voltages were all OK. We reconnected the wire harnesses but saw no indicators on the front panel. After checking the power supply harnesses we found two of the power connectors were not retained on the sheet metal of the power supply, so were not fully connected. We had to hold the power supply connectors to get the wire harness connectors fully connected.

After the power supply connectors were connected correctly we had lights on the front panel.

  • The controls on the front panel are a little different from the ones on the PDP-8/I, so we need to do a little studying.

  • The EXAM and STEP EXAM set the MA and increment the MA, but the MB is always zero.

  • FILL and STEP FILL set the MA and increment the MA, and set the MB, but the data is not stored in core.

  • Pressing I/O PRESET sets the INST FIELD to 2 and the DATA FIELD to 3. (Warren read the manual and said that this is normal.)

  • Switching the MODE switch between LINC and 8 always leaves the machine in 8 MODE. (The manual says to set the MODE switch and then press I/O Preset.)

  • Pressing DO, START or CONT turns on the RUN light and you can see cycling in the MA lights.

  • All of the LEFT and RIGHT SWITCHES work OK.

  • The INST FIELD switches have no effect.

We reassembled the peripherals in the processor and expansion cabinet. Looks very nice. The orange cabinets on the right are the recently restored are the PDP-8/I. This week we will scan and post the processor and RK8-F documentation. Time to do some studying and create a debugging plan. Debugging the core memory will be the top priority. Next week we will reconnect the cables to the TU56, and the VR14. Hopefully the TU56 will work OK.


Dan (the donor) brought more manuals, diags on paper tape, and his very nice color oscilloscope. Dan removed the noisy fan and will see if he can get some oil into the noisy bearing. If not, we will borrow one from one of the 11/45 I/O cabinets in the warehouse. After reading the manual and getting instructions from Warren, we found that some of the misbehaving front panel was due to operator (me) error. The Mode switch to change between LINC and 8 operation only has an effect after you press the I/O Preset switch, so that is working correctly.

When you press I/O Preset the INST FIELD is set to 1 and the DATA FIELD is set to 3. We though that this was wrong for an 8k machine. If you look at the front panel picture you will see three bits for the INST FIELD and DATA FIELD then an additional 2 bits. So part of this is for the 8 and all of it is for the LINC, and it is working correctly.

We recabled the VR14 and TU56. The Local Forward and Reverse switches on the TU56 do not get the correct behavior from the motors.

The Line fuse on the VR14 blew when we turned it on. We will replace the fuse and try a slow power up with a Variac.

We received lots of comments on reforming the capacitors in the power supply and strong recommendations from experts to just replace them. Since new caps are a different physical size, and would cost about $250 we decided to continue with reforming the originals.

We measured the voltage ripple on the backplanes near the power connectors.

Dan's 'scope displays these values at the side of the trace window. Very nice!

    • +5.0V = 5.01V (should be between +5.25V and +4.75V), 200 mV PTP ripple

    • +10.0V = 5.9V (should be between +12.00V and +8.00V), 200 mV PTP ripple

    • -15.0V = -14.2V (should be between -16.5V and -13.5V), 800 mV PTP ripple

    • -30.0V = -31.2V (should be between -33.00V and -27.00V), 800 mV PTP ripple

The 800mV of ripple on the -30 is a sign that the caps in the power supply were not working well enough, and the voltage is probably too noisy for the core to work. We tried to measure the capacitance of the power supply capacitors using Alex's ancient, but very nice, GenRad capacitor meter, but unfortunately the caps were too big to measure. Alex suggested that more power on time for the caps might improve their behavior.

We started the processor checkout and found that bits 4 and 11 in the Program Counter were always on. We looked at the flip-flops on the M221 modules in the processor that make up the PC register, and they the contents matched what was set on the console switches. We need to determine why the indicator lights on the front panel do not exactly reflect the internal state of the registers in the processor so we can continue debugging. After running the system for about four hours the ripple on the -30V was down to 180mV, so the capacitors are getting better. More run time will hopefully reduce the ripple to an acceptable level. If not, we will have to replace them.


