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PDP-8/e Power Supply Guide

From RetroTechCollection
A PDP-8/E power supply opened during restoration, fans removed (Tony Duell, CC BY 2.0)

The PDP-8/e runs from a DEC H724 linear power supply, or the H724A on 230 V mains.[1] It gives three regulated outputs (+5 V, −15 V and +15 V), an unregulated +8 V for the console lamps and a centre-tapped 28 V AC winding.[1] DEC describes the circuit in section 3.47 of the PDP-8/E, PDP-8/F, PDP-8/M Maintenance Manual, Volume 1 and gives troubleshooting data in section 4.8; its Illustrated Parts Breakdown lists every capacitor with its DEC part number.[2][3][4] The PDP-8/F and PDP-8/m use the different H740 switching supply, covered in PDP-8/m Capacitor Replacement Guide.[1]

Warning: DEC marks the metal terminals on the front of the H724 as carrying high voltage, and the filter capacitors are tens of thousands of microfarads each.[5][4] Turn off circuit breaker CB1 and unplug the computer before removing the supply.[6]

Outputs

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H724 parameters (DEC-8E-HMM1A, Tables 4-7 and 4-8)
Output Wire Limits Rating Ripple (max.) Fuse Adjust Feeds
+5 V Red 4.85–5.15 V (±3 %) 20 A 50 mV p-p F200, 25 A R21 (A2) CPU logic
−15 V Blue −14.25 to −15.75 V (±5 %) 8 A 50 mV p-p F300, 10 A R5 (A1) Memory
+15 V Orange 13.5–16.5 V (±10 %) 1 A 75 mV p-p F100, 1.5 A Tracks R5 Bus loads
+8 V (unregulated) Yellow 6–10 V 2 A – F150, 2.5 A – Console lamps
14 V AC – ±26 % 0.2 A – 0.5 A – Options
DC volts OK Grey 3.75–5 V – – – R29 (factory only) Power-fail logic

[7][8][9] Table 4-8 prints the +8 V fuse as 25 A; the circuit description and the parts breakdown both give F150 as 2.5 A, and the parts breakdown gives F100 as 1.5 A where Table 4-8 says 1 A.[8][2][9] The +15 V regulator has no adjustment of its own: its error amplifier is referenced to the −15 V output, so R5 sets both.[2]

How it works

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  • Relay K1, a 24 V DC relay controlled by the front-panel key switch through the interlock circuit, switches both sides of the mains to transformer T1. Thermal switch S1 opens the interlock at 90 °C ±5 °C and must be reset by hand.[2][3]
  • The +8 V lamp supply is a full-wave centre-tapped rectifier with no filter or regulator, fused at 2.5 A.[2]
  • The +15 V supply uses series pass transistor Q100, driven by emitter follower Q1 from the Q2/Q3 difference amplifier.[2]
  • In the −15 V supply a 723 regulator IC (VR1) drives Q300 and four parallel pass transistors Q301 to Q304, each with a 0.25 Ω 3 W emitter resistor to share the current.[2]
  • In the +5 V supply a 723 regulator IC (VR5) drives Q200 and the parallel pass transistors Q201 to Q206. An overvoltage circuit fires SCR Q207 if the output reaches 6.5 V, shorting the output and blowing F200.[2]
  • The voltage monitor negates POWER OK whenever +5 V falls below 4.3 V, +15 V below 12 V or −15 V above −13.5 V, which halts the timing generator.[2]

DEC also specifies at least 2 ms hold-up at full load after loss of mains, forbids loading any output beyond 175 % of its rating, and forbids paralleling supplies.[3]

Capacitor list

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From the H724/H724A power supply assembly in DEC's parts breakdown (figure 4). DEC's maintenance manual gives the same groups by circuit (C100 and C101 in the +15 V supply, C200 to C203 in the +5 V supply, C300 to C302 in the −15 V supply).[4][2]

H724 capacitors (DEC-PDP8E-IPB-1, figure 4, items 20, 39, 77, 97 to 100)
Reference Value Voltage DEC part Circuit
C100 6,000 µF 40 V 10-10186-00 +15 V raw filter
C101 10,800 µF 20 V 10-10185-00 +15 V output
C200, C201, C202, C203 30,000 µF 25 V 10-10184-00 +5 V raw filter
C204 1 µF 35 V 10-01776-00 +5 V circuit (listed with the SCR mounting hardware)
C205 18,000 µF 10 V 10-10197-00 +5 V output
C300, C301 14,000 µF 40 V 10-10187-00 −15 V raw filter
C302 10,800 µF 20 V 10-10185-00 −15 V output
C500, C501 0.1 µF 1,000 V 10-02153-00 (replacing 10-10193-00) Mains input

[4][2] The parts breakdown marks the C500/C501 part number change against ECO 00002. The two control boards carry only small capacitors: on the A1 board C1 and C3 (0.01 µF) and C2 (1.0 µF 35 V), and on the A2 board C10 and C11 (0.01 µF) and C12 (1.0 µF, listed at 25 V on etch revision D and 35 V on revision E); later etch revisions add more 0.01 µF positions.[10]

