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Professional 325 Capacitor Replacement Guide

From RetroTechCollection
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] The only other electrolytic part a restorer needs to deal with is the nickel-cadmium clock battery pack on the system module.[5]

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.[6] 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.[6]

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]

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).[5][7][8] The pack is charged at 10 mA from the +12 V supply whenever the computer is on.[7] 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.[8] 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.[9]
  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.[9]
  3. Before opening the cover, discharge and measure C1 and C2 as described in the warning above.[6]
  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.[6][10][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.[9]

After the recap

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Check the outputs at the 16-pin connector J2 with the system connected.[11]

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

[10][12][11] J2 also carries DCOK on pin 1 and POK on pin 3, with grounds on pins 10 to 16.[11] 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.[12][13] A recapped clock battery pack takes 48 hours of running to charge fully.[14]

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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. ↑ 5.0 5.1 DEC, MP-01394-00, KDF11-C large system unit motherboard parts list, PDF p. 11, item 8.
  6. ↑ 6.0 6.1 6.2 6.3 DEC, EK-PC350-TM-001, pp. 13-4 and 13-6, sections 13.3 and 13.4.1, with warnings on the input capacitors.
  7. ↑ 7.0 7.1 DEC, EK-PC350-TM-001, pp. 5-54 and 5-130, section 5.3.13 and Table 5-23.
  8. ↑ 8.0 8.1 DEC, EK-PC380-PS-003, pp. 88 to 89, section 2.11, battery pack, and Figure 2-43.
  9. ↑ 9.0 9.1 9.2 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).
  10. ↑ 10.0 10.1 DEC, EK-PC350-TM-001, p. 13-11, section 13.6, specifications.
  11. ↑ 11.0 11.1 11.2 DEC, EK-PC350-TM-001, p. 13-10, Table 13-3, system module connector J2.
  12. ↑ 12.0 12.1 DEC, EK-PC350-TM-001, pp. 13-5 to 13-9, sections 13.3.2 to 13.4.3 and Table 13-2.
  13. ↑ DEC, EK-PC300-V1-001, p. 10-7, section 10.4.2.2, overvoltage latch.
  14. ↑ 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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