Editing VAX-11/780 Power System Service

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[[File:DEC VAX-11 780 cabinet open, card cage and power supplies.jpg|thumb|right|320px|VAX-11/780 with the doors open; the H7100 supplies sit in a row below the CPU card cage (Patrick Finnegan, CC BY 2.0)]]
The power system of the '''[[VAX-11/780]]''': the cabinet power controllers and the power control bus, the H7100 DC supplies and their regulators, and the battery backup units. It is drawn from DEC's ''VAX-11/780 Power System Technical Description'' (EK-PS780-TD-001), the H7100 print set (MP-00480) and DEC's field service troubleshooting guide.<ref name="ps11" /><ref name="mp" /><ref name="ts713" />

Warning: the main cabinet takes three-phase mains at 120/208 V or 240/416 V and up to 150 A per phase at switch-on.<ref name="ps21" /> On systems with 866 or 861 power controllers, one phase is unswitched: it keeps feeding the H7112 memory battery backup, the memory power supply, the H7111 clock battery supply and one blower even after an emergency shutdown on the power control bus.<ref name="ps319" /> Disconnect each cabinet at the wall before working inside it.

== Power controllers and the power control bus ==
Each cabinet has its own power controller, fed from its own customer receptacle.<ref name="ps11" />
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+ '''VAX-11/780 power controllers (DEC section 1.3)'''
! Cabinet !! Shipped before !! Later
|-
| Main and SBI cabinets, 120/208 V || 866D (before June 1978) || 869D
|-
| Main and SBI cabinets, 240/416 V || 866E || 869E
|-
| Unibus expansion, 120 V || 861C (before September 1978) || 869C
|-
| Unibus expansion, 240 V || 861B || 869B
|}
<ref name="ps11" />

The controllers are chained by the three-wire power control bus, which meets DEC Standard 123: pin 1 power request, pin 2 emergency shutdown, pin 3 ground. Connecting line 1 to ground energises the controller's contactor and switches on its outlets; connecting line 2 to ground disconnects all of its switched outlets. With the controller's LOCAL/OFF/REMOTE switch at LOCAL, lines 1 and 3 are joined.<ref name="ps24" /><ref name="ps319" /> The whole system is switched from the five-position key switch at the right of the main cabinet's control panel, which also carries ATTN, RUN, POWER and REMOTE lamps and an AUTO RESTART switch.<ref name="ps32" />

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''869D and 869E controllers (DEC Table 2-2)'''
! Item !! 869D !! 869E
|-
| Input || 3-phase, 90–132 V phase to neutral || 3-phase, 180–264 V phase to neutral
|-
| Current || 24 A per phase || 12 A per phase
|-
| Circuit breaker || 30 A per pole, 4 poles || 15 A per pole, 4 poles
|-
| Outlets || 9 duplex, 120 V 20 A, J11–J19 || 9 duplex, 240 V 15 A, J11–J19
|-
| Overtemperature switch || Opens the contactor at 71 °C, resets at 49 °C || Same
|}
<ref name="ps23" /><ref name="ps24" />

DEC's caution is that no load outside the cabinet is to be plugged into a controller.<ref name="ps24" /> On an 866, the overtemperature sensor shuts down the switched outlets only; on an 869 it shuts down the whole cabinet, independently of the power control bus, while other cabinets keep running.<ref name="ps387" /> Each blower in the main cabinet, normally three, has its own air flow sensor.<ref name="ps388b" />

== H7100 DC power supplies ==
Up to six H7100 supplies fit in the main cabinet. A minimum "C" configuration uses three, a "D" configuration four, and an FP780 floating point accelerator adds one more.<ref name="ps388" /> Each supply has a thermal sensor on its output transistor heat sink.<ref name="ps319" />

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''H7100 specifications (DEC Table 2-6)'''
! Item !! Specification
|-
| Input || Single phase, 90–128 V or 180–256 V rms, 47–63 Hz; 10 A at 120 V, 5 A at 240 V; 1200 VA
|-
| Inrush || 80 A peak for half a cycle
|-
| Input protection || 2-pole circuit breaker, 20 A per phase
|-
| Output power || 500 W
|-
| Base supply || +5.1 V at 10–100 A
|-
| H7101 regulator option || −5.2 V at 3–30 A
|-
| H7102 regulator option || +12 V at 1–10 A
|-
| H7103 regulator option || +5.0 V at 2–20 A and −5.0 V at 0–0.2 A (battery-backed)
|-
| Regulation || ±5% on all outputs
|-
| Cooling || External forced air at 2.22 m/s
|-
| Size and weight || 17.8 × 27.9 × 40.6 cm, 15.9 kg
|}
<ref name="ps213" />

