RetroTechCollection

PDP-11/70 Power Supply Service

The PDP-11/70 power system is described in chapter 4 of DEC's PDP-11/70 Maintenance and Installation Manual (EK-11070-MM-002): two 861 power controls, two H7420 supplies with H744 and 5411086 regulators in the processor cabinet, and a separate power supply in each memory frame.[1] DEC Canada's 1170 Maintenance Service Guide (KA-K1170-MG-003) condenses the test points and limits onto two pages.[2]

H7420 power supply with four H744 regulators; positions A to D are the upper H7420, H to L the lower (DEC EK-11070-MM-002, Figure 4-25)

Warning: the 861 controls take three-phase mains (120 V or 240 V phase to neutral) at up to 24 A per phase, and DEC warns of dangerous potentials inside them. Remove the main power plug before attaching or removing test leads.[3][4]

AC power: the 861 power controls

edit
861-D and 861-E (EK-11070-MM-002, Table 4-2)
861-D 861-E
Supply 90–132 V phase to neutral, three-phase wye 180–264 V phase to neutral (312–458 V phase to phase)
Current 24 A per phase 15 A per phase
Full load 8,640 VA 10,800 VA
Input breaker 30 A, delayed action, manual reset 15 A
Outlets 14 (10 switched, 4 unswitched); 15 A each, 24 A per phase 14; 12 A each, 15 A per phase
Thermal switch Opens at 71.1 °C, resets at 49 °C

[3] The switched outlets follow the console switch through the remote control bus, and are cut if the thermal switch in the processor mounting box or the one on the side of the 861 trips; the memory cabinet's 861 follows the processor cabinet's.[5] DEC's 861 has no adjustments or fuses. Its preventive maintenance is cleaning and inspection, a check of the thermal switch by heating its sensing element while the controller runs, and a check of emergency shutdown by connecting pins 3 and 2 of the remote switching control bus.[4] DEC lists three failure modes:[4]

  • No output with the breaker tripped: a faulty breaker, a low-resistance load, or a failed component inside the controller.
  • No output with the breaker not tripped: bad cable connections, an open component in the line filter, a relay fault or a faulty breaker.
  • No control: bad cables, secondary breakers, relays, diodes or the thermal switch T1. Set the LOCAL/OFF/REMOTE switch to LOCAL: if the switched outlets still do not come on, the fault is inside the controller; if they do, check the control signals from outside.

Processor cabinet: H7420 supplies

edit

Each H7420 has a transformer giving 20 to 30 V AC (26 V nominal) to its regulators, a circuit breaker CB1 (20 A in the A and E versions, 15 A in the B and F), a terminal block jumpered for 115 V or 230 V input, two capacitor-varistor surge suppressors (C1/D1 and C2/D2), and fans.[6] DEC specifies more than 20 ms of output ride-through after a power failure.[7]

Processor regulators and test points (EK-11070-MM-002, Tables 4-1 and 4-21)
Regulator Supplies Measure at Voltage Max. ripple
H744 A (upper) Slots 2 to 5 (fitted only with the FP11 option) F02A2 +5 V 0.2 V p-p
H744 B (upper) Slots 1, 6 to 9 F09A2 +5 V 0.2 V p-p
H744 C (upper) Slots 10 to 15 F15A2 +5 V 0.2 V p-p
H744 D (upper) Slots 36 to 44 F44A2 +5 V 0.2 V p-p
H744 H (lower) Slots 20 to 22 F22A2 +5 V 0.2 V p-p
H744 J (lower) Slots 16 to 18 and the console F18A2 +5 V 0.2 V p-p
H744 K (lower) Slots 24 to 28 F28A2 +5 V 0.2 V p-p
H744 L (lower) Slots 29 to 35 F35A2 +5 V 0.2 V p-p
5411086 (upper) +8 V, slot 1 B01B1 +8 V ±1.2 V 0.24 V p-p
5411086 (upper) +15 V, slots 40 to 44 E13A1 +15 V ±1.5 V 0.45 V p-p
5411086 (lower) −15 V, slots 2, 17, 25, 27, 29 to 31, 33 to 35, 37 to 44 E13B2 −15 V ±1.5 V 0.45 V p-p

