DEC PDP-8/I Restoration


We did an initial survey of the system. There is some corrosion on the modules. The corrosion is not extensive and can be cleaned without damaging the modules.

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There is some corrosion on the AC power distribution terminals in the cabinets.

This can be cleaned. One of the red AC wires needs a new faston terminal installed.

The capacitors in the 704A and the two 779 power supplies need to be reformed.

We can use a Variac to reform the capacitors on the 779 power supplies. We did this for the PDP-8/S.

Warren recommended using an adjustable DC power supply to reform the capacitors on the 704A power supply.


We used a current limiting laboratory power supply to reform the capacitors in the 704A and 799 power supplies.

We set the current limit to 10 mA and the voltage to what the capacitor was rated for.

After a short time the current consumption went nearly to zero and the voltage went to the limit.

We performed this procedure for each capacitor in each power supply, and all worked OK.


The power switch on the console was not working, the power was always on.

After inspection we found that the switch was corroded, melted, and shorted.

We jumpered the power to always be on and disconnected the damaged switch.

We powered on the PDP-8/I power supply, and even with a load connected the voltages looked. OK.

We used a trick, connecting the Ground & +15V power backwards on the front panel to turn on all of the lights.

We were a little surprised to see that all of the lights worked except for EXECUTE and RUN.

The RUN light will be very useful, so we will take the front panel apart and replace the lights.


We started cleaning the modules in the system. A little work with a brass wire brush, a tooth brush, and some isopropyl did wonders.

We will need to finish cleaning the modules in the CPU, the TC01 Dectape controller, and all 5 of the TU55 tape drives.


We connected the power supply to the CPU to see if there was any life. We were rewarded with some lights and random CPU behavior.

At least we are familiar with the CPU design from resurecting the PDP-8/L, so this job should not be too dificult.

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Warren decided to skip this Saturday because of the weather. I left after three hours because there was about 6" of unplowed snow in front of the entrance.

I finished cleaning the processor boards. The oxidation on the PC traces cleans up easily with isopropyl and a tooth brush.

I had to scrub the gold fingers a bit to clean up some of the black deposits.

I powered on the system again to see if there was any improvement.

Now there are some changes to the lights when you press the start key, but not what you would expect.

I suspect that this system will be more challenging than the PDP-8/L to get running.

I reconnected the AC to the power supply for the first three TU55s.

The voltages looked OK, so I reconnected the DC outputs to the drives and the TC01.

Two of the drives have some strange behavior with the brakes and the motors when you press the forward or rewind buttons.

I will take better notes of their behavior next Saturday.

There was no AC to the power supply for the fourth and fifth TU55s.

I think that this power supply is fed from the corroded AC distribution block at the top of the cabinet.

We need to crimp on a new faston connector one one of the red wires. That may fix the AC power problem.


We repaired the broken AC wire where it connected to the terminal block at the top of the cabinet.

Now all five TU55 tape drives powered up.

We were also rewarded with lights on the display panel of the TC01 Dectape controller.

With drive #1 at the top left and going down and then left to drive #5, the state of the TU55 drives is:

Drive #1:

The drive defaults to the rewind state.

Both motors can be put in the tension and torque states.

Both brakes are inoperative.

Drive #2:

Everything works, except that there is never any back tension.

Drive #3:

The mounting bracket is broken off the brake drum.

We will see if we can disassemble the brake and spot weld the parts together.

Drive #4:

Everything works OK.

Drive #5:

The left motor is inoperative.

We discussed the best way to approach the processor restoration.

Since there is so much wrong with this processor we will install the modules, one at a time, in the PDP-8/L for testing.

Once we test and repair the processor modules we will reinstall them in the PDP-8/I and start the system debug process.

When we get the processor working OK we can start on the TC01 Dectape controller.

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We started testing the modules from the 8/I in the 8/L.

This will save us a lot of debugging time by having know good modules (well at least know to work in the 8/L) in the 8/I.

We tested all of the M220 modules first because you can't get much debugging done without working registers.

One of the M220 modules has defective SN7474 chip and the other has a defective SN7453.

I will repair these this week so they are available for testing next Saturday.

We also had issues with the M216 modules in the 8/L, and found two defective ones.

These modules control instruction and memory states so it is very important that they work correctly.

The M310 modules control timing, so again they are important.

We found four defective ones. We had three working spares, and will repair the fourth.


We tested the repaired M220 and M310 modules in the 8/L.

One of the M220 modules works OK and the M310 works OK.

