Editing DECsystem-10 Power Supply Guide

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[[File:DECsystem-1090 KL10 and MH10 memory cabinets.jpg|thumb|right|340px|DECsystem-1090: KL10 processor cabinets and MH10 memory cabinets, each with its own power control and supplies (Retro-Computing Society of Rhode Island, CC BY-SA 3.0)]]
Every cabinet of a '''[[DECsystem-10]]''' has its own power control, bulk supplies and regulators, and the cabinets switch on and off together over DEC's power control bus. DEC described the supplies, their control and protection, and their adjustment in its ''KL10 Maintenance Guide'' (EK-OKL10-MG-003), its power system description for the KL10-C processor (EK-PWR20-SD-001) and its ''MH10 Maintenance Manual'' (EK-MH10-MM-003).<ref name="mgps" /><ref name="pwr" /><ref name="mh" /> The KL10-C description was written for the DECSYSTEM-20 model 2040, whose cabinets differ in packaging from the tall DECsystem-10 cabinets; the supply and regulator types are the same families that DEC's KL10 guide lists for all KL10-based systems.<ref name="pwr" /><ref name="mgps" /><ref name="wp" />

Warning: these systems run from three-phase mains at up to 60 A per phase, and the KL10 processor's ECL supply stores its energy in a 3.9 farad capacitor bank. Isolate the cabinet at its power control and at the wall, and let the capacitors discharge, before working inside.<ref name="pwr1" /><ref name="pwr3c" />

== DEC supply types and limits ==
DEC's field service guide gives these output limits, measured at the backplane of the option the supply powers:<ref name="mgps" />
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''General power supply specifications (KL10 Maintenance Guide, general information p. 10)'''
! Supply type !! Output !! Maximum !! Minimum !! Maximum ripple (mV)
|-
| 702 || Variable || – || – || 600
|-
| 703 || +10 / +1.8 / −3.0 V || +11.0 / +1.9 / −3.15 || +9.4 / +1.7 / −2.85 || 300 / – / –
|-
| 705 || +10 / −15 V || +11.0 / −16.5 || +9.4 / −14.5 || 300 / 700
|-
| 706 || +50 V || +54.0 || +49.0 || 1500
|-
| 723 || +8 V || +9.5 || +7.8 || 600
|-
| 725 || −15 V || −18.0 || −14.7 || 900
|-
| 728 || +10 / −15 V || +11.0 / −16.0 || +9.5 / −14.5 || 700 / 700
|-
| 732 || Variable || – || – || 600
|-
| 739 || +53 / +65 V || +55 / +65 || +52 / +63 || 250 / 250
|-
| 742 / 7420 || +25 / −15 / +3 V || +30 / −16.5 / +3.5 || +20 / −13.5 / +2.5 || –
|-
| 744 / 7440 || +5 V || +5.05 || +4.95 || 150
|-
| 745 || −15 V || −15.05 || −14.95 || 450
|-
| 754 || +20 / −5 V || +20.2 / −5.05 || +19.8 / −4.95 || 450 / 150
|-
| 761 || −2 / −5.2 V || colspan="2" | No limits given || –
|-
| 770 || +15 V || +15.05 || +14.95 || 450
|-
| 778 || −15 V || −16.5 || −14.5 || 700
|-
| 7131 / 7131A || +5 / +12 / −2 / −5.2 V || +5.07 / +12.18 / −2.03 / −5.28 || +4.93 / +11.82 / −1.97 / −5.12 || 50 / 100 / 50 / 50
|}
<ref name="mgps" />

== Power control ==
=== The power control bus ===
Each cabinet has a DEC power controller (the 863 and 861 in the KL10-C processor, the 857 in MH10 memory cabinets). They are chained by a three-wire DEC power control bus: connecting line 1 to ground (line 3) turns on every cabinet on the bus, and grounding line 2 (EMERGENCY SHUTDOWN) shuts every cabinet down.<ref name="mh4" /><ref name="pwr2" /> On the 857 the bus connects at J5, with J6 and J7 in parallel for the neighbouring cabinets, and a REMOTE/LOCAL switch must be at REMOTE for the cabinet to follow the processor's power switch.<ref name="mh4" /><ref name="mh2" />

=== KL10 processor ===
The KL10-C processor takes 120/208 V or 240/416 V three-phase five-wire power, within +6% and −12% of nominal and ±1 Hz, at up to 60 A per phase at 60 Hz (30 A at 50 Hz), about 21.6 kVA. DEC asks for a supply separate from lighting and air conditioning.<ref name="pwr1" /> The 863 power control applies the switched three-phase power; it lights three orange lamps for the incoming phases, and a red POWER LED on the switch panel shows switched power.<ref name="pwr2" />

