RetroTechCollection

PDP-11/44 Power Supply Guide

The PDP-11/44 is powered by an H7140 switching power supply mounted at the rear of its BA11-A box: the H7140-AA for 120 V and the H7140-AB for 240 V, with -DA and -DB variants that give −15 V instead of −12 V for memory.[1][2] DEC describes it in the BA11-A Mounting Box and Power System Technical Manual (EK-BA11A-TM-003).[3]

The BA11-A mounting box of the PDP-11/44, with the H7140 power supply at the rear (DEC EK-11044-UG-003, Figure 1-2)

Warning: the H7140 rectifies the mains to a bus of about 300 V DC. DEC warns that it holds dangerous voltages and currents: after removing AC power wait five minutes, then measure across pins 1 and 2 of TB1 with a meter on the 300 V DC range, and work on the supply only when it reads less than 20 V. The front-panel keyswitch does not remove AC; only unplugging the line cord does.[4][5]

Ratings

edit
H7140-AA and -AB input (EK-BA11A-TM-003, Table 1-1)
H7140-AA H7140-AB
Line voltage 90–128 V, single phase 180–256 V, single phase
Frequency 47–63 Hz 47–63 Hz
Current 15 A RMS max., 55 A peak 9 A RMS max., 33 A peak
Inrush 65 A peak at 128 V 130 A peak at 256 V
Power 1,350 W at maximum load 1,350 W

[1]

H7140-AA and -AB outputs (EK-BA11A-TM-003, Table 1-2)
Output Load range Static regulation Ripple (p-p) Current limit Overvoltage limit
+5.1 V logic 6 to 120 A ±0.10 V ±100 mV 125 to 140 A +6.5 V
+15 V logic 0 to 3 A ±0.30 V ±300 mV 3.2 to 4.0 A 18.0 V
−15 V logic 0 to 3 A ±0.30 V ±300 mV 3.2 to 4.0 A 18.0 V
+12 V memory 0 to 5 A ±0.24 V ±240 mV 5.5 to 6.5 A +15.0 V
−12 V memory 0 to 1 A ±0.24 V ±240 mV 1.8 to 3.0 A −15.0 V
+5.1 V memory (+5 B) 0 to 10 A ±0.10 V ±100 mV 10.5 to 13.5 A +6.5 V

[6] The +5.1 V logic output needs a minimum load of 6 A. Its 120 A rating is reduced by the current drawn from the ±15 V outputs: DEC's formula is I(+5.1 V) = 120 − 5[(I(+15 V) − 1) + (I(−15 V) − 1)].[6][7]

How it works

edit

DEC's manual divides the H7140 into an input assembly, three removable module assemblies, an output assembly, a mother module that interconnects them and a power distribution module that feeds the backplanes.[8]

Input assembly

edit

An EMI filter FL1 and circuit breaker CB1 feed a rectifier that works as a voltage doubler on 120 V (switch S1) and a full-wave bridge on 240 V, charging input capacitors C1 and C2. A resistor R1 limits inrush until the bus reaches 230 V, when relay K1 shorts it out.[9]

Bias and interface circuits

edit

The bias and interface module switches the 300 V bus at 20 kHz through transformer T1 on the mother module to make +12 V and −15 V bias for the other circuits, monitors the 300 V bus, controls turn-on from the keyswitch (normal and standby), shuts the supply down when the temperature sensor on the output heat sink closes, and makes the line clock signal.[9][10]

Memory voltages

edit

The memory inverter module converts the 300 V bus to +35 V at 30 kHz (shutting down if the +35 V reaches 48 V); the memory regulator module makes +5 B, +12 V and −12 V (or −15 V) with three switching regulators, each with current limiting and a crowbar.[10]

Logic voltages

edit

The logic inverter makes +5.1 V at up to 120 A and ±15 V for the processor; it is switched off in standby.[9][10] When the optional H7750 battery backup unit is fitted, it supplies the +35 V during a mains failure, so the memory voltages and fans keep running.[10]

Power checks

edit
  1. Check that the 872 power controller's AC indicator is lit, its breaker is on, and its LOCAL/OFF/REMOTE switch is at LOCAL-ON or REMOTE-ON.[11]
  2. Look at the DC ON indicator on the front panel: on means all DC voltages are within limits, blinking means at least one is out, off means DC power is off.[5]
  3. Measure +5 V at the connector end of the +5 V flexprint cable to the CPU backplane. Do not short the +5 V and ground buses together.[11]
  4. Measure the other voltages at connector P1 of the power flexprint cable.[11][12]
CPU backplane power connector P1 (EK-KD11Z-TM-001, Table 4-2)
Pins Function
1 to 10 +5 B (memory +5 V)
11 to 16 +12 V B
17, 18 −12 V B
19 LTC
20 BUS AC LO L
21 BOOT ENAB L
22 BUS DC LO L
23, 24 Ground sense, +5 sense
25, 27 −15 V
26, 28 +15 V

