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MicroVAX II Power Supply Guide

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DEC MicroVAX II floor-standing system (ITESM Monterrey)
MicroVAX II in a floor-standing enclosure (Victoroy, CC BY-SA 4.0)

A MicroVAX II is powered by a mains switching supply in its enclosure: the H7864 in the BA23, a 460 W two-regulator supply in the BA123, and two H7864-A supplies in the H9642 cabinet.[1] DEC treated these supplies as field-replaceable units and published their output specifications, but no schematic or parts list for them is in DEC's MicroVAX II maintenance guide. DEC says the H7864-A is not adjustable and contains no replaceable printed circuit boards.[2] What is known about repairing them comes from restorers.

Warning: these supplies rectify the mains directly. Their primary-side capacitors hold a lethal charge after the supply is unplugged; discharge and measure them before touching the board.[3]

H7864 (BA23)

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H7864 specifications (MicroVAX II Maintenance Guide, Table 1-1)
Output Voltage Current Overcurrent Ripple and noise
+5 V +5.1 V ±2.5% 4.5 A minimum, 36.0 A maximum Must not trip at 37 A, must trip at 42 A (averaged over 1 ms) 50 mV p-p
+12 V +12.1 V ±2.5% 0 A minimum, 7.0 A maximum Must not trip at 7.2 A, must trip at 8.0 A (averaged over 1 s); start-up surges of 9 A for 10 s, 10 A for 5 s and 11.5 A for 1 s allowed, trips at 13 A for 500 µs 75 mV p-p

[1] The supply is rated at 230 W in total, so +5 V and +12 V cannot both be loaded to their maximum at once. It also has two +10 V 0.45 A outputs for the front and rear fans, which a jumper raises to +12 V; DEC's thermal and acoustic figures for the system assume the 10 V setting.[1][4] The supply asserts BDCOK H and BPOK H on the backplane when its outputs are stable, and generates BEVENT L, the line-clock interrupt, which the LTC switch on the control panel enables.[1]

Connectors and mains

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At the rear are the remote power control connector, an IEC mains inlet, a circuit breaker and the VOLT SEL switch: 120 V covers 88–128 V AC and 240 V covers 176–256 V AC. The rear fan cable is part of the supply.[5] At the front are J7 (front control panel power), J8 (mass storage power), J9 (backplane power, to J1 of the signal distribution panel) and J10 (front fan).[5][2] The DEC FRU number for the H7864-A is 30-21749-01; DEC requires a replacement supply to have the same part number as the one removed.[6]

Removing the H7864

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DEC's procedure for the BA23:[2]

  1. Remove the front and rear covers and all cables.
  2. Remove the rear chassis retaining bracket and slide the subsystem out through the back.
  3. Remove the subsystem storage cover, then the fan cowling and its holder if fitted.
  4. Disconnect the backplane power connector from J9 on the supply and J1 on the signal distribution panel, the mass storage power connector from J8, and the front fan and control panel connectors from J10 and J7 (keyed and latched).
  5. Remove the five screws holding the supply, lift it onto the Q22-bus module cover and disconnect the rear fan.

When refitting, connect the rear fan first. Its cable is not keyed: the curve of the connector must match the curve of the fan housing, and the cable is routed over the top of the rear fan.[2]

BA123 supply

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The BA123-A supply is a 460 W unit with two regulators, A and B. Each feeds half the backplane slots and some of the mass-storage devices, and each is limited to 230 W, 4.5–36 A at +5 V and 7 A at +12 V. Two further +12 V outputs, independent of the main 460 W, drive the two external fans and the temperature sensor above the card cage.[7][4] The mains input has a circuit breaker and a voltage selector with the same 88–128 V and 176–256 V ranges; DEC asks for at least 90 V at the outlet on the 120 V setting to allow for the voltage drop in the line cord.[7] The FRU list gives the supply as 30-23616-01, with separate cables from each regulator to the backplane and drives.[8] The BA123 also has a card-cage door interlock switch wired to the temperature sensor board, which is itself cabled to the power supply.[9]

