PDP-11/20 Power Supply Service

DEC drawing D-CS-H720-0-1: the H720 transformer, rectifiers, 22,000 µF reservoir capacitors and switching transistors, with the parts list

The H720 is the power supply of the PDP-11/20. It sits across the rear of the BA11-C basic box and the BA11-E extension box and provides every voltage the system units need: switching-regulated +5 V and −15 V, unregulated supplies, the line-clock signal and the AC LO and DC LO power-fail signals. DEC documented it in the H720 Power Supply and Mounting Box Manual (DEC-11-HR5B-D) and its volume of engineering drawings, which has the circuit schematic, the regulator board schematics and the parts lists.

Warning: the H720 runs from mains and its two unregulated supplies are filtered by 22,000 µF capacitors at about 25 V. Unplug the box, and let the reservoir capacitors discharge and confirm it with a meter, before touching the regulator board or the capacitor terminals. A shorted tool across those terminals delivers a very large current.

Models

H720 models (DEC-11-HR5B-D, Table 2-1 and data sheets)

Model	Mains	+5 V rating	Notes
H720-A	120 V ±10%, 47–63 Hz, 6 A	16 A	Fused input; console switch breaks both mains lines
H720-B	218, 225, 233 or 240 V ±10%, 3 A	16 A	As A, transformer jumpered for the line voltage
H720-C, -D	As A/B or E/F	–	Door-mounted versions on a metal frame, used with some peripheral controllers
H720-E	120 V ±10%, 6 A	22 A	Circuit breaker, remote/local power control, thermal cut-out, paired switching transistors
H720-F	As B	22 A	As E for 240 V

On the E and F models the console POWER switch only grounds a control line; on the A and B models it breaks both mains lines. They have no overcurrent sensing on +5 V.

Outputs and protection

H720 output specification (PDP-11/20 System Manual Table 2-7; H720 data sheet)

Output	Rating	Protection
+5 V regulated	±5%; 16 A (A, B), 22 A (E, F)	Current limit 18–22 A, about 30 A into a short (A, B); crowbar limits the output to 6.3 V
−15 V regulated	±3%, 10 A	Current limit 12–16 A, 25 A into a short; output limited to the unregulated −22 V
−22 V unregulated	±20%, 1 A	–
+8 V	Full-wave rectified, unfiltered, 1.5 A	Console lamps
LTC	Line-frequency sine wave clipped at ground and +5 V	For the KW11-L line clock
AC LO, DC LO	Power-fail signals, wired together across every H720 on the Unibus	On power-down DC LO follows AC LO by 7 ms

The descriptive text of the H720 manual gives the unregulated outputs as +8 V at 1.5 A and −25 V at 1.5 A, against −22 V at 1 A in its own data sheet and in the system manual; measure the unregulated negative rail against the higher tolerance. With a basic 11/20 installed (KA11, KY11-A, MM11-E and KL11), 3.4 A at +5 V, 1.7 A at −15 V and 0.8 A of the unregulated supply are left for options.

How the H720 works

Mains and unregulated supplies

Mains enters through fuse F1, runs to the OFF/POWER/PANEL LOCK switch on the console, then to two accessory outlets at the rear and through fuse F2 to the transformer primary. The primary is split, with jumpers for 120 V or 240 V working, and one pair of fans is wired across each half so the fans always see 120 V. Two full-wave bridges on the centre-tapped secondary give +25 V and −25 V unregulated, each filtered by a 22,000 µF capacitor; taps of about 8 V RMS either side of the centre tap give the unfiltered lamp supply.

Switching regulators

The +5 V and −15 V regulators are free-running switchers. On the +5 V side the switching transistor Q1 on the heat sink feeds a choke and capacitor; a differential amplifier compares the output with a zener reference, turns Q1 off when the output reaches 5.1 V and on again at 4.9 V, so the output ripples between those limits around an average of 5.0 V. The switching period varies from 20 µs to 50 µs with load. Potentiometer R38 on the regulator board sets the +5 V level. The −15 V regulator, built around Q2, works the same way between 14.9 V and 15.1 V.