One of the M220 modules has yet another problem, this time a SN7474. I will replace that IC this week.

We continued testing the modules from the 8/I in the 8/L.

We found 8 more broken modules today.

Some we replaced with spares from the parts 8/L, the rest will be repaired.


We tested the M160 modules that I repaired in the 8/L. All but one worked. We have enough good ones for the 8/I and a spare.

One of the repaired M220 modules works OK now so it was reinstalled in the 8/I.

We found two more bad chips on one of the repaired M220 modules. I will replace those this week.

We tested the repaired M707 in the 8/L. It is still broken and not debugged. We actually have two that need repair.

At this point the 8/I is starting to show some signs of life.

There were more than one instruction indicator on at the same time so we knew that there were some bad M115 modules.

We tested the M115, M117, and M700 modules in the 8/L. We found 4 bad M115, 2 bad 117, and a bad M700.

The 8/I is actually responding to some of the key switches.

We measures the signals from the switches on the backplane pins and found that some were not working and some were flaky.

Warren dumped some isopropyl into the broken switches and they are working now.

We started with some basic memory Examine/Deposit tests and found that MB bits 7 & 8 were always on.

We pulled M228 modules and tested them in the 8/L.

We found three bad M228 modules. I will repair two this week and one was replaced with a spare.


We tested the G228 modules that I repaired this week. They work OK and were installed in slots A37 and B36.

We finished testing all of the modules from the 8/I in the 8/L.

We found a G020 that caused the odd bit to be stuck on and replaced it with a spare from with a spare from the parts 8/L.

We found  a G221 that didn't decode the addresses correctly and replaced it with a spare from the parts 8/L

We measured all of the resistors on the G624 modules. The looked OK.

At this point the processor looked like it was actually responding to the front panel switches.

Examining the memory almost always yields just zeros and a deposit doesn't change memory.

We measured the voltage between the MEMORY SUPPLY - and the MEMORY SUPPLY + on the 8/L and compared it to the 8/I.

The voltage on the 8/I was very high. Since the voltage was not 30V or 0V we thought that the shunt transistor on the G085 module was probably OK.

We tested the G826 from the parts 8/L in the 8/L and the resulting memory voltage was only about 1.0V different so this module is probably OK.

We tested the G826 from the 8/I in the 8/L and the resulting memory voltage was the same as what we saw when it was installed in the 8/I.

We installed the G826 from the parts 8/L in the 8/I and now the memory voltage looked reasonable.

Well, the memory still doesn't work.

Just to be sure we looked at the output of the MEM ENABLE, FIELD, READ, INHIBIT, and WRITE flip-flops.

They all seem to be working OK. This took a long time to get working correctly on the 8/L.

The READ(1) and WRITE(1) signals to the G228 modules looked OK.

In single ended mode the signals on the X R/W SOURCE, X R/W RETURN, and the Y R/W SOURCE, Y R/W RETURN look OK.

We will look at these in differential mode next week.

We looked at the STROBE\ signal from the M360 in slot B23. This signal looked OK, but is was very late compared to the MEM START signal.

It is possible that the M360 delay is adjusted too long. That will be a project for next week.

We also noticed that two of the upper bits in the MA do not increment correctly so there is still a problem in one of the M220 boards.

I replaced three ICs on one of the spare M220 modules, but it still doesn't work. Warren took one to work on.


Adjust the MEMORY SUPPLY - and the MEMORY SUPPLY + voltage to 22.5V with trimpot R28 on the G826 module in slot AB2. (Done)

Adjust the M360 in B23 so that the STROBE\ signal is 500 ns after the MEM START signal. (Done)

Check the width of the STROBE FIELD 0 signal. It should be less than or equal to 80 ns. (Done)

Debugging the inoperative memory.

Based on a recommendation from Anders Sandahl we looked at the output of the sense amplifier on pin E1 of the G020 module in slot A31 and the STROBE FIELD 0 signal.

The first time we looked at the signal there was nothing that resembled a 0 or a 1 in the data.

The signal should have resembled that shown on page 5-13 of the PDP-8/I Maintenance Manual Volume I.

Anders also recommended that we pull the core stack and measure the diodes and wire continuity.

We tested all of the diodes on the core stack. There must be hundreds, and they were all OK.

Much to our disappointment we found that the sense wires for bits 7, 8, and 11 were open and should have been about 16 Ohms.

It will be a tremendous amount of work to disassemble the core and repair the broken wires.