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Professional 325 Capacitor Replacement Guide

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DEC H7862 power supply schematic sheet 1 (D-CS-5415086-0-1)
DEC schematic of the H7862 power supply, sheet 1: input capacitors C1 and C2 at left, the +12 V (C8), +5 V (C13) and −12 V (C17) outputs at right

The Professional 325 takes all its power from an H7862 switching power supply, the same unit DEC fits to the whole Professional 300 series.[1][2] DEC's field maintenance print set includes the parts list and schematic of the supply's circuit board (5415086, revision D), so every capacitor in it can be identified.[3][4] DEC describes the 325 as identical to the 350 apart from a short list of differences, and the system module, video generator and 128 KB memory board in the PC350 print set carry only two small aluminium electrolytics and four solid tantalums between them; they are listed on Professional 350 Capacitor Replacement Guide.[5][6] The system module also carries the nickel-cadmium clock battery pack.[7]

Warning: the H7862 runs its two input capacitors at 250 to 350 V DC, and DEC warns that they hold a high voltage for up to five minutes after the power is turned off.[8] DEC's procedure before removing the supply cover: switch off, wait five minutes, then measure from the case of Q2 (the TO-3 transistor nearest the open side of the case) to the end of bleeder resistor R4, starting on a range of at least 500 V DC. Proceed with caution only when it reads 20 V or less.[8]

H7862 electrolytic capacitors

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H7862 circuit board 5415086 electrolytics (DEC parts list K-PL-5415086-0-DBP, revision D)
Reference Value Voltage Tolerance DEC part Position on the schematic
C1, C2 1,100 µF 200 V +50 / −10 % 10-16390-00 Input capacitors after bridge D1, each with a 100 kΩ 1 W bleeder (R3, R4)
C13 18,000 µF 6.3 V 20 % 10-18971-01 +5 V output filter after L2
C8 10,000 µF 16 V 20 % 10-18971-00 +12 V output filter after L1
C56 330 µF 20 V +100 / −10 % 10-13393-00 +12 V output, after L3
C17 560 µF 20 V +100 / −10 % 10-12607-00 −12 V input to regulator Z1 (7912)
C20 1,000 µF 35 V +50 / −10 % 10-19537-01 Start-up supply from transformer T4, input to regulator Z2 (7812)
C18, C21, C28, C32 10 µF 35 V +50 / −10 % 10-17472-00 C18 on the −12 V output; C21 on the 7812 output; C28 at the PWM controller's supply; C32 in the switch driver circuit
C26 1 µF 100 V +50 / −10 % 10-05507-00 Soft-start capacitor of PWM controller E5 (3527A)

[3][4] DEC writes its part numbers without the first hyphen on the parts list (1016390-00 and so on). The list marks all of the above as aluminium electrolytics ("AL EL").[3]

The list is for board revision D, as drawn in the PC350 print set. The copy of the H7862 print set in DEC's later PC380 drawing set has the board at assembly revision H and schematic revision L: ECO 5415086-ML010 adds a capacitor, C59, and changes C31 from 0.082 µF to 0.033 µF. That schematic shows the same values as above for C1, C2, C8, C13, C20, C21, C32 and C56, but the scan has no parts list for the later revision, so check the parts on a late board against this list.[9]

Other capacitors on the board

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The rest are film, mica or ceramic parts and do not normally need replacing: C3 4.7 µF 400 V metallised Mylar; C4, C5 0.0022 µF 600 V polypropylene; C6, C7, C23 3,300 pF 200 V metallised Mylar; C10, C15 1 µF 100 V metallised Mylar; C9, C14, C19, C54 0.47 µF 25 V ceramic; C41 0.47 µF 50 V ceramic; fourteen 0.22 µF 50 V ceramics (C12, C16, C24, C27, C34, C35, C38, C44 to C46, C48, C51 to C53); and small Mylar, mica and ceramic parts (C11, C22, C25, C29, C30, C31, C33, C36, C37, C39, C40, C42, C43, C47, C55).[3]

Clock battery

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The system module's clock and 50-byte configuration RAM run from a three-cell nickel-cadmium pack, DEC part 12-19245-00, listed as "BATTERY, 3CELL 3.75V .18MA NICAD", on a two-wire cable to connector J3 (+3.6 V on pin 1).[7][10][11] The pack is charged at 10 mA from the +12 V supply whenever the computer is on.[10] To remove it, take off the top cover, slide the system module about 2 inches towards the rear of the cabinet, and unplug the battery cable from the module.[11] A replacement must be rechargeable, because the charging circuit feeds it continuously; see Battery Explosion, Capacitor or Corrosion Damage for cleaning any leakage.

