digital equipment corporation · maynard, massachusetts
PDP-8 (Straight 8) Computer Backplane Restoration

The second items I restored on the Straight 8 are the backplanes. I removed all the cards for cleaning and to allow the backplane to be cleaned and inspected. Pictures were taken and cards kept in order to allow the cards to be put back in the same location. I didn't want to have additional problems if the identical cards aren't really interchangeable after they have aged.

The cooling fan assemblies at the bottom of each backplane were disassembled, cleaned, oiled, and reinstalled. See the pictures below for more information. The cut power wires were repaired.

The cards were all washed using hand dish washing soap, rinsed, and blown off with compressed air. My compressor has a normal filter, water separator and a desiccant filter to hopefully get just air. Most of the cards were immersed for cleaning. Cards with potentiometers and transformers that may be damaged or trap water were just cleaned with damp brush. After cleaning the boards were put back in the backplane and the cooling fans turned on for several hours to make sure the boards were dry.

The margin switches at the bottom of the backplanes were tested. Even with wiggling the contact resistance of some of the switches was over an ohm and changed with slight movement. This was corrected by cleaning the switch contacts. To do this the switch assembly was removed and the switches removed from the housing, opened, cleaned, new grease applied, and reassembled. The contact resistance was at the limit that I can measure of .02 ohms after cleaning. The switches have silver plated contacts. On some the corrosion had removed some of the plating. In the long term the damaged switches are likely to develop high contact resistance again. They will likely need to be replaced if I can find suitable switches or replated. More information on the process with the pictures below.

The next step was checking that all cards were present and in the correct slot. Several cards were in different slots than shown in the manual. By checking the wiring to the slot I verified that the cards were in the wrong slot. All the cards were present except for the W070 teletype connector. The manual shows a S603 should be in PB2 but no wires go to that card slot. Apparently later machines have a change to the logic which required a card in PB2.

I put the plastic washers I found in the power supply in the backplane pivot so the fans wouldn't rub on the base when opening the backplanes. I then checked the backplane for problems. I straightened several pins that were bent and shorting. I also fixed the broken wire.

More information in descriptions on pictures below.

The following picture links also have descriptions of what is shown in the pictures.

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Fan Assembly pictures
Before Cleaning Top ( 90K)  Bottom (100K)  Partially Disassembled (110K)  Disassembled (133K)
Ouch ( 99K)  Ouch Removed (112K)
After Cleaning top ( 94K)  Bottom (100K)

Margin Switch Assembly pictures
Margin Switches (189K)  Wiring (188K)  Removed (173K)  Moving Contacts (120K)  Fixed Contacts (118K)
Wire Protection ( 83K)

Backplanes and Cards
Ready for Cards (153K)  Cleaned Cards (117K)
Canadian Card (121K)  Card Front (100K)  Repaired Card (153K)  Card Front (157K)  Debugging Needed (130K)
Oops? (108K)  W300 Back (117K)  W300 Installed (176K)
Labels (189K)  More Labels (135K)  and More (188K)
Core Stack (114K)  Core Contact Corrosion (188K)  Corrosion Closeup (152K)
Bent Pins (186K)  Repaired Wire (244K)
Memory Cards (170K)  Processor Cards (171K)  Front (113K)  Backplanes Open (185K)

Next front panel restoration Up to straight 8 restoration

Feel free to contact me, David Gesswein djg@pdp8online.com with any questions, comments on the web site, or if you have related equipment, documentation, software etc. you are willing to part with.  I am interested in anything PDP-8 related, computers, peripherals used with them, DEC or third party, or documentation. 

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