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PDP-8/m Troubleshooting

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PDP-8/M with a paper tape reader at VCF Berlin 2019 (stiefkind, CC0)

Fault-finding on the PDP-8/m and PDP-8/F. The processor, memory and Teletype logic are the PDP-8/E's, and DEC's tables for them are on PDP-8/e Troubleshooting. The main difference is the H740 switching power supply.[1][2] DEC's own advice is that the most likely source of an H740 fault is the DC regulator module, and that the quickest remedy is to replace it; the fault can also be a short in the system unit or in the AC input circuit.[3]

Rules for working on the H740

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DEC's precautions:[4]

  1. Never adjust an output beyond 105 % of nominal, and adjust slowly: an overvoltage crowbar fires and blows the DC output fuses.
  2. Use a calibrated voltmeter, preferably digital, and set the outputs to +15.0, +5.0 and −15.0 V under load at the end of the DC cable.
  3. Turn the power off and unplug the supply before servicing it, and discharge input capacitors C1 and C2 (a 10 to 100 Ω, 10 W resistor speeds this up).
  4. The regulator module is not grounded to the chassis internally, so shorts to ground can be found by disconnecting the DC output cable.
  5. Output fuses F1 and F2 are soldered to the board and can be changed without removing the module.

Regulator module faults

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Regulator module troubleshooting chart (DEC-8E-HMM1A, Table 4-10)
Problem Causes listed by DEC
No +5 V and no +15 V F1 open; D14 (bridge rectifier) or transformer open; +5 V adjusted too high so the crowbar has fired
+5 V too low Q5, D9, Q10, Q9, Q11, D12 or D10 shorted; C5 or C7 shorted; R49, R50, R46 or R44 open; Q6, Q7, Q8 or D11 shorted; Q9 (printed "A9"), Q10 or D9 open; R51 or R50 open
+15 V too high Q1 shorted; D8 open; R35 or R36 open
+15 V too low Q3, Q4, Q5 or D8 shorted; R56, R35 or R34 open; C19 shorted
−15 V too low F2 open; D14 or transformer open; Q25, D4, Q26, Q21, Q27, D7 or D5 shorted; C14 or C12 shorted; R22, R26, R25 or R29 open; Q22, Q23, Q24 or D6 shorted; Q25, Q26 or D4 open; R26 or R27 open; −15 V adjusted too high
AC LO L will not go high Q13, Q14 or Q15 shorted; Q16 or D3 open; R7, R3, R6 or R8 open; C9 shorted
AC LO L will not go low, or erratic at power-on and power-off Q13, Q14 or Q16 open; Q15 or D3 shorted; R12, R13, R7 or R10 open

[5] The component references are those of the regulator board parts list.[6]

How the H740 behaves

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Switching ripple

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The +5 V regulator is a power Schmitt trigger switching at about 10 kHz; a 200 mV peak-to-peak sawtooth on +5 V and a 30 V rectangular wave across the commutating diode D10 are expected.[7]

Current limits and crowbars

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The +15 V supply limits at about 1.5 A (1.3 to 1.7 A in DEC's specification); the +5 V supply shuts its regulator down on overcurrent.[7][8] DEC's circuit description gives the +5 V crowbar as firing above 6.3 V, through D12 and SCR Q11, blowing F1; the specification gives 5.7 to 6.8 V. The −15 V crowbar fires at 17.5 to 20.5 V.[7][8]

Thermostat

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A thermostat on the regulator board heat sink opens the transformer and fan power when it overheats and closes again by itself once it cools. DEC's manual gives 100 °C to open and about 64 °C to close; the specification gives 98 to 105 °C and 56 to 69 °C.[7][8]

Power-fail signals

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AC LO and DC LO are derived from C9, charged from the 28 V AC through D1 and D2. In DEC's specification DC LO goes high at 74 to 80 V AC input and AC LO 8 to 11 V higher; on falling mains AC LO drops at 80 to 86 V and DC LO 7 to 10 V lower, and the outputs stay in regulation down to 70 V.[7][8] If the +5 V or +15 V output drops, POWER OK is negated and the timing generator halts.[7]

Other 8/F and 8/M points

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  • The KP8-E power-fail and auto-restart option as first sold did not work with the PDP-8/F supply.[2]
  • An 8/E KC8-EA console will not light in an 8/F or 8/M: the H740 has no +8 V lamp supply.[2]
  • A PDP-8/M with only the minimal operator's panel needs the KP8-E or the MI8-E bootstrap loader to start a program; without either it cannot be started from the panel.[9]
  • For processor, memory and Teletype faults use DEC's tables on PDP-8/e Troubleshooting. The first-level spares for the 8/F and 8/M are the same modules as the 8/E's (M8300, M8310, M8330, G104, G227) plus LEDs, switches and the 54-09728 regulator board.[10]
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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. 3-172 (File:PDP-8E Maintenance Manual Volume 1 (DEC-8E-HMM1A-D-D).pdf).
  2. ↑ 2.0 2.1 2.2 Douglas W. Jones, "What is a PDP-8/F?", PDP-8 Summary of Models and Options, faqs.org.
  3. ↑ DEC, DEC-8E-HMM1A-D-D, p. 4-26, section 4.9.2, troubleshooting chart.
  4. ↑ DEC, DEC-8E-HMM1A-D-D, p. 4-26, section 4.9.1, troubleshooting rules and precautions.
  5. ↑ DEC, DEC-8E-HMM1A-D-D, p. 4-27, Table 4-10, regulator module troubleshooting chart.
  6. ↑ Digital Equipment Corporation, drawing E-IA-5409728-0-0, "Regulator Board", in PDP-8/M Engineering Drawings, November 1973, PDF p. 30 (File:PDP-8M Engineering Drawings (November 1973).pdf).
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 DEC, DEC-8E-HMM1A-D-D, pp. 3-177 to 3-181, section 3.48, PDP-8/F and PDP-8/M power supply.
  8. ↑ 8.0 8.1 8.2 8.3 Digital Equipment Corporation, engineering specification SP-5409728-0-8, "Power Supply Specification – H740", sheets 8 to 13, in H740 Power Supply Engineering Drawings, revision H (File:H740 Power Supply Engineering Drawings (Rev H, 1974).pdf).
  9. ↑ Douglas W. Jones, "What is a PDP-8/M?", faqs.org.
  10. ↑ DEC, DEC-8E-HMM1A-D-D, pp. 5-5 to 5-6, Tables 5-5 and 5-6.
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