Five seconds after the POWER ON switch is pressed, the 863 opens its normally closed CROBAR and POWER WARNING lines and the logic is released; at power-down CROBAR lags POWER WARNING by 5 ms. At least 13 seconds must pass between power-down and the next power-up.<ref name="pwr2" /> The 863 shuts the system down and holds it off for at least 13 seconds on any of eight faults, each shown by an LED on the 863 and by the FAULT lamp on the switch panel: insufficient air flow at four sensors, an over-temperature input (not used), a tripped circuit breaker to the CPU supply, and two cooling-assembly doors open.<ref name="pwr2" /> An override switch in the 863 lets maintenance staff power up through a fault; the POWER lamp then blinks.<ref name="pwr2" />

=== MH10 memory cabinets ===
The 857 in each MH10 starts the cabinet's supplies through contactor K1. Four air-flow switches in the cabinet set an over-temperature latch that powers the cabinet down, and interlock switches on the stack and logic door panels do the same if a door is opened with power on. OVERRIDE switch S1 bypasses the air-flow shutdown (the OVERTEMP lamp still lights) and S2 bypasses the door interlocks (the PWR SWD ON lamp then flashes as a warning). A recycle delay stops the cabinet powering up again less than four seconds after power-off.<ref name="mh4" />

== The KL10 ECL supply ==
The KL10 CPU's ECL logic runs from −5.2 V and −2.0 V. DEC builds them in three stages:<ref name="pwr2b" /><ref name="pwr3" />
* The H760 ferroresonant raw supply, with transformer primaries in three-phase delta and full-wave three-phase rectified outputs, gives −11 V at 490 A and ±15 V at 2 A. It tolerates line variation of +10% to −30%. Its ±15 V filter capacitors are C1, 50,000 µF, and C2, 30,000 µF. The H760A takes 120 V three-phase at 60 Hz and the H760B 240 V at 50 Hz, with different transformer tap connections and fan feeds.
* The capacitor assembly (DEC 7009878) sits between the H760 and the series pass assembly: thirteen 0.3 F capacitors in parallel, 3.9 F in all, at about −11 V. On power-off it discharges through a load of a fraction of an ohm, giving 20 ms before the voltage falls to about −7.5 V so the machine can save its state.
* The H761 regulated series pass assembly holds −5.2 V and −2.0 V with linear series-pass regulators of up to 35 A each, on G8010 (−5.2 V) and G8011 (−2.0 V) control cards with heat sinks, circuit breakers, overvoltage protection and current limiting. LEDs on the back of the control cards show that each output is present; DEC notes that apart from tripped breakers, LEDs off are the only sign that a regulator card has failed.
<ref name="pwr2b" /><ref name="pwr3" />

== H7420 supplies and regulators ==
The H7420 is DEC's multipurpose bulk supply: one transformer with seven separate 25 V AC (±20%) secondaries for regulators, +8 V at 1 A and +15 V ±1.5 V at 3 A from its power control board, DC LO sensing and a 115 V convenience outlet. The H7420-A takes 120 V and the H7420-B 240 V; the jumpering for 115 V or 230 V is printed on the supply's label.<ref name="pwrt" /><ref name="mh4" /><ref name="mh2" /> The regulators plug into it:
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''Regulators (KL10-C power system description, Table 1-3)'''
! Regulator !! Output !! Rating !! Input fuse !! Notes
|-
| H744 || +5 V ±60 mV (±50 mV in the 1985 field guide) || 25 A, 150 mV ripple || 15 A || Ripple (switching) regulator; current limit at about 30–35 A; crowbar SCR fires above about +6.0 V
|-
| H745 || −15 V ±50 mV || 10 A, 450 mV ripple || 15 A || Used on the DIA20 and DMA20 in the KL10-C I/O cabinet
|-
| H754 || +20 V and −5 V, ±5% || 8 A at +20 V, 1 A at −5 V || 10 A || Shunt-regulated pair from a 25 V supply; overcurrent limit about 10 A; crowbar on each output
|-
| H770 || +15 V || – || – || Powers the air-flow sensors; switched on only after the fans are up to speed
|}
<ref name="pwrt" /><ref name="mh4" /><ref name="pwr2" />

The H744 and H754 crowbars short the output to ground when they fire and stay on until the capacitors have discharged, so a crowbarred regulator reads 0 V until power is cycled.<ref name="mh4" />

=== MH10 power system ===
DEC's MH10 manual lists one 857 power control, three H7420 supplies, H744 +5 V regulators, four H754 +20/−5 V regulators and one H770 +15 V regulator. The manual gives the number of H744s as nine in its component list and seven in the regulator section.<ref name="mh4" /> Single-phase 115 V or 220 V feeds the 857, which has a rotary input-voltage selector; the top cooling fans run from the 857 and the others from 115 V outputs of the H7420s.<ref name="mh4" />

