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PDP-11/34 General Maintenance

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Wire-wrap side of a PDP-11/34's backplanes, with the power harness at the left (Autopilot, CC BY-SA 3.0)

Routine care of a PDP-11/34 covers its site, inspection of the box and backplane, power checks, the power budget of the processor backplane, and DEC's first-time start-up procedure, which doubles as a quick health check after any work. DEC's PDP-11/34 System User's Manual (EK-11034-UG-001) gives the installation and start-up procedures and the PDP11/34A Power System description (EK-1134A-TM-002) the power distribution rules.[1][2] Supply repairs are on PDP-11/34 Power Supply Restoration and fault finding on PDP-11/34 Troubleshooting Guide.

Site

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DEC designed the 11/34 for 5–50 °C at 10–95% relative humidity without condensation, but systems with magnetic tape, card readers or disks need 15–27 °C at 40–60% humidity; DEC's nominal conditions are 20 °C and 45%.[3] DEC's site advice also covers filtered air at slightly raised pressure to keep dust out, low humidity encouraging static, and interference from sources such as arc welders and transmitters.[3] The system runs from 115 V or 230 V ±10%, 47–63 Hz; a BA11-K or BA11-L box takes a NEMA 5-15 plug (115 V, 15 A) or 6-15 plug (230 V, 15 A), and a cabinet system an 861-C (115 V) or 861-B (230 V) power controller.[4]

Inspection

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DEC's initial inspection, done with the box pulled out on its slides, is a good starting point for a machine that has been stored:[5]

  1. Check the outside for damage and the box for loose or missing screws and nuts.
  2. Work each front-panel switch to make sure it moves freely.
  3. Examine the backplane for bent pins.
  4. Check the power harness and console harness connections at the power supply and the console.
  5. Remove any shipping brackets.

Jonathan Chapman, restoring a BA11-K, also removed the front panel to clean the switches, and found the backplane full of dust, paper-tape chad and green flakes and the regulators' clear covers opaque with dirt.[6]

Backplane

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The DD11-PK has its connector pins soldered into the board and joined partly by printed tracks and partly by wire-wrap. Jörg Hoppe found that on one backplane many wire-wrap pins had never been wetted by solder, so they lost contact with the tracks as the machine was handled. Moving a pin slightly while measuring the resistance of a signal path finds them.[7] Keep the bus grant chain intact: fit a G727 grant card in connector D of every empty SPC slot, and when removing a DMA controller restore the CA1 to CB1 NPG wire in that slot or fit a G7273.[8][9]

Power checks

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Regulator outputs in the BA11-K (DEC limits)
Supply Regulator Limit Ripple, peak to peak
+5 V, processor backplane H744 or H7441 in slot 2 ±5% (4.75–5.25 V) 200 mV
+5 V, other backplanes H744 or H7441 in slot 3 ±5% 200 mV
−15 V H745 in slot 1 ±5% 450 mV

[2][10] Measure at the backplane with a scope, ground lead on a backplane ground pin as close as possible to the point measured; DEC warns to be sure the ground clip is on the right pin.[10] Each regulator has an adjustment potentiometer.[11]

Processor backplane power budget

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The +5 V regulator for the processor backplane is an H744 (25 A) in the 11/34 and an H7441 (32 A) in the 11/34A.[2] Henk Gooijen gives these +5 V loads for the 11/34A processor backplane, and warns that with all the options fitted the backplane is at its limit:[9]

11/34A +5 V loads (Henk Gooijen)
Module Current at +5 V
M8265 and M8266 processor 11.5 A
M7859 KY11-LB console interface 3.0 A
M8267 FP11-A floating point 7.0 A
M8268 KK11-A cache 4.0 A
M9312 bootstrap/terminator 2.0 A
M7856 DL11-W console 2.0 A

[9] Each MOS memory board also takes 0.5 A of refresh current from whichever +5 V regulator feeds the +5B bus.[2]

Backplane +5B jumpers

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MOS memory has separate +5B, +15B and −15B refresh buses on the backplane. In a 10½-inch box without H775 battery backup, jumper them to +5 V, +15 V and −15 V with #20 insulated wire. With the 54-10864-YA distribution board (11/34A) put the +5 V to +5B jumper on the CPU backplane only: a second one elsewhere puts the two +5 V regulators in parallel, which DEC forbids.[2]

First-time start-up

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DEC's start-up procedure assumes a DL11-W or equivalent console interface, an on-line terminal, a KY11-LA or KY11-LB console and all M9301 switches on.[12]

  1. Set the power switch to OFF and plug in the line cord. DEC warns that OFF does not remove AC from parts of the machine while the cord is plugged in.
  2. Set HALT/CONT to CONT and the power switch to ON.
  3. Check that the fans turn, DC ON lights, BATT lights or blinks if battery backup is fitted, the terminal prints the registers and a $ prompt (press BOOT/INIT if M9301 switch 2 is off), and RUN is on.
  4. Run the quick verification routine (see PDP-11/34 Troubleshooting Guide).
  5. Boot a diagnostic from a peripheral and let it run for two passes.

[12]

Preventive maintenance checklist

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  1. Clean the box, fans and regulator covers; check the fans turn.[6]
  2. Inspect the backplane for bent pins and the harness connectors for seating.[5][11]
  3. Measure +5 V and −15 V at the backplane.[10]
  4. Check grant cards and NPG wires after any change of modules.[8][9]
  5. Run the quick verification routine and the CPU and memory diagnostics.[12]
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References

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  1. ↑ Digital Equipment Corporation, PDP-11/34 System User's Manual, EK-11034-UG-001, July 1977, chapter 5 (File:PDP-11-34 System User's Manual (EK-11034-UG-001).pdf).
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Digital Equipment Corporation, PDP11/34A Power System, EK-1134A-TM-002, second edition July 1977, sections 1.4, 1.6 and 2.5 to 2.7 (File:PDP-11-34A Power System Description (EK-1134A-TM-002).pdf).
  3. ↑ 3.0 3.1 DEC, EK-11034-UG-001, sections 5.2 to 5.2.6, pp. 5-1 to 5-2.
  4. ↑ DEC, EK-11034-UG-001, section 5.3 and Figures 5-1 and 5-2, pp. 5-2 to 5-4.
  5. ↑ 5.0 5.1 DEC, EK-11034-UG-001, section 5.6, initial inspection, p. 5-10.
  6. ↑ 6.0 6.1 Jonathan Chapman, "Repairing the PDP-11/10 Power Supply", Glitch Works, 5 June 2019, users.glitchwrks.com. Cleaning a BA11-K and H765.
  7. ↑ Jörg Hoppe, "Trying to fix a DEC PDP-11/34 backplane – diagnosis", retrocmp.com, retrocmp.com.
  8. ↑ 8.0 8.1 DEC, EK-11034-UG-001, section 6.1.3, p. 6-2.
  9. ↑ 9.0 9.1 9.2 9.3 Henk Gooijen, "PDP-11/34", pdp-11.nl. Processor backplane loads, grant cards and NPG wire.
  10. ↑ 10.0 10.1 10.2 Digital Equipment Corporation, PDP-11 Mainframe Troubleshooting Guide, December 1976, DECAID PWRB-1 (File:PDP-11 Mainframe Troubleshooting Guide (December 1976).pdf). Written for the 11/05 to 11/45; same regulator types.
  11. ↑ 11.0 11.1 DEC, PDP-11 Mainframe Troubleshooting Guide, DECAID PWRB-3.
  12. ↑ 12.0 12.1 12.2 DEC, EK-11034-UG-001, section 5.8, first-time start-up procedure, pp. 5-21 to 5-23.
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