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VAXstation 2000 Power Supply Guide

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H7848 circuit board (5416897): fuse F1 at top left, primary capacitors C8 and C9, output capacitors on the right (DEC print set MP-02389-01)

The H7848 power supply of the VAXstation 2000 and MicroVAX 2000, drawn from DEC's maintenance guide (EK-VSTAA-MG-001), the VAXstation 2000 print set (MP-02400-01) and the H7848 print set (MP-02389-01).[1][2] The same supply is used in the system box and in both expansion boxes: DEC's spares list names only the H7848-BA and H7848-BB, and the maintenance guide's power flowchart covers all three boxes.[3][4]

Warning: the H7848 is a mains switching supply. The primary side holds two 300 µF 250 V electrolytics, C8 and C9.[5] Unplug the box and let these discharge before touching the board; see Capacitor Failure Symptoms for what failed capacitors look like.

Variants and ratings

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H7848 variants
Variant Input Rating Module board assembly
H7848-BA 120 V 104 W 70-22655-03
H7848-BB 240 V 104 W 70-22655-04

[6][7]

The supply assembly includes its own DC fan (70-22656), the power switch S1 (a 16 A double-pole rocker, 12-22355-09), an AC input harness, a DC output harness, a daisy-chained floppy power harness, the cable to the AC power switch and a voltage selector jumper.[2][7] The board layout marks a W1 point for 120 V and a W3 point for 240 V.[8] The input fuse on the board is F1, a 3AG 4.0 A 250 V normal-blow fuse (DEC 90-07219-02).[9]

Output voltages

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Voltages on the system module's 9-pin power connector and on a drive power connector (DEC, EK-VSTAA-MG-001, Figure 2-19)

DEC's instruction is to measure with the power cables connected.[10]

9-pin power connector (DEC Figure 2-19)
Pin Voltage Tolerance as printed
1, 2, 3 Ground
4 −12.0 V −0.6 V
5 +5.1 V −0.25 V
6 +5.1 V −0.25 V
7 +12.1 V −0.60 V
8 9.0 V to pin 9
9 9.0 V to pin 8

[10]

DEC prints each tolerance with a minus sign only, so the figure does not say whether a reading above nominal is also out of tolerance. The 4-pin drive power connectors carry +5.1 V on the red wire, +12.1 V and two grounds.[10]

Troubleshooting

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DEC's flowchart for a supply problem in any of the three boxes (Figure 2-18):[4]

  1. Check that there is power at the wall outlet. If not, the fault is in the building wiring.
  2. Check that the power cord is plugged into the box and the outlet.
  3. Open the box and check the power cables. In an expansion box the 9-pin cable is not used. In a system box with only one hard disk, one of the 4-pin drive cables is not used. Every other power cable must be connected.
  4. Switch on and measure the voltages on the connectors, with the cables connected, against the table above. DEC's field service action is to replace the supply if any voltage is out of tolerance.

A system box with no hard disk must have its resistor load board fitted, or the supply cannot regulate (see below).[11] When the hard disk in the expansion box shows as off line, DEC's maintenance guide first checks that the box has power and is switched on and that its data cable is in port B, then troubleshoots the box's supply.[12]

Resistor load board

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The H7848 needs a minimum load. In a diskless system box, or one with only the RX33 diskette drive, the supply is too lightly loaded to regulate, so DEC fitted a resistor load board that stands in for the drives and draws enough power for the supply to regulate. Remove the load board when a hard disk is fitted in the system box. The system box load board must be part 54-17163-02; the -01 version goes in the expansion boxes.[11][3]

Resistor load board 5417163 parts list
Designators Part Value
R1–R15 13-20602-28 24 Ω 2 W 5% metal oxide
R16–R27 13-19471-16 150 Ω 2 W 5% metal oxide
C1–C5 10-17472-00 10 µF 35 V aluminium electrolytic, +75/−10%
C6–C10 10-14265-00 0.22 µF 50 V ceramic
J1, J2 12-11342-04 4-pin Mate-N-Lok header
W1–W10 (-01) or W7–W10 (-02) 90-09185-00 Wire jumpers

[13]

To remove the system box load board: take off the shield, remove the three screws holding the drive mounting bracket, unplug the power cable from the load board and lift it out. When fitting a drive in its place, discard the standoffs the load board sat on.[11]

Removing the supply from the system box

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The four screws under the chassis that hold the supply (DEC, EK-VSTAA-MG-001, Figure 3-31)
  1. Switch off, disconnect all cables from the rear of the system box and, on a MicroVAX 2000, take off the DEC423 converter.[14]
  2. Remove the shield as described on VAXstation 2000 Maintenance Guide.
  3. Unplug the power cables from the rear of the drives. For more room, remove the three drive mounting screws and slide the drives part way out.
  4. Stand the system on its side and remove the four screws that hold the supply to the bottom of the chassis.
  5. Slide the supply out of the chassis.[15]

Refitting is the reverse. The expansion box supplies have their own removal procedures in sections 3.17.1.2 and 3.17.2.3 of the maintenance guide.[16]

H7848 circuit board capacitors

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DEC's parts list for the H7848 circuit board, 5416897, released 6 November 1986.[5][17]