Procedure

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  1. Remove the top cover. Unplug the 16-pin system module power connector from the rear of the supply by its white pull tab, and the 4-pin drive power connectors from its side.[12]
  2. Pull out the two release clips on the side of the supply near the fan and lift the supply out. When refitting, press down in the centre to latch it.[12]
  3. Before opening the cover, discharge and measure C1 and C2 as described in the warning above.[8]
  4. Replace the electrolytics with parts of the same capacitance and at least the same voltage rating, observing polarity. C1 and C2 sit on the rectified mains at 250 to 350 V DC, and C13 filters the +5 V output, which supplies up to 20 A.[8][13][4]
  5. Check fuse F1 (0.5 A, 250 V) and the two 5 Ω NTC inrush thermistors R105 and R106 while the board is out.[3]

DEC's own service documents treat the whole supply as a field replaceable unit: it "is not adjustable and does not contain replaceable circuit boards", and only the fan (part 12-13185-03) is replaced separately.[12]

After the recap

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Check the outputs at the 16-pin connector J2 with the system connected.[14]

H7862 outputs (EK-PC350-TM-001, section 13.6 and Tables 13-2 and 13-3)
Output J2 pins Regulation Load range DCOK asserted above Overvoltage limit Overcurrent limit
+5 V 6 to 9 ±5 % 5 to 20 A 4.7 ±0.2 V 7.0 V 21 to 29 A
+12 V 5 ±5 % 1 to 8 A 11.1 ±0.3 V 14.5 V 11 to 15 A
−12 V 4 ±5 % 0.1 to 1 A 10.8 ±0.3 V (magnitude) −14.0 V 1.5 to 2.5 A

[13][15][14] J2 also carries DCOK on pin 1 and POK on pin 3, with grounds on pins 10 to 16.[14] The H7862 has no adjustment. If an output trips the overvoltage latch the supply stays off until the power is removed; DEC's 1985 manual says to leave it off for at least five minutes.[15][16] A recapped clock battery pack takes 48 hours of running to charge fully.[17]

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References

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  1. ↑ Digital Equipment Corporation, PC350 Field Maintenance Print Set, MP-01394-00, 1982, PC350 unit assembly parts list, PDF p. 5 (File:Professional 350 PC350 Field Maintenance Print Set (MP-01394-00).pdf).
  2. ↑ Digital Equipment Corporation, Professional 300 Series Technical Manual, Volume 1, EK-PC300-V1-001, June 1985, p. xix, chapter 10 "H7862 Power Supply describes the Professional 300 Series power supply" (File:Professional 300 Series Technical Manual Volume 1 (EK-PC300-V1-001).pdf).
  3. ↑ 3.0 3.1 3.2 3.3 3.4 DEC, MP-01394-00, parts list K-PL-5415086-0-DBP, revision D, "H7862 Pwr Supply Circuit Board", sheets A1 to A4, PDF pp. 55 to 58.
  4. ↑ 4.0 4.1 4.2 DEC, MP-01394-00, drawing D-CS-5415086-0-1, revision D, "H7862 Circuit Board", sheets 1 and 2, PDF pp. 59 to 60 (File:DEC H7862 power supply schematic sheet 1 (D-CS-5415086-0-1).png).
  5. ↑ Digital Equipment Corporation, Professional 300 Series Technical Manual, Volume 1: Kernel System, EK-PC300-V1-001, June 1985, pp. 1-2 to 1-3, section 1.1.4 and Table 1-1 (File:Professional 300 Series Technical Manual Volume 1 (EK-PC300-V1-001).pdf). The 325 and 350 are identical apart from the listed differences.
  6. ↑ DEC, MP-01394-00, parts lists 5415101-0-DBP (CT100 system module, PDF p. 15), 5415138-0-DBP (CT100 video generator, PDF p. 40) and 5415084 (CT RAM, 128 KB, PDF p. 91).
  7. ↑ 7.0 7.1 DEC, MP-01394-00, KDF11-C large system unit motherboard parts list, PDF p. 11, item 8.
  8. ↑ 8.0 8.1 8.2 8.3 DEC, EK-PC350-TM-001, pp. 13-4 and 13-6, sections 13.3 and 13.4.1, with warnings on the input capacitors.
  9. ↑ Digital Equipment Corporation, PC380 engineering drawings, MP-01922, June 1984: H7862 print set MP-01320 revision B, contents (PDF p. 40); board assembly E-UA-5415086-0-0 revision H with ECO 5415086-ML010 (PDF p. 50); schematic D-CS-5415086-0-1 revision L (PDF pp. 51 to 53) (File:Professional 380 PC380 Engineering Drawings (MP-01922, June 1984).pdf).
  10. ↑ 10.0 10.1 DEC, EK-PC350-TM-001, pp. 5-54 and 5-130, section 5.3.13 and Table 5-23.
  11. ↑ 11.0 11.1 DEC, EK-PC380-PS-003, pp. 88 to 89, section 2.11, battery pack, and Figure 2-43.
  12. ↑ 12.0 12.1 12.2 Digital Equipment Corporation, Professional 380 Pocket Service Guide, EK-PC380-PS-003, June 1986, pp. 79 to 80, section 2.8, power supply removal (File:Professional 380 Pocket Service Guide (EK-PC380-PS-003).pdf).
  13. ↑ 13.0 13.1 DEC, EK-PC350-TM-001, p. 13-11, section 13.6, specifications.
  14. ↑ 14.0 14.1 14.2 DEC, EK-PC350-TM-001, p. 13-10, Table 13-3, system module connector J2.
  15. ↑ 15.0 15.1 DEC, EK-PC350-TM-001, pp. 13-5 to 13-9, sections 13.3.2 to 13.4.3 and Table 13-2.
  16. ↑ DEC, EK-PC300-V1-001, p. 10-7, section 10.4.2.2, overvoltage latch.
  17. ↑ Digital Equipment Corporation, Professional 300 Series Technical Manual, EK-PC350-TM-001, December 1982, p. 5-9, section 5.2.12 (File:Professional 300 Series Technical Manual (EK-PC350-TM-001, 1982).pdf).
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