[12] DEC's field procedure for a DC failure is to replace the whole H7140.[11] Jumper W10 on the M7090 console interface enables monitoring of the ±12 V memory supplies; with it out, the ±12 V outputs are not monitored.[13]

Removing the supply

edit

DEC's procedure, in outline:[14]

  1. Unplug the supply's power cord from the AC distribution connector.
  2. Remove the two 8-32 screws in each chassis angle at the rear of the mounting box, then remove the fan assembly as in DEC's paragraph 5.3.
  3. Remove the bottom cover (four 6-32 screws) and the cover plate under the supply (four 6-32 screws).
  4. Remove the ground lead screw, loosen the clamps on the ground and +5 V flexprint cables at the bus bars, slide the cables away and bend them up towards the backplane.
  5. Unplug the power flexprint connector P1 from J11 and the console interface cable P3 from J1 (release the tabs), and the ground lead of P3.
  6. Unplug any extra backplane connectors from J2 to J4 of the power distribution board.
  7. Remove the side screws; the supply then tends to slide forward. Slide it out about 5 cm, unplug the fan cable from J2 and J3 on the power supply board, and slide it out.

When refitting, reconnect the fan leads before the supply is fully home and keep the I/O and bus cables clear.[14]

Capacitors

edit

DEC's technical manual names the input capacitors C1 and C2 and the regulator output capacitors (for example C4 in the +5 B regulator and C11 in the +12 B regulator) but gives no values.[9][10] Component values are in DEC's BA11-A field maintenance print set (MP-00832), which bitsavers holds as a 35 MB scan.[15]

Battery backup

edit

The H7750 connects to J9 on the supply's mother module through cable 17-00177; DEC warns that if any pin of J9 misses the plug, both the battery unit and the power supply will be damaged. A second cable (70-08288) from J3 or J4 of the H7140 gives overtemperature control of the H7750.[16] The front-panel BATT indicator is on when the battery is at least 90 % charged, blinks slowly while charging, blinks fast while the battery is carrying the memory through a mains failure, and is off when the battery is discharged or absent.[5]

edit

References

edit
  1. ↑ 1.0 1.1 DEC, EK-BA11A-TM-003, pp. 1-4 to 1-5, Table 1-1, H7140-AA, -AB power supply specifications.
  2. ↑ "BA11-A mounting box", Computer History Wiki, gunkies.org/wiki/BA11-A_mounting_box. H7140 -AA, -AB, -DA and -DB variants.
  3. ↑ Digital Equipment Corporation, BA11-A Mounting Box and Power System Technical Manual, EK-BA11A-TM-003, August 1983 (File:BA11-A Mounting Box and Power System Technical Manual (EK-BA11A-TM-003).pdf).
  4. ↑ DEC, EK-BA11A-TM-003, pp. 2-20 to 2-21, H7750 installation, caution and TB1 measurement.
  5. ↑ 5.0 5.1 5.2 Digital Equipment Corporation, PDP-11/44 System User's Guide, EK-11044-UG-003, July 1981, pp. 2-1 to 2-2, Table 2-1 (File:PDP-11-44 System User's Guide (EK-11044-UG-003).pdf).
  6. ↑ 6.0 6.1 DEC, EK-BA11A-TM-003, p. 1-6, Table 1-2, H7140-AA, -AB output specifications.
  7. ↑ DEC, EK-11044-UG-003, p. 3-9, section 3.2.2 and Table 3-8.
  8. ↑ DEC, EK-BA11A-TM-003, p. 4-30, section 4.3, power supply component locations.
  9. ↑ 9.0 9.1 9.2 9.3 DEC, EK-BA11A-TM-003, pp. 3-3 to 3-8, sections 3.2 to 3.2.2, power supply operations, input assembly and bias and interface circuits.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 DEC, EK-BA11A-TM-003, pp. 3-12 to 3-15, sections 3.2.2.4 to 3.2.4, standby, temperature sensing, memory voltages and regulators.
  11. ↑ 11.0 11.1 11.2 11.3 DEC, EK-BA11A-TM-003, pp. 4-1 to 4-2, section 4.1, power checks.
  12. ↑ 12.0 12.1 Digital Equipment Corporation, PDP-11/44 System Technical Manual, EK-KD11Z-TM-001, December 1980, p. 4-24, Table 4-2 (File:PDP-11-44 System Technical Manual (EK-KD11Z-TM-001).pdf).
  13. ↑ DEC, EK-11044-UG-003, p. 3-13, section 3.3.1.4, voltage monitoring.
  14. ↑ 14.0 14.1 DEC, EK-11044-UG-003, pp. 5-12 to 5-15, section 5.4, H7140 removal and replacement.
  15. ↑ Directory listing, bitsavers.org/pdf/dec/unibus/BA11-A/, retrieved 2 October 2026.
  16. ↑ DEC, EK-11044-UG-003, pp. 5-30 to 5-34, section 5.6, H7750 battery backup unit installation.
Retrieved from "https://wiki.retrotechcollection.com/index.php?title=PDP-11/44_Power_Supply_Guide&oldid=21738"