Aluminium electrolytics
Designators Value Tolerance DEC part
C8, C9 300 µF 250 V ±20% 10-26365-01
C11, C21, C31, C35, C37 1000 µF 35 V +50/−10% 10-19537-01
C18, C19, C20, C26, C27 10 µF 35 V +50/−20% 10-18000-04
C24 330 µF 25 V +50/−20% 10-18000-11
C46 470 µF 25 V +50/−20% 10-18000-17
Solid tantalum
Designators Value DEC part
C12, C22 1.0 µF 35 V ±10% 10-01776-00
C39, C41 2.2 µF 20 V ±10% 10-02627-00
C32 10 µF 20 V ±10% 10-04813-00
AC line capacitors
Designators Value DEC part
C1 0.47 µF 250 V AC ±20% 10-24317-02
C2 0.33 µF 250 V AC ±20% 10-24317-01
C5 0.22 µF 250 V AC ±20% 10-24317-03
C3, C4, C6, C7 2200 pF 250 V AC ±20% 10-24375-01

[5][17]

The rest of the board's capacitors are ceramic, film and high-voltage parts: C10 2200 pF 600 V polypropylene, C17 330 pF 1000 V, C47 1000 pF 1000 V, C29 and C30 3300 pF 200 V Mylar and C42 1000 pF 100 V.[5][17] The layout above shows C8 and C9 as the two large cans at the left of the board, and C11, C21, C24, C31, C35, C37 and C46 among the output components on the right.[8]

Main semiconductors and magnetics

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H7848 circuit board, selected parts
Designator Description in DEC's parts list DEC part
E1 3527A regulating pulse-width modulator 19-16819-00
Q4 VMOS N-channel FET, 800 V 5 A 15-24264-01
D1 Rectifier, 420 V 6 A 11-18205-00
D6, D10 Ultra-fast rectifier, 800 V 1 A 11-15112-00
D12 Ultra-fast rectifier, 100 V 16 A 11-20323-01
D15 30 A Schottky rectifier, TO-218 11-22399-01
Z1 7812 regulator, +12 V 19-12048-05
Z2 7912 regulator, −12 V 19-14517-01
Z3 7905 regulator, −5 V 19-14437-00
Z4 +9 V regulator 19-25554-01
T1 High-frequency power transformer, 100 kHz 16-25559-01
T2 Power transformer, 115/230 V 47–63 Hz 16-24445-01
L1, L2 EMI filter chokes, 12 mH 1.9 A RMS 16-26142-01
F1 Fuse, 3AG 4.0 A 250 V normal-blow 90-07219-02

[5][9]

References

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  1. ↑ Digital Equipment Corporation, VAXstation 2000 and MicroVAX 2000 Maintenance Guide, EK-VSTAA-MG-001, January 1987 (File:VAXstation 2000 and MicroVAX 2000 Maintenance Guide (EK-VSTAA-MG-001).pdf).
  2. ↑ 2.0 2.1 Digital Equipment Corporation, H7848 power supply field maintenance print set, MP-02389-01, table of contents (PDF p. 1) (File:H7848 Power Supply Field Maintenance Print Set (MP-02389-01).pdf). Drawings for the module board assembly, DC fan, harnesses, power switch cable and voltage selector jumper.
  3. ↑ 3.0 3.1 DEC, EK-VSTAA-MG-001, p. 3-82, section 3.18. Recommended spares list.
  4. ↑ 4.0 4.1 DEC, EK-VSTAA-MG-001, p. 2-40, section 2.4.13 and Figure 2-18. Power supply troubleshooting flowchart.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 DEC, MP-02389-01, H7848 circuit board parts list 5416897-0-DBP, sheet A1 (PDF p. 7). Capacitors C1 to C42, released 6 November 1986.
  6. ↑ Digital Equipment Corporation, VAXstation 2000 field maintenance print set, MP-02400-01, revision A1, 1987, VS410 unit assembly parts list, sheet A1 (PDF p. 41) (File:VAXstation 2000 Field Maintenance Print Set (MP-02400-01).pdf). H7848-BA 120 V and H7848-BB 240 V, 104 W.
  7. ↑ 7.0 7.1 DEC, MP-02389-01, H7848 power supply parts list (PDF p. 4). Power switch S1, module board assemblies 70-22655-03 and -04, DC fan assembly.
  8. ↑ 8.0 8.1 DEC, MP-02389-01, H7848 circuit board unit assembly D-UA-5416897 (PDF p. 6). Component layout.
  9. ↑ 9.0 9.1 DEC, MP-02389-01, H7848 circuit board parts list, sheet A2 (PDF p. 8). Transistors, magnetics, regulators, fuse F1.
  10. ↑ 10.0 10.1 10.2 DEC, EK-VSTAA-MG-001, p. 2-41, Figure 2-19. Power connector pin voltages.
  11. ↑ 11.0 11.1 11.2 DEC, EK-VSTAA-MG-001, p. 3-31, section 3.9. Resistor load board removal from the system box.
  12. ↑ DEC, EK-VSTAA-MG-001, p. 2-27, section 2.4.9.3. Troubleshooting the hard disk expansion box.
  13. ↑ DEC, MP-02400-01, load board parts list 5417163-0-DBP, sheet A1, August to September 1986 (PDF p. 50).
  14. ↑ DEC, EK-VSTAA-MG-001, p. 3-4, section 3.2. Shield removal, steps 1 to 3.
  15. ↑ DEC, EK-VSTAA-MG-001, pp. 3-34 to 3-35, section 3.11 and Figure 3-31. Power supply removal from the system box.
  16. ↑ DEC, EK-VSTAA-MG-001, pp. 3-1 to 3-2, Table 3-1. FRU section listings.
  17. ↑ 17.0 17.1 17.2 DEC, MP-02389-01, H7848 circuit board parts list, sheet A3 (PDF p. 9). C32, C46 and C47.
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