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PDP-11/20 General Maintenance

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KY11-A console of a PDP-11/20: the right-hand switch register keys, the control keys and the state lamps (Don DeBold, CC BY 2.0)

Routine care of a PDP-11/20 covers its site and cooling, the BA11 box, the H720 supply, the KA11 processor clock, the core memory adjustments and DEC's diagnostics. DEC's maintenance philosophy for the 11/20 was to describe how the machine should behave so that a technician could recognise a fault; the procedures below are the ones DEC gives in the KA11 Processor Manual, the MM11-E Core Memory Manual, the H720 Power Supply and Mounting Box Manual and the PDP-11/20 System Manual.[1][2] Faults are covered in PDP-11/20 Troubleshooting Guide and supply repairs in PDP-11/20 Power Supply Service.

Site and environment

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DEC allows an ambient temperature of 10–50 °C and 20–95% relative humidity without condensation, and recommends keeping the room at 20–30 °C to extend the life of the system. Leave service clearance of 8¾ in at the front and 14⅞ in at the back of a DEC cabinet.[3] A basic system on 115 V needs a circuit of at least 8 A (6 A for the computer, 2 A for the Teletype) and dissipates 400 W, plus 150 W for an ASR-33; a 230 V system needs at least 5 A.[3]

The ASR-33 must stand within 8 ft of the computer because of its signal cable, and it takes its mains from the outlet at the rear of the mounting box. Expansion boxes are joined to the basic box only by a BC11 Unibus cable, and the total Unibus length, inside and outside the boxes, must not exceed 50 ft.[3]

The mounting box

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The BA11 box holds the system units with their pins upwards and the modules hanging down. It has four cooling fans and a filter, an insulated top cover that stops debris falling onto the wire-wrap pins, and a foam-lined bottom cover that retains the modules and forms part of the air plenum.[4][5] The fans are always wired across 120 V, on both 120 V and 240 V supplies.[6]

  • Keep both covers in place when the machine runs; the bottom cover is part of the cooling path.[4]
  • Clean the fan filter and the fans, and check that all four fans turn.
  • With the top cover off and the power off, look over the backplane pins for dust, swarf and loose wire-wrap ends before the first power-up of a newly acquired machine.

Power supply checks

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Measure the regulated outputs at the backplane before any other work and after any change to the box:

H720 regulated outputs (PDP-11/20 System Manual Table 2-7)
Output Nominal Tolerance Acceptable range
+5 V +5.0 V ±5% 4.75 to 5.25 V
−15 V −15.0 V ±3% −14.55 to −15.45 V
Unregulated −22 V ±20% −17.6 to −26.4 V

[7] The +5 V regulator switches between 4.9 V and 5.1 V, so about 200 mV of sawtooth ripple on +5 V is normal; the level is set with potentiometer R38 on the regulator board.[8][9] See PDP-11/20 Power Supply Service.

Processor clock adjustment

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DEC's procedure sets the basic operating frequency of the KA11:[10]

  1. Load address 0.
  2. Deposit 000001 (the WAIT instruction) in location 0.
  3. Load address 0 again.
  4. Set ENABLE/HALT to ENABLE.
  5. Press START. The processor now executes WAIT continuously and its clock runs steadily.
  6. Put an oscilloscope probe on pin E03F1, the SCLK L output of the M728 Timing and States module. One complete cycle should take 280 ns.
  7. If needed, adjust trimpot R8 on the M728, at location CDEF03.

Core memory adjustments

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The MM11-E has three adjustments. DEC says they rarely need changing unless a module has been replaced, and requires them to be made at 20–30 °C with a Tektronix 453 or 547 oscilloscope and preferably a 0.5% digital voltmeter.[11]

MM11-E adjustments (DEC-11-HR3A-D, Table 3-1 and section 3.2)
Adjustment Trimpot Test point Setting
V REF (X-Y current) Current generator G225 E1U2 Within +0.70 to +1.90 V at 30 °C; set midway between the high and low failure points of a worst-case pattern test, with at least 0.80 V of working margin
V SLICE (sense threshold) V levels module G103 D2F1 −5.3 V ±0.2 V; set whenever a G103 is replaced
STROBE DEL Control and timing M729 E1K1, E2S1 T STROBE 210 ns ±5 ns; check when an M729 or G225 is replaced

[11] DEC calls the STROBE DEL setting critical and says to change it only when absolutely necessary. V REF drifts with temperature, rising as the room warms.[11]

Diagnostics

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DEC's MAINDEC-11 diagnostics test particular devices and functions. DEC names Processor Test 17 as an overall system exerciser that isolates a fault to a device; the device's own test then narrows it further. If every diagnostic passes but the user's programs or equipment still fail, DEC advises looking first at those programs and equipment.[12]