[8][9] DEC's manual sets the H744s to +5 V ±250 mV at the backplane; the Canadian service guide's table tightens this to ±0.1 V.[10][2] The H744 crowbar fires at 5.5 V at the regulator; never adjust a regulator up to that figure.[11] DEC's wire colours for PDP-11 power are +5 V red, +8 V white, +15 V grey, −15 V blue, −5 V brown, +20 V orange, AC LO yellow, DC LO violet and ground black.[12]

Adjusting an H744

edit
  1. Power down and pull the processor frame fully out of the rack, keeping the cables free.
  2. Power up and measure each +5 V output at its backplane pin (table above) with a digital voltmeter.
  3. Adjust to 5 V ±250 mV with the screw above the regulator's indicator lamp.
  4. Power down and push the frame back.

[10] If a newly fitted regulator crowbars at power-up, turn it off, turn the adjustment fully anticlockwise, power up and bring it up slowly to the correct voltage.[13]

Fault isolation

edit

Each regulator has a lamp that lights while its output is in range.[14]

  1. If one lamp in a group is out, suspect that regulator; confirm by swapping H744s.
  2. If a whole group is out, the fault is in that H7420 or the 861: check the 861 lamps and breakers, the H7420's power indicator and CB1.
  3. Check the two LEDs on the 5411086 board (both lit means AC LO and DC LO are not asserted); the first two steps of the 5411086 removal procedure are needed to see them.
  4. Measure every output at the backplane. If all are correct and the machine still misbehaves, check the backplane wiring.
  5. If an output is wrong, check the regulator fuse, the transformer assembly, the harness and the load. If +8 V, +15 V or −15 V is wrong, check the 5411086 and its fuses.

[14] Both H7420s plug into the same 861, so make sure you unplug the right power cord before working inside one.[15]

Memory cabinet supplies

edit
Memory regulators (EK-11070-MM-002, Tables 4-17 to 4-19)
Regulator Output Tolerance at backplane Crowbar (max. at regulator) Max. current Ripple
H744 (MJ11, two per frame) +5 V ±250 mV 5.5 V 25 A 0.2 V
H754 (MJ11, two per frame) +20 V and −5 V ±1 V; ±250 mV 21.5 V; −5.5 V 8 A; 1 to 8 A 1.0 V
H7441 (MK11) +5 V ±250 mV 5.5 V 32 A 0.2 V
7014251 (MK11, three per frame) +5 V and ±12 V ±250 mV; ±600 mV 6.0 V; 13 V 4 A; 2 A 1.0 V

[11] On an MJ11 measure at the M8149 test points: TP1 and TP2 for the two H744s (+5 V), TP3 and TP4 for the H754s' +20 V, TP5 and TP6 for their −5 V, with TP7 as ground.[16] The H754 has two potentiometers that interact: R17 sets the whole 25 V between the +20 V and −5 V outputs and R21 sets −5 V to ground. DEC's procedure is to set +25 V across TP3 and TP5 (or TP4 and TP6) with R17, then −5 V to ground with R21, then recheck both.[13] On an MK11 measure +5 V at J18 and J21 and the ±12 V outputs at J18 and J21 as listed in DEC's Table 4-18; all must be within 5 %.[16][17] The MK11's three H775-D battery backup units keep the MOS memory refreshed through a power failure; DEC issued field change M8160-R4 for H775-D breakers tripping during power fail.[18][19]

Repairing H744 and H754 regulators

edit

DEC's regulator troubleshooting chart:[13]

H744, H754 regulator faults (EK-11070-MM-002, Table 4-20)
Symptom Probable cause
No output Q2 to Q5; D5; D1 (burnt underneath); crowbar Q6 and Q7; E1 (723 regulator); output set above the crowbar point
Blown fuse F1 Pass transistor Q2 and associated parts; excessive load

