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.

Click on the image for a larger view.

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.