Removing the supply

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DEC's procedure for getting at the heat sink assembly:[6]

  1. Turn off CB1 and unplug the mains lead. Take a rack-mounted machine out of the rack and put it on a table.
  2. Remove the five Phillips screws holding the supply: two at the front of the chassis, one at the rear and two at the back.
  3. Unplug the OMNIBUS power harness and the switch power harness.
  4. Lift the supply and slide it back just far enough to unplug the blue and yellow wires from the front panel, then lift it out.
  5. Remove the protective screen from the side of the supply (12 countersunk screws).
  6. Remove the nylon plug and six screws from the heat sink bracket and lift the heat sink assembly out.

On refitting, the yellow wire goes to the top tab terminal of the front panel.[6]

Testing

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  1. Measure the outputs at their connectors with the load connected, against the table above. DEC treats any output out of tolerance as a defective supply.[11]
  2. Check ripple with an oscilloscope against the same table.[7]
  3. Adjust −15 V (and with it +15 V) with R5 on A1, and +5 V with R21 on A2. Leave R29 alone; DEC marks it factory-adjustable only.[8]
  4. Set these two outputs before working on memory faults: the memory's inhibit current varies in proportion to −15 V, and its slice level in proportion to +5 V.[12]

DEC gives the static resistance of the supply's thermistor as about 23 Ω.[8]

Restoration notes

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Tony Duell photographed a full restoration of a PDP-8/E system, including the CPU supply dismantled, its control boards, regulated and unregulated tests on the bench, and the fans cleaned and refitted.[13] David Gesswein's H724 page gives the same outputs and fuse ratings as DEC.[14]

The A1 and A2 control boards of a PDP-8/E supply (Tony Duell, CC BY 2.0)
DEC's location drawing of the H724: filter capacitors at left, heat sink with Q100, Q200 and Q300, A1 and A2 boards with R5, R21 and R29, and the thermal cut-out (DEC-8E-HMM1A, Figure 4-7)
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References

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  1. ↑ 1.0 1.1 1.2 Digital Equipment Corporation, PDP-8/E, PDP-8/F, PDP-8/M Maintenance Manual, Volume 1: Processor, DEC-8E-HMM1A-D-D, September 1973, p. 3-172, section 8, "Power Supplies" (File:PDP-8E Maintenance Manual Volume 1 (DEC-8E-HMM1A-D-D).pdf).
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 DEC, DEC-8E-HMM1A-D-D, pp. 3-172 to 3-176, section 3.47, "PDP-8/E Power Supply", Figures 3-122 to 3-127.
  3. ↑ 3.0 3.1 3.2 DEC, DEC-8E-HMM1A-D-D, pp. 4-25 to 4-26, sections 4.8.1 to 4.8.8.
  4. ↑ 4.0 4.1 4.2 4.3 Digital Equipment Corporation, Illustrated Parts Breakdown, PDP8E 12 Bit Computer, DEC-PDP8E-IPB-1, figure 4, H724/H724A power supply assembly, items 20, 39, 77 and 97 to 100 (File:PDP-8E Illustrated Parts Breakdown (DEC-PDP8E-IPB-1).pdf).
  5. ↑ DEC, DEC-8E-HMM1A-D-D, p. 4-24, Figure 4-7 (File:DEC H724 power supply component locations (Figure 4-7).png).
  6. ↑ 6.0 6.1 6.2 DEC, DEC-8E-HMM1A-D-D, pp. 4-13 to 4-14, section 4.5.5, heat sink assembly removal.
  7. ↑ 7.0 7.1 DEC, DEC-8E-HMM1A-D-D, p. 4-23, section 4.8 and Table 4-7, H724 power supply parameters.
  8. ↑ 8.0 8.1 8.2 8.3 DEC, DEC-8E-HMM1A-D-D, p. 4-24, Table 4-8, component troubleshooting aid.
  9. ↑ 9.0 9.1 DEC, DEC-PDP8E-IPB-1, figure 4, items 24 to 30, fuses.
  10. ↑ DEC, DEC-PDP8E-IPB-1, figures 5A to 5C (A1 control board) and 6A to 6C (A2 control board).
  11. ↑ DEC, DEC-8E-HMM1A-D-D, pp. 4-5 to 4-6, Table 4-2, processor preventive maintenance schedule.
  12. ↑ DEC, DEC-8E-HMM1A-D-D, pp. 4-20 to 4-22, sections 4.7.5 to 4.7.8.
  13. ↑ Tony Duell, PDP-8/E restoration photographs, Wikimedia Commons category "Detailed views of disassembled DEC PDP-8/E" (from Flickr, CC BY 2.0).
  14. ↑ David Gesswein, "PDP-8 h724 Power Supply Information", pdp8online.com.
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