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PDP-8/m Capacitor Replacement Guide

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DEC assembly drawing and parts list E-IA-5409728-0-0 for the H740 regulator board (PDP-8/M Engineering Drawings, November 1973)

The PDP-8/m and PDP-8/F take all their DC power from the H740 supply, and almost all of the H740's capacitors are on one part, the regulator board assembly 54-09728 (DEC drawing E-IA-5409728-0-0).[1][2] DEC's parts list for that board gives a value for every capacitor, so the list below is complete for the revision drawn. The PDP-8/e uses the H724 instead; its capacitors are listed in PDP-8/e Power Supply Guide.

Warning: input capacitors C1 and C2 are 24,000 µF each. DEC tells servicers to unplug the supply and discharge them before working on it, and suggests a 10 to 100 Ω, 10 W resistor to speed the discharge.[3]

Capacitor list

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H740 regulator board 5409728 capacitors (DEC parts list E-IA-5409728-0-0)
Reference Value Voltage Tolerance and type as listed DEC part Circuit (from DEC's description)
C1, C2 24,000 µF 50 V −10 % / +75 % 1010702 Input capacitors on the rectified 28 V AC
C7 6,000 µF 10 V −10 % / +75 %, "S.TANT" 1010704 +5 V output smoothing
C14 3,000 µF 25 V −10 % / +75 %, "S.TANT" 1010703 −15 V output
C9 20 µF 60 V 5 %, tantalum 1010716 Voltage monitor filter (AC LO and DC LO)
C16 100 µF 20 V solid tantalum 1004815 +15 V output
C5, C17 6.8 µF 35 V 10 %, solid tantalum 1005306 C5 in the +5 V regulator
C12, C19 2.2 µF 35 V 10 %, solid tantalum 1002431 C12 in the −15 V regulator, C19 in the +15 V regulator
C3, C6, C10, C13 1 µF 100 V 10 % 1011032 Regulator circuits
C8, C15 0.22 µF 50 V – 1010274 Regulator circuits
C4, C11 0.033 µF 100 V 10 %, Mylar 1000050 Regulator circuits
C20 2,200 pF 250 V ceramic 1000055 –
C18 1,000 pF 100 V dipped mica 1000042 –

[1][4][5] DEC's specification for the H740 calls for computer-grade electrolytics, "Type 36D or better".[6] The parts list's abbreviation for C7 and C14 reads "S.TANT", the same as for the small solid tantalum parts; check the parts on your board before ordering. The circuit column follows DEC's circuit description and troubleshooting chart: C1 and C2 as the input capacitors, C7 as the +5 V output smoothing capacitor, C9 on the rectified 28 V AC for the voltage monitor, C16 on the +15 V output, and C5, C12, C14 and C19 as parts whose short drops the +5 V, −15 V or +15 V output.[4][3][5]

Before you start

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  • The board drawn is regulator board 54-09728 at etch revision E. Load ratings in DEC's H740 specification differ between revisions (B1, C, D and YA), so record your board's revision.[1][7]
  • DEC's parts list also covers the heat sink, the thermostat S1 (opens at 98 to 105 °C, resets at 56 to 69 °C), fuses F1 (15 A) and F2 (10 A), chokes L1 (100 µH 20 A) and L2 (100 µH 10 A) and the power semiconductors.[1][7]

Procedure

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DEC's rules for working on the H740 regulator module:[3]

  1. Turn the power off, unplug the supply and discharge C1 and C2.
  2. The regulator module lifts out of the top of the power chassis on six screws, with the chassis still bolted into the computer.
  3. The DC regulator module is not grounded to the chassis inside, so shorts to ground can be found with the DC output cable disconnected.
  4. The output fuses F1 and F2 are soldered in and can be replaced without removing the module.
  5. Tighten all hardware, including the capacitor, choke and semiconductor fixings, to about 12 inch-pounds, using identical replacement hardware.
  6. Where a power semiconductor mounts on the heat sink, apply a thin coat of Wakefield 128 compound or Dow silicone grease. DEC says insulating wafers are not needed.

Fit replacements of the same value and at least the same voltage rating.

After the recap

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  1. Load the supply and measure at the end of the DC cable with a calibrated meter, preferably digital.[3]
  2. Set the outputs to their centre values: +15.0 V, +5.0 V and −15.0 V under load. Turn the adjustments slowly and never beyond 105 % of nominal, or the overvoltage crowbar will fire and blow the output fuses.[3] The adjustment potentiometers are R26, R35 and R50 (100 Ω); R35 is part of the +15 V sampling divider with R34 and R36.[1][4]
  3. Check ripple. A 200 mV peak-to-peak, 10 kHz sawtooth on +5 V is normal for this switching regulator.[4] DEC's specification allows 4 % peak-to-peak ripple and noise on +5 V and 3 % on −15 V.[7]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Digital Equipment Corporation, drawing E-IA-5409728-0-0, "Regulator Board", assembly and parts list (etch board revision E), in PDP-8/M Engineering Drawings, November 1973, PDF p. 30 (File:PDP-8M Engineering Drawings (November 1973).pdf; File:DEC H740 regulator board 5409728 assembly drawing.png).
  2. ↑ Digital Equipment Corporation, PDP-8/E, PDP-8/F, PDP-8/M Maintenance Manual, Volume 1: Processor, DEC-8E-HMM1A-D-D, September 1973, pp. 5-5 to 5-6, Tables 5-5 and 5-6 (File:PDP-8E Maintenance Manual Volume 1 (DEC-8E-HMM1A-D-D).pdf). Regulator board assembly 54-09728.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 DEC, DEC-8E-HMM1A-D-D, p. 4-26, section 4.9.1, H740 troubleshooting rules and precautions.
  4. ↑ 4.0 4.1 4.2 4.3 DEC, DEC-8E-HMM1A-D-D, pp. 3-177 to 3-181, section 3.48, H740 circuit description and Figures 3-128 to 3-132.
  5. ↑ 5.0 5.1 DEC, DEC-8E-HMM1A-D-D, p. 4-27, Table 4-10, regulator module troubleshooting chart.
  6. ↑ DEC, SP-5409728-0-8, sheet 18, section 5.1, component selection.
  7. ↑ 7.0 7.1 7.2 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).
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