Working on modules

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  • DEC's recommended kit for the KA11 is an oscilloscope (Tektronix 453 or equivalent), a volt-ohmmeter (Triplett 630 or equivalent), a W984A double extender board and two single extenders cut from one, the KM11 maintenance set (W130 and W131 modules), an IC test clip, and a soldering iron rated under 40 W.[13]
  • Backplane pins are identified by four characters: column letter, row number, then the module pin (for example F4A1). The MM11-E, PC11 reader and punch, and KL11 use a three-character form, so their prints apply in any slot.[14]
  • The KM11 maintenance set lets the processor be single-clocked, disables time-out, and shows the ISR and BSR time states, MSYN, BSYN, traps and condition codes. DEC calls it one of the most valuable tools for troubleshooting the KA11.[12]
  • Removing the KA11 system unit involves the console mounting screws, the fan door, the three power bus modules, the Teletype and reader control cables, the M920 Unibus connector and the KY11-A console, and finally six screws along the side of the system unit. DEC warns that the KY11-A is a very tight fit and is easily damaged on removal.[15]
  • When adding BA11 extension boxes, connect the AC LO and DC LO of each extra supply to the system so that a power failure in any box is seen by the processor.[16]

Preventive maintenance checklist

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  1. Clean the filter and fans; confirm all four fans run.[5]
  2. Check both box covers are fitted.[4]
  3. Measure +5 V and −15 V at the backplane against the table above.[7]
  4. Check the SCLK period at E03F1 if the processor has been worked on or modules swapped.[10]
  5. Run the processor and memory MAINDECs, then Processor Test 17.[12]
  6. After replacing an MM11-E module, check the V REF, V SLICE and STROBE DEL settings.[11]
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References

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  1. ↑ Digital Equipment Corporation, KA11 Processor Manual, DEC-11-HR2B-D, fifth printing May 1972, chapter 5, section 5.1 (File:KA11 Processor Manual (DEC-11-HR2B-D).pdf).
  2. ↑ Digital Equipment Corporation, H720 Power Supply and Mounting Box Manual, DEC-11-HR5B-D, fifth printing November 1972 (File:H720 Power Supply and Mounting Box Manual (DEC-11-HR5B-D).pdf).
  3. ↑ 3.0 3.1 3.2 DEC, DEC-11-HR5B-D, pp. 4-2 to 4-3, sections 4.3.1 to 4.3.4, space, environmental, power and cable requirements.
  4. ↑ 4.0 4.1 4.2 DEC, DEC-11-HR5B-D, p. 3-1, section 3.2, mounting box description.
  5. ↑ 5.0 5.1 DEC, DEC-11-HR5B-D, pp. 4-4 to 4-5, data sheets for models AA/AB and BA/BB (components: four cooling fans and a filter).
  6. ↑ DEC, DEC-11-HR5B-D, p. 2-1, section 2.2.1, AC power distribution.
  7. ↑ 7.0 7.1 Digital Equipment Corporation, PDP-11/20 System Manual, DEC-11-HR1B-D, Table 2-7, p. 2-5 (File:PDP-11-20 System Manual (DEC-11-HR1B-D).pdf).
  8. ↑ DEC, DEC-11-HR5B-D, p. 2-2, section 2.3.1, switching mode regulation.
  9. ↑ DEC, DEC-11-HR5B-D, p. 2-5, section 2.4.1, R38.
  10. ↑ 10.0 10.1 DEC, DEC-11-HR2B-D, section 5.9, processor clock adjustment.
  11. ↑ 11.0 11.1 11.2 11.3 Digital Equipment Corporation, MM11-E Core Memory Manual, DEC-11-HR3A-D, first printing August 1970, chapter 3, Table 3-1 and sections 3.2.1 to 3.2.5, pp. 3-1 to 3-2 (File:MM11-E Core Memory Manual (DEC-11-HR3A-D).pdf).
  12. ↑ 12.0 12.1 12.2 DEC, DEC-11-HR2B-D, section 5.11, maintenance tips: diagnostic programs and the KM11 maintenance set.
  13. ↑ DEC, DEC-11-HR2B-D, section 5.2, Table 5-2, test equipment and tools.
  14. ↑ DEC, DEC-11-HR2B-D, section 5.4, module identification and layout, Figure 5-1.
  15. ↑ DEC, DEC-11-HR2B-D, section 5.10 and Figure 5-2, removal of the KA11 system unit and console.
  16. ↑ DEC, DEC-11-HR2B-D, section 5.7, multiple box systems.
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