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PDP-8/m General Maintenance

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PDP-8/M with the KC8-ML console (Dave Ruske, CC BY 2.0)

Routine care of the PDP-8/m and PDP-8/F. DEC services them from the same manual as the PDP-8/e, and the processor, memory and Teletype procedures on PDP-8/e General Maintenance apply unchanged: the three-monthly schedule, the module order and jumpers, and the memory strobe setting.[1][2] What differs on the 8/F and 8/M is the box, the air flow and the H740 power supply.

Schedule

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DEC's processor preventive maintenance is due every three months or 500 hours: clean the cabinet and air filter, lubricate the slides, inspect wiring, module seating and the power supply for leaky capacitors and overheated resistors, measure the supply outputs under load, and run every MAINDEC for at least three minutes or two passes.[2]

Air flow and cooling

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  • The 8/F and 8/M carry their air filter on the side of the computer. DEC ships it separately, along with the chassis tracks for rack mounting.[3]
  • DEC's H740 specification asks for at least 375 linear feet per minute of air through the heat sink and 250 over the capacitors, chokes and transformer, with a rated heat sink temperature of 95 °C. Depending on the version the supply has an integral 5-inch or 3-inch fan, or relies on an external fan.[4]
  • A thermostat on the heat sink cuts power to the transformer and fan if it overheats and resets itself when it cools (98 to 105 °C open, 56 to 69 °C close in DEC's specification).[5]

Keep the filter clean and the fan turning freely; the regulator is cooled entirely by forced air.[4]

H740 checks

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Measure the H740's outputs at each preventive maintenance, under load, at the end of the DC cable, with a calibrated digital meter.[6]

H740 outputs (DEC specification 5409728-0-8 and DEC-8E-HMM1A)
Output Set to Adjustment range Ripple and noise
+5 V 5.0 V ±5 % min. 4 % p-p (200 mV, 10 kHz sawtooth typical)
−15 V −15.0 V ±5 % min. 3 % p-p
+15 V 15.0 V ±5 % min. –

[6][5][7] Adjust slowly and never beyond 105 %: the +5 V and −15 V crowbars blow the output fuses.[6] DEC's specification recommends no more than 3 % loss in the distribution wiring between the supply and the load.[5] The capacitors and their replacement are on PDP-8/m Capacitor Replacement Guide.

Front panel

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The KC8-ML console uses LEDs, so it has no lamps to replace; the H740 has no +8 V supply for the incandescent KC8-EA console of the 8/E.[8] DEC's first-level spares for the 8/F and 8/M include LEDs, slide switches and a momentary slide switch.[9]

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References

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  1. ↑ Digital Equipment Corporation, PDP-8/E, PDP-8/F, PDP-8/M Maintenance Manual, Volume 1: Processor, DEC-8E-HMM1A-D-D, September 1973, p. 1-2 (File:PDP-8E Maintenance Manual Volume 1 (DEC-8E-HMM1A-D-D).pdf).
  2. ↑ 2.0 2.1 DEC, DEC-8E-HMM1A-D-D, pp. 4-5 to 4-6, Table 4-2, processor preventive maintenance schedule.
  3. ↑ DEC, DEC-8E-HMM1A-D-D, p. 2-4, section 2.2.1.1, unpacking (PDP-8/F and PDP-8/M air filter and chassis tracks).
  4. ↑ 4.0 4.1 DEC, SP-5409728-0-8, sheet 16, sections 3.3 to 3.5, cooling.
  5. ↑ 5.0 5.1 5.2 Digital Equipment Corporation, engineering specification SP-5409728-0-8, "Power Supply Specification – H740", sheets 8 to 12, in H740 Power Supply Engineering Drawings, revision H (File:H740 Power Supply Engineering Drawings (Rev H, 1974).pdf).
  6. ↑ 6.0 6.1 6.2 DEC, DEC-8E-HMM1A-D-D, p. 4-26, section 4.9.1, H740 rules and precautions.
  7. ↑ DEC, DEC-8E-HMM1A-D-D, p. 3-177, section 3.48.3, +5 V ripple.
  8. ↑ Douglas W. Jones, "What is a PDP-8/F?", PDP-8 Summary of Models and Options, faqs.org.
  9. ↑ DEC, DEC-8E-HMM1A-D-D, pp. 5-5 to 5-6, Tables 5-5 and 5-6, first-level spares.
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