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DECstation 5000 Capacitor Replacement Guide

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Capacitors taken out of an H7878 supply: the 820 µF 200 V input capacitor on the left, the 8200 µF and 3300 µF output capacitors, and the smaller output-stage parts (Rob Jarratt)

The capacitors most likely to have failed in a DECstation 5000 are in its power supply. DEC's maintenance guides treat the supply as a single replaceable unit and give no component list for it or for the system modules.[1][2] The values below come from owners' repair logs: Rob Jarratt's rebuild of the H7878 from a Model 240, which is the supply DEC lists as functionally identical to the Model 200's 30-32506-01, and a Vintage Computer Federation forum thread on the H7826-AA used in the Model 100 series.[3][4] Neither is a full parts list. Check every value against the part on your own board before ordering.

Warning: both supplies rectify the mains onto input capacitors charged to over 300 V DC, and parts of the primary-side control circuit sit at a dangerous voltage relative to neutral. Unplug the supply and confirm the input capacitors have discharged before working on it.[3][4]

Signs of failure

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  • The supply shuts down after running for a while, or will not start; the fans twitch once and stop.[3][4]
  • Low voltage across the input capacitors: Jarratt read about 170 V DC where about 330 V was correct, with one capacitor apparently open.[3]
  • Mottling on the underside of the board under the input capacitors, or a capacitor top slightly domed.[3]
  • Electrolyte on the output riser board; in Jarratt's supply C407 had leaked and left a shiny residue.[3]
  • A capacitor vented from the bottom, where it cannot be seen without lifting it. This was the fault on an H7826 from a 5000/133.[4]

See also Capacitor Failure Symptoms.

H7878 / 30-32506-01 (Model 200, 240 and 260)

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The output riser board in Jarratt's photographs is marked Astec and carries LM339 and LM393 comparators; Jarratt took it to be doing the output regulation. Its designators run in the 400s (C407, IC401, R414).[3]

Capacitors replaced in Rob Jarratt's H7878 (2017)
Position Value as found Location and notes
C10, C11 820 µF 200 V (Nippon Chemi-Con KMG, 105 °C, in Jarratt's photograph) Input rectifier smoothing. Held down with epoxy, with card insulating discs underneath; one had failed
C21 220 µF 25 V Output stage
C24 330 µF 16 V Output stage
C26 10 µF 25 V +12 V output; a small purple Sanyo part marked only "10 25"
C36 2.2 µF 50 V Output stage
C40, C41 120 µF 25 V Output stage
No designator visible 3300 µF (10 V SXE in the photograph) Across the +5 V output; high ESR
No designator visible 8200 µF (10 V SXE in the photograph) Across the +5 V output; ESR relatively high
C407 120 µF 25 V Output riser board; leaked, high ESR
C408 120 µF 16 V Output riser board; heat marks around it

[3]

Jarratt also replaced two small capacitors with high ESR early in the repair without recording their positions, and noted that C15, a Rifa 3300 pF class Y paper capacitor, looked slightly cracked; he left it in place.[3]

H7826-AA (Model 120, 125, 133 and 150)

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DEC rates this supply at 196 W.[2] No designator list has been published for it. Repair reports in the VCF thread:

  • The 5.1 V output has three 1800 µF capacitors; on one 5000/133 supply one was very leaky and had vented from the bottom.[4]
  • Three 120 Ω 2 W resistors in parallel beside them are a minimum load and darken the board with heat; Twylo planned to replace them with 5 W parts.[4]
  • Brown, narrow, tall 1800 µF 35 V capacitors had leaked in two other H7826 supplies seen by dreamlayers.[4]
  • C24, the start-up supply capacitor beside one of the big input capacitors, charges from the rectified mains through R7 and R8 and powers the controller until the transformer winding takes over. dreamlayers found it bad.[4]

Rob Jarratt replaced many of the output-area electrolytics in his H7826 and one heat-browned capacitor with high ESR on the 5019574 daughterboard, but the fault that kept it off was a corroded track.[5]

Procedure

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These steps follow Jarratt's H7878 repair.[3]

  1. Remove the supply from the system unit (see DECstation 5000 General Maintenance) and open its case.
  2. The power LED is wired into the board and does not come out with it. Desolder the LED wires, then partly unbolt the heat sink to free the board.
  3. Photograph the mains input connections before disconnecting them.
  4. Test each electrolytic for ESR. The large input capacitors are glued with epoxy; Jarratt cut one off and worked the other free. Keep the card insulating discs under them.
  5. Take the output riser board out to reach C407 and C408 and to clean under it; replacing parts on it in place is awkward.
  6. Fit replacements of the same capacitance and at least the same voltage, observing polarity, and clean off any electrolyte residue before refitting the riser board.
  7. Test with a load. Unloaded, the supply only twitches its fans. Jarratt used a 10 W 15 Ω resistor on the +5 V output at first, then a MicroVAX 2000 load board.
  8. Refit the LED wires and the supply, and run the self-test several times.

Checks after the repair

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KN02 supply limits (DEC KN02 specification, Table 3-1)
Rail Tolerance Range
+5.1 V ±4% 4.90 to 5.30 V
+12.1 V ±4% 11.62 to 12.58 V
−12.1 V ±4% −11.62 to −12.58 V

[6]

Measure under load and check ripple on the +5 V and +12 V outputs with an oscilloscope grounded to mains earth (the chassis), since the whole supply is earth-referenced once fitted.[3] After his repair Jarratt read 12.4 V on the 12.1 V output, inside the KN02 tolerance.[3][6] The supply's green LED should light, and DEC's self-test should complete with all diagnostic LEDs off.[1][7]

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References

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  1. ↑ 1.0 1.1 Digital Equipment Corporation, DECstation/DECsystem 5000 Model 200 Series Maintenance Guide, EK-PM38C-MG-002, April 1993, p. 5-20 (File:DECstation 5000 Model 200 Series Maintenance Guide (EK-PM38C-MG-002).pdf). 244 W supply, LED, part numbers 30-32506-01 and H7878A "functionally identical".
  2. ↑ 2.0 2.1 Digital Equipment Corporation, DECstation 5000 Model 100 Series Maintenance Guide, EK-PM32G-MG-003, April 1993, p. 5-26 (File:DECstation 5000 Model 100 Series Maintenance Guide (EK-PM32G-MG-003).pdf). H7826-AA, 196 W.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 Rob Jarratt, Rob's Old Computers: "DECstation 5000 Model 240 H7878 Power Supply Failure" (27 August 2017), "H7878 Possible Fault Found" (28 August 2017), "PSU Failing Under Moderate Load" (17 September 2017) and "PSU Fixed" (1 October 2017), robs-old-computers.com. Designators and values of the capacitors replaced, photographs of the removed parts and the riser board.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 "DECstation 5000/133 Power Supply Repair (H7826)", Vintage Computer Federation forum, posts by Twylo and paul (September 2017) and dreamlayers (2 October 2020), forum.vcfed.org. Community repair reports.
  5. ↑ Rob Jarratt, "Repair Progress on the H7826 Power Supply" (5 November 2017) and "H7826 Power Supply Repaired" (23 September 2018), Rob's Old Computers, robs-old-computers.com.
  6. ↑ 6.0 6.1 Digital Equipment Corporation, DS5000/200 KN02 System Module Functional Specification, 1990, Table 3-1 (File:DECstation 5000 Model 200 KN02 System Module Functional Specification.pdf).
  7. ↑ DEC, EK-PM38C-MG-002, p. 9-3, Table 9-1. LED code 00 for a completed self-test.
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