=== Status indicators ===
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+ '''H7100 front-panel indicators'''
! Indicator !! Meaning
|-
| POWER NORMAL (green) || Outputs correct
|-
| PLUG-IN REGULATOR FAILURE (red) || Fault in one or more of the plug-in regulators: +5 V, +5 VB or −5 VB, +12 V
|-
| OVERCURRENT (red) || +5 V load of 120 A or more
|-
| OVERVOLTAGE (red) || +5 V at 6.2 V or more
|-
| POWER INVERTER FAILURE (red) || Main +5 V inverter failed
|-
| OVERTEMPERATURE (yellow reflective flag) || Internal temperature of 150 °F (66 °C) or more
|}
<ref name="ps215" /><ref name="ts713" />

=== Replacing supplies and regulators ===
DEC's field kit held a configurable H7100 (the base supply with its bias/control board, giving +5.1 V), an H7101 −5.2 V regulator board (5412560) and an H7102/H7103 +12 V and ±5 V battery-backed regulator board (5412556/5412558). DEC's guidelines:<ref name="psb" />
# A failed regulator board is replaced with the board from the kit.
# A failed base supply is replaced with the kit's H7100.
# If the base supply has failed but its regulators are good, the regulators are moved into the new H7100: remove the blank J6 or J7 from the new supply, reconnect the regulators with the old supply's cables, fit J6 or J7 in the rear panel cut-out, reuse the old insulator boards and fit the decal that shows the regulator complement (3614762-00 for an H7101, 3614762-01 for H7102 and H7103).
The cables were ordered separately: 7014545 (12-wire output, H7101), 7014546 (output harness, H7102/H7103), 7014548 (7-wire input, H7103) and insulator board 1700021-04.<ref name="psb" />

=== Board electrolytics ===
The H7100 print set lists these aluminium electrolytics on the supply's three circuit boards:<ref name="mp" />
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''H7100 circuit board electrolytics (print set MP-00480)'''
! Board !! Reference !! Value !! Rating !! DEC part
|-
| Motherboard 54-12550 (rev. H) || C5, C6 || 2800 µF || 55 V || 1014337-00
|-
| Bias/control 54-12554 (rev. M) || C5–C8 || 560 µF || 20 V || 1012607-00
|-
| Bias/control 54-12554 (rev. M) || C11 || 600 µF || 25 V || 1011849-01
|-
| Status card 54-12562 (rev. C) || C1, C2 || 8 µF || 25 V || 1012084-01
|-
| Status card 54-12562 (rev. C) || C4 || 25 µF || 25 V || 1000075-00
|}
<ref name="mp29" /><ref name="mp35" /><ref name="mp20" />

The bias/control board also has solid tantalums (1 µF 35 V at C10, C49, C104 and C105, among others).<ref name="mp35" /> Read the values printed on any capacitor mounted off these boards before ordering.

== Battery backup units ==
Both battery units use sealed lead-acid batteries, DEC 12-12499-00. DEC gives the H7111 battery's shelf life as 40 days at 70 °C and 6000 days at 0 °C.<ref name="ps219" /><ref name="ps220" />
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''Battery backup supplies (DEC Tables 2-8 and 2-9)'''
! Item !! H7111 time-of-day clock supply !! H7112 memory battery backup
|-
| Input || 90–128 V (H7111A) or 180–256 V (H7111B) || 90–128 V or 180–256 V, 50 or 60 Hz
|-
| Input fuse || 1/8 A slow-blow || 1.0 A (120 V) or 0.5 A (240 V) slow-blow
|-
| Output || +5 V, 0–25 mA || 27–45 V DC, 250 W
|-
| Batteries || One, 5 Ah, 2 A fuse || Three in series (30–48 V), 15 A fuse
|-
| Charge time || Under 24 hours to 90% || 16 hours maximum to 90%
|-
| Backup time || 100 hours minimum || 10 minutes minimum
|}
<ref name="ps219" /><ref name="ps220" />

== Power-fail signals ==
The power monitoring signals of the console's H780 supply and of H7100 supplies 1, 2 and 3 are ORed into the LSI-11 console's BPOK and BDCOK lines. DEC's troubleshooting guide says the H7100 AC LO and DC LO signals should sit at −9.5 V or more negative for proper noise immunity; noisy or marginal levels cause false power-fail traps in the console.<ref name="ts1142" /> See [[VAX-11/780 Troubleshooting Guide]].