We didn't find the allowable ripple in the PDP-12 documentation, but Warren said that it is in the Classic PDP-8 docs. It says 700mV on the +10V and -15V, so we are well within those levels now. The -30V ripple is supposed to be below 50mV when measured on the output of the regulator modules. So next Saturday we will measure the -30V on the G624 modules.


Today we removed the front panel for test and repair. That was a bit of a project because of the long flexprint cables that go from the front panel to the processor chassis in the back of the cabinet. We used Warren's flip-chip tester to activate the individual inputs to the M900 flip-chips. We found an SN7400 that was bad, IC E3 on the M900 in slot N29, that was causing the PC04 light to always be on. We found that the AC03, NP, and LI11 bulbs were bad, and a few were not making good contact. We replaced the bad bulbs and wiggled the others. We swapped the STEP EXAM switch that had broken pivot pins with the INST FIELD 0 switch that we don't need. So, now we think that the front panel is working and we can rely on it for debugging.

This is Warren testing the PDP-12 front panel. This system allows you to enter one instruction in the left switches and the execute it with the DO switch.

Even with the core memory not working this let us try a few instructions. In 8-Mode the CLA, ION, IOF, and CAM work. The INA works, but increments from bit 10 instead of bit 11. The rotate instructions do some really strange things. The Instruction Register has bits 6, 8, and 9 stuck on so that will affect our debugging. We think that some of the SN7474 ICs on the M216 cards that make up the IR are defective. We have replaced LOTS of SN7474 ICs during other repair projects. We will test and repair the the M216 flip-chips on Saturday.


Bits 6, 8 & 9 of the IR were always on, so we suspected that this might be the cause of some of the rotate instructions misbehaving. We swapped the M216 flip-chips in slots H39 & H40, and the stuck bits moved to bits 0, 2 & 3. This meant that the M216 that was in slot H40 was bad. We replaced it with a repaired and tested M216 from spares. The IR works OK now. We tested some of the PDP-8 and LINC instructions to see what works.




LINC Broken: We can read and write the relay register and the relays change state. Two of the relays have high resistance contacts, so they probably need to be cleaned. We have seen LOTs of problems with SN7474 chips on other restorations. We already found a bad SN7474 in this system so we pulled all of the M216 flip-chips and tested them. We found a bad SN7474 chips on the M216 flip-chip in slot E8. This M216 controls the core memory states, so it was likely the problem with the memory not working. With the M216 in the core memory replaced with a repaired and tested spare, the core memory now works! We started going through the troubleshooting guide in Maintenance Volume-II. We noticed that the PC04 light didn't work, so we pulled it and measured its resistance. The bulb measured about 50 Ohms, so we put back in, and now it works, um, no it doesn't. We filled memory with 7777 using FILL STEP and AUTO, and then looked at memory with STEP EXAM and AUTO. We saw a flickering pattern, so we knew that some locations were not working OK. After experimenting we found that 1xxXXX did not work. We swapped the G221 in slot C09 with a repaired spare. All of the bottom 4k of memory works OK now. A JMP instruction, 5000, puts 5000 in the PC and then in the MA. There is some gating logic to only use the lower 9 bits for the address, so that must be including the upper three bits. Maybe the RCB EN MA0-4 H signal on the RCB schematic sheet is causing the problem with the JMP instruction. That signal comes from the M113 in slot K28 that uses just SN7400 ICs.


We looked at the prints to see which flip-chip might be causing the problem with the JMP instruction. We speculated that the M113 (three SN7400 ICs) in slot K28 was the problem. We tested it with Warren's flip-chip tester and found that it was OK. We continued testing the M113 flip-chips. The M113 in slot J33 had the F2 output stuck low. See sheet INS signal (L JMP * XEQ 0) L. We replaced it with a tested spare, and now the JMP instruction works. The M113 in slot K30 had a bad V2 output where the U2 input was ignored. See sheet CST, something to do with AUTO. We tried some instructions and found that ISZ will clear the AC during the Execute cycle. We looked through sheet RCL of the schematics, found all of the flip-chips that generate the RCL LOAD AC H signal, and tested them. We found that the M119 in slot H28 had the J2 pin stuck high (active). This would cause other instructions to load the AC. We replaced the M119 with a repaired and tested spare and now ISZ works OK.