PDP-8 General Maintenance

PDP-8 serial 85 drawn out of its rack with the side panel removed, University of Iowa (Douglas W. Jones, CC0)

Routine care of the original PDP-8 (the Straight-8), from chapter 9 of DEC's PDP-8 Maintenance Manual (F-87, 1966) and from two published restorations: David Gesswein's and the University of Iowa's work on serial number 85. The power supply has its own page, PDP-8 Power Supply Restoration; faults are covered in PDP-8 Troubleshooting.

DEC's schedule

DEC's preventive maintenance has three parts: mechanical checks (module seating, cleaning, inspection); marginal checks, which aggravate borderline circuits so that they fail under test instead of in service; and checks of the power supply, sense amplifiers and memory currents. DEC asks for the mechanical checks at least once a month, and for everything else on a schedule set by the reliability the installation needs, typically every 600 operating hours or every four months, whichever comes first. The elapsed-time meter on the back of the Type 708 power supply records operating hours for this purpose. DEC stresses the air filter above all: a clogged filter overheats the machine.

Tools

Maintenance equipment (F-87, Table 9-1)

Item	DEC's example
Multimeter	Triplett 630-NA or Simpson 260
Oscilloscope	Tektronix 547 with a 1A1 plug-in
Current probe and amplifier	Tektronix P6016 with Type 131 amplifier
Module extender	DEC W980 FLIP CHIP extender (one supplied)
Wire-wrap tools	Gardner-Denver 14H1C wrapping tool, 500130 unwrapping tool
Diagnostics	MAINDEC 801 to 803 (processor and memory), 810, 812 and 814 (I/O), on paper tape

The Iowa team built their own W982 extender boards from H807 connectors.

Mechanical checks

DEC's monthly routine:

1. Clean the inside and outside of the cabinet with a vacuum cleaner or cloths moistened with non-flammable solvent.
2. Clean the air filter. In a cabinet machine it is at the bottom of the cabinet, behind the fan housing held by two captive screws. In a table-top machine, remove the metal strip below the power supply control panel (one screw each side) and slide the filter out of the back; on the first 30 table-top machines the filter lies on top of the power supply instead. Wash it in soapy water, dry it, spray it with filter coating and refit it.
3. Lubricate door hinges, slides and casters with light machine oil and wipe off the excess.
4. Inspect for completeness and condition; touch up paint.
5. Inspect all wiring and cables for cuts, fraying, kinks and strain, and repair them.
6. Check keys, switches, knobs, lamp assemblies, connectors, transformers, fans, capacitors and the elapsed-time meter for security.
7. Check that every module is fully seated.
8. Inspect the power supply capacitors for leaks, bulges or discoloration and replace any that show them.

Restoring a machine that has stood for decades

Cards and backplanes

• Remove the cards in order and photograph them so each goes back in its own slot. Gesswein did not assume that identical aged cards were interchangeable.
• Gesswein washed the cards in hand dish-washing soap, rinsed them and blew them dry with filtered compressed air. Cards with potentiometers or transformers that could trap water were cleaned with a damp brush instead. He refitted the cards and ran the fans for several hours to finish drying them.
• Check every card against its slot. Gesswein found several in the wrong slots, confirmed by tracing the backplane wiring, and noted that his machine's backplane differed from the manual (the manual shows an S603 in PB2 where his had no wiring).
• Look for bent pins shorting on the wire-wrap side and for broken wires.
• The flexible ribbon cables (W034D) that join the two half-backplanes can decay: on serial 85 the insulation had turned to goo.

Fans

There are three fans at the bottom of each half-backplane. Both restorations took the blades off, cleaned the fans, opened the motor hubs and oiled them.

Marginal-check switches

The row switches at the bottom of each backplane select normal or marginal-check power for each row of modules. They have silver-plated contacts. Gesswein measured contact resistance over 1 Ω on some, changing with slight movement; after he dismantled, cleaned and regreased them it was 0.02 Ω, the limit of his meter. Some contacts had lost plating to corrosion and may need replacing later. On serial 85 the same switches stopped power reaching parts of the backplane until they were repaired.

Console lamps and switches

• Test each lamp with a meter and mark the failed ones. Gesswein replaced 46 bulbs, including some that failed when the panel was moved and some that read good on the meter but did not light; the cause was corrosion of the thin lead wires. The original 1762F (CM1762) is hard to find in quantity, and he used the CM2187 as a substitute. Iowa replaced 28 lamps and found one dead lamp caused by a broken trace.
• Broken switch handles can be pinned. Gesswein drilled the handles and fitted metal pins in a jig, then filed the inside of some handles where they bound at the end of their travel.
• Contact cleaner helps the console switches only partly; Gesswein expected to dismantle them as he did the margin switches.

Marginal checks

Marginal checking runs MAINDEC diagnostics while the +10 V or −15 V supply to one row of modules is raised or lowered, so that weak transistors fail on the bench. Raising +10 V catches low-gain transistors; lowering it catches leaky transistors and simulates high temperature; moving −15 V changes the collector supply of every module in the row. Each machine shipped with its factory margins, which are the baseline for later checks. A margin that shrinks by 0.5 V a year is normal ageing; a fall of 0.7 V in six months calls for investigation.

Warning: DEC forbids taking the −15 V margin beyond −18 V; it can seriously damage the logic.

1. Set every normal/marginal switch to normal (down).
2. Set the marginal-check supply toggle to +10 (down) and its output to 10 V on the panel meter.
3. Power down, switch the first row to be tested to MC (up), and power up.
4. Run a MAINDEC that exercises that row (DEC's Table 9-4 lists which test covers which row).
5. Lower the marginal-check voltage until the program fails and record it; return to 10 V. Raise it until the program fails and record that; return to 10 V.
6. Power down, return the switch to normal, and repeat for every row.
7. Repeat the whole procedure for −15 V with the toggle up and the supply set to 15 V.

The marginal-check connectors on each assembly are colour coded: orange +10 V marginal, red +10 V normal, black ground, blue −15 V normal, green −15 V marginal.

Memory adjustments

Memory current check

Warm the machine up for about an hour, ideally at 25 °C. Measure the read/write current with a clip-on current probe on the yellow wire from the current-setting resistor to the G209 selectors, synchronised on READ at MC16D, and set it to the value on the memory array label with R16 on the read/write G808. Measure and set the inhibit current the same way with the inhibit G808. Correct the target by 1 mA for each degree away from 25 °C (subtract above, add below).

Sense amplifier check

Run the memory checkerboard test (MAINDEC 802) while marginal-checking the sense amplifiers' +10 V line, and set the slice level with R3 on the G008 master slice control until the positive and negative margins are about equal. The sense amplifiers are at MA25 to MA30 and MB25 to MB30, with the parity amplifier at MA31 and the G008 at MB31.

Related pages

• PDP-8
• PDP-8 Power Supply Restoration
• PDP-8 Troubleshooting
• PDP-8 Documentation
• Recommended Tools