Protection

• Overcurrent. The current drawn through the +5 V switch is sensed across 0.05 Ω in the +25 V line. On overload the regulator switches off until the current falls, then tries again, and short-circuit current is about 30 A. The −15 V side limits at 14–16 A.
• +5 V crowbar. If +5 V rises towards 6 V, zener D11 fires SCR Q27, which shorts the output through R44 and turns Q1 off until the output capacitors have discharged. If Q1 has failed short, the SCR stays on, the overcurrent circuit tries to turn Q1 off, and if that fails fuse F2 on the regulator board opens.
• −15 V shutdown. Whenever +5 V is below about 4.5 V, transistor Q3 turns the −15 V regulator off and a Darlington pair (Q12, Q15) crowbars the −15 V line, which then sits at about −1 V to −2 V. This protects the core memory, which must not see −15 V without the full +5 V. There is no overvoltage protection on −15 V.
• AC LO. Asserted when the +25 V raw supply falls to about 18 V or the −25 V raw supply to about 20 V. From AC LO the supply holds up for 2 ms at full load. The circuit has hysteresis: asserted at 100 V mains, it is not released until the mains returns to 103 V.
• DC LO. A divider between +5 V and −15 V watches for the 1:3 ratio between them; DC LO is asserted if either rail sags, or at once when the +5 V low-sense circuit trips. The AC LO and DC LO outputs of every H720 in a system are wired together on the Unibus.

Parts

Capacitors

H720 capacitors from DEC's drawings (A and B models)

Ref	Value	Board	Position	DEC part no.
C3, C4	22,000 µF 50 V	Chassis	+25 V and −25 V unregulated legs	10-09189
C5, C6	1 µF 150 V	Chassis	–	10-00063
C1, C2	2 × 0.1 µF 1000 V	Chassis	–	10-02153
C12, C14, C16	330 µF 10 V	Regulator board 5408475	+5 V output	–
C11, C13, C15, C17	150 µF 20 V	Regulator board 5408475	−15 V output	–
C2	39 µF 10 V 10%	Regulator board 5408475	−15 V overcurrent circuit	–

The chassis values and part numbers are from the parts list on schematic D-CS-H720-0-1 revision D; the regulator board values are printed on schematic D-CS-5408475-0-1, which has no separate parts list in the scan. The E and F models use regulator board 5409267, drawn on the next sheet of the same set; check that sheet before ordering parts for an E or F supply. See Capacitor Failure Symptoms for what aged electrolytics do.

Fuses and semiconductors

H720 fuses and power semiconductors (D-CS-H720-0-1 parts list)

Ref	Part	DEC part no.
F1 (120 V)	AGC 15 A slow-blow	90-07227
F2 (120 V)	AGC 5 A slow-blow	90-07222
F1 (240 V)	AGC 10 A slow-blow	90-07225
F2 (240 V)	AGC 4 A slow-blow	90-07220
Q1, Q2	2N5301 switching transistors (heat sink)	15-09737
D1, D2	DM15 bridge rectifiers	11-05799
D3, D4	1N3899	11-09781
T1	Transformer MMC 2940	16-09810

Restoring a stored supply

1. Unplug the box. Confirm with a meter that C3 and C4 have discharged.
2. Check F1 and F2 and their ratings against the table above, and the fuses on the regulator board. A fuse that has been replaced by a larger one hides a fault.
3. Check the transformer jumpers for the local mains voltage; the jumper table is printed on D-CS-H720-0-1.
4. Inspect C3, C4 and the regulator-board electrolytics for leakage and bulging, and measure them out of circuit. Replace any doubtful part with one of the same capacitance and at least the same voltage rating.
5. Bring the supply up slowly on a variable transformer with a resistive load on +5 V, watching the input current. A supply that seems dead may be cycling in its overcurrent or crowbar mode, which DEC describes as separate modes of oscillation.
6. Measure +5 V (4.75–5.25 V) and −15 V (14.55–15.45 V) at the backplane, and set +5 V with R38.
7. Check with an oscilloscope that AC LO and DC LO change state on power-up and power-down before installing the processor and memory.

Related pages

• PDP-11/20
• PDP-11/20 General Maintenance
• PDP-11/20 Troubleshooting Guide
• PDP-11/20 Documentation
• PDP-11/34 Power Supply Restoration (the later H765 and H744 system)
• Capacitor Failure Symptoms