== Setting the voltages ==
DEC's MH10 procedure, with power on:<ref name="mh5" />
# Find the regulators and backplane power tabs on the power wiring diagram D-IC-MH10-0-PW.
# Set +5 V at the backplane tabs with the potentiometer on top of each H744.
# Set +20 V with R17 on the side of each H754.
# Set −5 V with R21 on the top of each H754.
# Repeat the last two steps until neither needs further adjustment.
All three must be within ±5% at the backplane, and DEC asks for adjustments to be made at 20–30 °C ambient.<ref name="mh5" /> R17 sets the full 25 V span between +20 V and −5 V and R21 then sets −5 V to ground, which is why the two interact. For the H754 on its own, DEC's method is to power down, disconnect the load, power up, set 25 V across the outputs with R17 and −5 V to ground with R21, then reconnect the load and check again.<ref name="mh4" />

== Capacitors ==
DEC's power documentation gives values for the largest capacitors: the H760's 50,000 µF and 30,000 µF ±15 V filters and the thirteen 0.3 F capacitors of the ECL capacitor assembly.<ref name="pwr3" /> The regulator and supply component values are on DEC's schematics, which the power system description lists by drawing number (for example D-CS-H744-0-1, D-CS-H754-0-1, D-CS-H7420-0-1 and D-CS-7009878-0-1 for the capacitor assembly) as part of the field maintenance print set.<ref name="pwrt" /> Bitsavers holds drawings and illustrated parts breakdowns for several of these units.<ref name="bitsps" />

== Related pages ==
* [[DECsystem-10]]
* [[DECsystem-10 General Maintenance]]
* [[DECsystem-10 Troubleshooting Guide]]
* [[DECsystem-10 Documentation]]
* [[Capacitor Failure Symptoms]]

== References ==
<references>
<ref name="mgps">Digital Equipment Corporation, ''KL10 Maintenance Guide, Volume I'', EK-OKL10-MG-003, April 1985, general information p. 10, "General Power Supply Specifications" ([[:File:KL10 Maintenance Guide Volume 1 (EK-OKL10-MG-003).pdf]]).</ref>
<ref name="pwr">Digital Equipment Corporation, ''DECSYSTEM-20 Power System, System Description'', EK-PWR20-SD-001, April 1976, foreword ([[:File:KL10-C Power System Description (EK-PWR20-SD-001).pdf]]). Scope: the KL10-C in 2040 configurations.</ref>
<ref name="pwr1">DEC, EK-PWR20-SD-001, pp. PWR20/1-4 and 1-9, Table 1-2. KL10-C electrical requirements.</ref>
<ref name="pwrt">DEC, EK-PWR20-SD-001, pp. PWR20/1-3 and 1-10 to 1-15, Tables 1-1, 1-3 and 1-4. Supplies, regulators, ratings and reference drawings.</ref>
<ref name="pwr2">DEC, EK-PWR20-SD-001, pp. PWR20/2-1 to 2-12. Block diagram, CROBAR and POWER WARNING timing, fault conditions, switch panel, power-fail protection and restart.</ref>
<ref name="pwr2b">DEC, EK-PWR20-SD-001, pp. PWR20/2-1 to 2-3. H760, capacitor assembly and H761.</ref>
<ref name="pwr3">DEC, EK-PWR20-SD-001, pp. PWR20/3-74 to 3-78. H760 power supply and capacitor assembly.</ref>
<ref name="pwr3c">DEC, EK-PWR20-SD-001, p. PWR20/3-78. Capacitor assembly discharge.</ref>
<ref name="mh">Digital Equipment Corporation, ''MH10 Maintenance Manual'', EK-MH10-MM-003, August 1977 ([[:File:MH10 Core Memory Maintenance Manual (EK-MH10-MM-003).pdf]]).</ref>
<ref name="mh2">DEC, EK-MH10-MM-003, pp. 2-2 to 2-3. AC power, line voltage jumpers, 857 power control interconnection.</ref>
<ref name="mh4">DEC, EK-MH10-MM-003, pp. 4-34 to 4-40, section 4.4.5. MH10 power system, 857 power control, air-flow and door interlocks, H7420, H744, H754 and H770.</ref>
<ref name="mh5">DEC, EK-MH10-MM-003, pp. 5-1 to 5-3, sections 5.3 and 5.4.1. Voltage measurements and adjustment procedure.</ref>
<ref name="wp">"PDP-10", ''Wikipedia'', [https://en.wikipedia.org/wiki/PDP-10 en.wikipedia.org/wiki/PDP-10]. Packaging of the 10xx and 20xx KL10 systems.</ref>
<ref name="bitsps">Directory listing, [http://bitsavers.org/pdf/dec/powerSupply/ bitsavers.org/pdf/dec/powerSupply/], retrieved 2 October 2026: H742, H744, H745 and H754 engineering drawings and parts breakdowns, 861 power controller manual.</ref>
</references>

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