== Restoration experience ==
A German museum restoring a VAX-11/780-compatible Robotron K 1840 had its power supplies repaired before the machine could be switched on, and checked every board for shorts before applying power. Another owner planned to dismantle and test his five supplies one at a time, and built a new power distribution unit so that the cabinet could run from 120/240 V split-phase instead of 120/208 V three-phase.<ref name="vcf" /> DEC's own power-up order is on [[VAX-11/780 Maintenance Guide]].

== Related pages ==
* [[VAX-11/780]]
* [[VAX-11/780 Maintenance Guide]]
* [[VAX-11/780 Troubleshooting Guide]]
* [[VAX-11/780 Documentation]]
* [[DECsystem-10 Power Supply Guide]]
* [[Capacitor Failure Symptoms]]

== References ==
<references>
<ref name="ps11">Digital Equipment Corporation, ''VAX-11/780 Power System Technical Description'', EK-PS780-TD-001, March 1979, p. 1-1 ([[:File:VAX-11 780 Power System Technical Description (EK-PS780-TD-001).pdf]]). Controllers, key switch, H7100 supplies.</ref>
<ref name="ps21">DEC, EK-PS780-TD-001, pp. 2-1 to 2-2, Table 2-1. Main cabinet power and surge current.</ref>
<ref name="ps23">DEC, EK-PS780-TD-001, p. 2-3, Table 2-2. 869D/E input, current and circuit breakers.</ref>
<ref name="ps24">DEC, EK-PS780-TD-001, pp. 2-4 to 2-5, Table 2-2 (cont.). Thermoswitch, power control bus pins, outlets and loads caution.</ref>
<ref name="ps213">DEC, EK-PS780-TD-001, pp. 2-13 to 2-14, Table 2-6, H7100 DC Power Supply Specifications.</ref>
<ref name="ps215">DEC, EK-PS780-TD-001, p. 2-15, Table 2-6 (cont.). Status indicators.</ref>
<ref name="ps219">DEC, EK-PS780-TD-001, pp. 2-19 to 2-20, Table 2-8, H7111 TODC Battery Power Supply.</ref>
<ref name="ps220">DEC, EK-PS780-TD-001, pp. 2-20 to 2-21, Table 2-9, H7112 Memory Battery Backup Power Supply.</ref>
<ref name="ps32">DEC, EK-PS780-TD-001, pp. 3-1 to 3-2, Figure 3-1, VAX-11/780 control panel.</ref>
<ref name="ps319">DEC, EK-PS780-TD-001, p. 3-19. Power control bus operation; unswitched phase on 866 and 861 systems; thermal sensors.</ref>
<ref name="ps387">DEC, EK-PS780-TD-001, p. 3-87, section 3.4.4. Power controller overtemperature sensor.</ref>
<ref name="ps388b">DEC, EK-PS780-TD-001, p. 3-88, section 3.4.5. Air flow sensors.</ref>
<ref name="ps388">DEC, EK-PS780-TD-001, pp. 3-88 to 3-89, section 3.5.1. H7100 supplies by configuration.</ref>
<ref name="psb">DEC, EK-PS780-TD-001, pp. B-1 to B-2, appendix B, H7100 power supply and regulator replacement.</ref>
<ref name="mp">Digital Equipment Corporation, H7100 power supply field maintenance print set, MP-00480, January 1983 ([[:File:VAX-11 780 H7100 Power Supply Print Set (MP-00480, January 1983).pdf]]).</ref>
<ref name="mp20">DEC, MP-00480, status card parts list 5412562-0-DBP, revision C (PDF p. 20).</ref>
<ref name="mp29">DEC, MP-00480, motherboard STAR (H7100) parts list 5412550-0-DBP, revision H (PDF p. 29).</ref>
<ref name="mp35">DEC, MP-00480, H7100 bias control parts list 5412554-0-DBP, revision M, sheet A1 (PDF p. 35).</ref>
<ref name="ts713">Digital Equipment Corporation, ''Guide to VAX-11/780 System Troubleshooting'', EY-2217E-SG-0001, November 1985, p. 7-13, H7100 Power Regulator LEDs ([[:File:Guide to VAX-11 780 System Troubleshooting (EY-2217E-SG-0001).pdf]]).</ref>
<ref name="ts1142">DEC, EY-2217E-SG-0001, p. 1-142. Power fail traps and AC LO/DC LO levels.</ref>
<ref name="vcf">"Who is still running a VAX/780?", Vintage Computer Federation forum, January 2024 to October 2024, [https://forum.vcfed.org/index.php?threads/who-is-still-running-a-vax-780.1246196/ forum.vcfed.org], pages 1 and 3.</ref>
</references>

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[[Category:Repair Guides]]