DEC's bench checks: a blown F1 usually means a shorted pass transistor Q2 or driver Q3 or Q4; a fired crowbar gives 1 to 1.5 V output with a loud "singing"; a shorted D5 usually destroys Q2, Q3 and Q4, so check it before powering up after replacing them. A regulator with no output but an intact F1 is probably driving a short. On a dead regulator with 25 V AC input and no load, look for 30 V DC across the filter capacitor C1, +15 V at pin 12 of E1 and 6.8 to 7.5 V at pin 6. After repair, bring it up on a Variac with no load, set it to 5.10 V (H744) or 25 V between +20 V and −5 V (H754) at full load, and check that the crowbar fires between 6.00 and 6.65 V (H744).[13] The H744's parts and the experience of restorers who have recapped them are on PDP-11/34 Power Supply Restoration, whose supply uses the same regulator.

edit

References

edit
  1. ↑ Digital Equipment Corporation, PDP-11/70 Maintenance and Installation Manual, EK-11070-MM-002, May 1979, pp. 4-1 to 4-2, sections 4.1 and 4.2 (File:PDP-11-70 Maintenance and Installation Manual (EK-11070-MM-002).pdf).
  2. ↑ 2.0 2.1 Digital Equipment of Canada, 1170 Maintenance Service Guide, KA-K1170-MG-003, April 1988, pp. 26 to 27, regulator voltages, tolerances and test points (File:PDP-11-70 Maintenance Service Guide (KA-K1170-MG-003).pdf).
  3. ↑ 3.0 3.1 DEC, EK-11070-MM-002, pp. 4-9 to 4-11, section 4.3 and Table 4-2, 861 power controller specifications.
  4. ↑ 4.0 4.1 4.2 DEC, EK-11070-MM-002, p. 4-72, section 4.5.1, 861 maintenance.
  5. ↑ DEC, EK-11070-MM-002, p. 4-3, section 4.2.1, processor cabinet.
  6. ↑ DEC, EK-11070-MM-002, p. 4-33, section 4.4.4, H7420.
  7. ↑ DEC, EK-11070-MM-002, pp. 4-42 to 4-43, Table 4-8, H7420 specifications.
  8. ↑ DEC, EK-11070-MM-002, p. 4-4, Table 4-1, processor power supply voltage regulators.
  9. ↑ DEC, EK-11070-MM-002, p. 4-101, Table 4-21, H7420 voltage measurements.
  10. ↑ 10.0 10.1 DEC, EK-11070-MM-002, p. 4-104, section 4.5.3.2, H744 voltage adjustment.
  11. ↑ 11.0 11.1 DEC, EK-11070-MM-002, p. 4-80, Table 4-19, regulator specifications.
  12. ↑ DEC Canada, KA-K1170-MG-003, p. 42C, wire colours for PDP-11 voltages.
  13. ↑ 13.0 13.1 13.2 13.3 DEC, EK-11070-MM-002, pp. 4-82 and 4-87 to 4-91 and 4-100, H754 and H744 adjustment, Table 4-20, regulator bench tests and installation.
  14. ↑ 14.0 14.1 DEC, EK-11070-MM-002, pp. 4-104 and 4-106, sections 4.5.3.3 and 4.5.3.4, H7420 fault isolation.
  15. ↑ DEC, EK-11070-MM-002, p. 4-106, power supply access procedure, caution.
  16. ↑ 16.0 16.1 DEC, EK-11070-MM-002, p. 4-79, Tables 4-17 and 4-18.
  17. ↑ DEC, EK-11070-MM-002, p. 4-77, section 4.5.2.1, memory power supply voltage measurements.
  18. ↑ DEC, EK-11070-MM-002, p. 4-62, section 4.4.7, MK11 power supply.
  19. ↑ DEC Canada, KA-K1170-MG-003, pp. 20 to 22, field change order and tech tip indexes.
Retrieved from "https://wiki.retrotechcollection.com/index.php?title=PDP-11/70_Power_Supply_Service&oldid=21722"