Fault reports

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  • Blown switching transistor. Robert Jarratt's H7864 failed with a loud pop and no visible damage. He replaced a blown TO-3 transistor on the primary side of the main transformer, but the supply still did not switch, and he went on to trace its circuit and compare it with a working unit.[3]
  • RIFA filter capacitors. John H. Reinhardt removed RIFA X2 capacitors from his H7864 that had gone amber and had visible cracks in their cases.[10] Another list member describes RIFA filter capacitors as failing often and violently.[11] See Capacitor Failure Symptoms.
  • Running without a load. DEC specifies a 4.5 A minimum load on +5 V.[1] Tony Duell, answering Jarratt, said he had not found a DEC supply that failed destructively without a load; an unloaded switcher is more likely to trip its crowbar, shut down and try again in a cycle.[12] Bench-test with a resistive load of at least 4.5 A on +5 V.

Bringing up a stored machine

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  1. Unplug the machine. Check the VOLT SEL switch matches the mains, and reset the circuit breaker if it has tripped.[5][13]
  2. Open the supply only if you are prepared to work on a mains switcher; inspect it for cracked mains filter capacitors and leaking electrolytics.[10]
  3. Power up with the CPU, memory and console only. The DC OK light on the control panel should glow green; the CPU module's green DC OK LED and the display at the rear should start counting down from F.[13][14]
  4. If the display stays at F or E, the CPU is not seeing DC OK or POK. DEC lists the supply, a Q22-bus device holding the line, the backplane and the supply cable among the causes. See MicroVAX II Troubleshooting Guide.[14]
  5. Measure +5 V and +12 V at the backplane: 4.97–5.23 V and 11.80–12.40 V.[2]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Digital Equipment Corporation, MicroVAX II Maintenance Guide, AZ-GM3AA-MN, first edition October 1985, section 1.4.1 and Table 1-1, pp. 1-22 to 1-24 (File:MicroVAX II Maintenance Guide (AZ-GM3AA-MN).pdf).
  2. ↑ 2.0 2.1 2.2 2.3 2.4 DEC, AZ-GM3AA-MN, section 8.6, power supply (H7864-A) removal, and Figures 8-22 to 8-25.
  3. ↑ 3.0 3.1 Robert Jarratt, "Testing H7864 (MicroVAX II) PSU With No Load", cctalk mailing list, 1 November 2015, mail-archive.com, and follow-up msg07744.
  4. ↑ 4.0 4.1 DEC, AZ-GM3AA-MN, sections 5.4.2 and 5.5.2, pp. 5-10 to 5-13. Power limits.
  5. ↑ 5.0 5.1 5.2 DEC, AZ-GM3AA-MN, p. 1-25 and Figure 1-15. H7864 rear and front connectors.
  6. ↑ DEC, AZ-GM3AA-MN, Table 8-1, BA23-A field replaceable units.
  7. ↑ 7.0 7.1 DEC, AZ-GM3AA-MN, section 1.4.2, Table 1-2 and Figure 1-17, pp. 1-26 to 1-28. BA123-A power supply.
  8. ↑ DEC, AZ-GM3AA-MN, Table 9-1, BA123-A FRUs.
  9. ↑ DEC, AZ-GM3AA-MN, sections 9.8 and 9.9 and Figure 9-17. Door interlock switch and temperature sensor.
  10. ↑ 10.0 10.1 John H. Reinhardt, quoted in Antonio Carlini, "Re: Is this a RIFA (uV3100-10 PSU)?", cctalk mailing list, 24 October 2022, mail-archive.com.
  11. ↑ Joshua Rice, "Re: H7821 power supply in MicroVAX 3100, SCSI disk enclosures and others", cctalk mailing list, 31 January 2020, mail-archive.com.
  12. ↑ Tony Duell, "RE: Testing H7864 (MicroVAX II) PSU With No Load", cctalk mailing list, 1 November 2015, mail-archive.com.
  13. ↑ 13.0 13.1 Digital Equipment Corporation, MicroVAX II 630QB Owner's Manual, AZ-GLCAA-MN, Tables 2-1, 2-2 and 3-2 (File:MicroVAX II 630QB Owner's Manual (AZ-GLCAA-MN).pdf).
  14. ↑ 14.0 14.1 DEC, AZ-GM3AA-MN, Table 7-1 and Figure 7-1, pp. 7-3 to 7-7.
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