Editing DECstation 3100 Capacitor Replacement Guide

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[[File:DEC H7821-00 power supply main board capacitor locations (Malte Dehling).png|thumb|right|340px|Capacitor positions on the main board shared by the H7821-00 and H7822-00 supplies, component side up; designators are Dehling's own (Malte Dehling)]]
The electrolytic capacitors most at risk in a '''[[DECstation 3100]]''' are in its power supply. DEC published no component list for the supply. Malte Dehling, who repairs the VAXstation 3100 family, recorded the original values in the two supplies used across that family and lists the DECstation 3100 among the machines with the H7822-00 (larger BA42-B enclosure) and the DECstation 2100 among those with the H7821-00 (smaller BA42-A enclosure).<ref name="dehling" /> The KN01 system module's own capacitors are drawn on DEC's schematic.<ref name="sch" /> Check the model label on your supply before ordering.

Warning: the supply rectifies the mains onto capacitors rated 250 V and 400 V. Unplug it and confirm they are discharged before working on the board.<ref name="dehling" />

== Failure signs ==
Dehling describes the supplies as "plagued with the usual leaky capacitor issues"; most are fixed by cleaning the board with isopropanol and replacing the capacitors.<ref name="dehling" /> Look for electrolyte on the board and the base of the capacitors, and for bulged or vented cans; see [[Capacitor Failure Symptoms]].

== H7822-00 capacitor list ==
The H7822-00 has two large boards. One is identical to the H7821-00 board, so a full recap needs both lists.<ref name="dehling" /> Neither board has designators or polarity markings; the numbers below are Dehling's, keyed to his drawings.<ref name="dehling" />

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''Main board, shared with the H7821-00 (Dehling)'''
! Designator !! Quantity !! Original part
|-
| C1–C5 || 5 || 1800 µF 25 V 105 °C, low ESR
|-
| C6, C11 || 2 || 330 µF 25 V 85 °C
|-
| C7–C10 || 4 || 10 µF 35 V 85 °C
|-
| C12 || 1 || 470 µF 25 V 85 °C
|-
| C13, C14 || 2 || 470 µF 250 V 105 °C
|-
| C15 || 1 || 39 µF 16 V 85 °C, on a small daughterboard
|-
| C16 || 1 || 47 µF 16 V 105 °C, on a small daughterboard
|}
<ref name="dehling" />

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''Second board, H7822-00 only (Dehling)'''
! Designator !! Quantity !! Original part
|-
| C20 || 1 || 220 µF 400 V 85 °C
|-
| C21 || 1 || 330 µF 50 V 85 °C
|-
| C22 || 1 || 47 µF 16 V 105 °C
|-
| C23–C26 || 4 || 1800 µF 25 V 105 °C, low ESR
|}
<ref name="dehling" />

[[File:DEC H7822-00 power supply second board capacitor locations (Malte Dehling).png|center|thumb|600px|Second board of the H7822-00, component side up (Malte Dehling)]]

Dehling's notes on the parts:<ref name="dehling" />
* He replaces every capacitor with a 105 °C part of the same or higher voltage.
* C1–C5 and C23–C26 must be low-ESR types.
* For C15 he fits 33 µF or 47 µF in place of the 39 µF original.
* On C15 the negative lead is the one nearer the main board.
His drawing of the second board labels two positions C22; the four large output capacitors are the group of four round outlines in the right half, listed as C23–C26 in his table.<ref name="dehling" />

== KN01 system module ==
DEC's schematic gives the system module's decoupling as text on sheet 12: two 100 µF 10 V capacitors and nine 10 µF 15 V capacitors across the +5 V supply, single 10 µF 15 V capacitors on the +12 V, −12 V and −9 V inputs and on the clock supply, and 193 small 0.047 µF 50 V capacitors.<ref name="sch" /> The drawing does not say whether the larger ones are tantalum or aluminium electrolytic, and neither DEC's documents nor Dehling's notes report failures of them. Check their type on the board before replacing any.

== Procedure ==
# Unplug the system unit, take the supply out and let it stand so the input capacitors discharge; confirm with a meter.
# Photograph both boards and mark the polarity of every capacitor before removing it, since the boards carry none.<ref name="dehling" />
# Clean electrolyte and dirt off the board with isopropanol.<ref name="dehling" />
# Fit replacements from the lists above: 105 °C, same or higher voltage, low ESR where marked.<ref name="dehling" />
# Test. Dehling finds the supply runs acceptably without a load, with the fans turning and the voltages about right, but its LED does not light until it has some load.<ref name="dehling" />
# Refit and check the rails at the system module against DEC's limits below.

== Checks after the repair ==
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''KN01 supply limits (DEC Table 3-1)'''
! Rail !! Tolerance !! Range !! Maximum load
|-
| +5 V || ±5% || 4.75 to 5.25 V || 15.6 A
|-
| +12 V || ±5% || 11.40 to 12.60 V || 0.89 A
|-
| −12 V || ±5% || −11.40 to −12.60 V || 0.11 A
|-
| −9 V || ±5% || −8.55 to −9.45 V || 0.18 A
|}
<ref name="fs3" />
The power-up self-test should then count down to 0 with no FAILURE message; see [[DECstation 3100 Troubleshooting Guide]].<ref name="og7" />

== Related pages ==
* [[DECstation 3100]]
* [[DECstation 3100 General Maintenance]]
* [[DECstation 3100 Troubleshooting Guide]]
* [[DECstation 5000 Capacitor Replacement Guide]]
* [[Capacitor Failure Symptoms]]

== References ==
<references>
<ref name="dehling">Malte Dehling, "DIGITAL VAXstation 3100 PSU Repair", GitHub repository README and drawings h7821-00.svg and h7822-00.svg, May 2022, [https://github.com/mdehling/vs3100-psu-repair github.com/mdehling/vs3100-psu-repair]. Machines using each supply, original capacitor values, replacement notes. Community source.</ref>
<ref name="sch">Digital Equipment Corporation, KN01 system module schematic, drawing 54-19463-01, October 1988, sheet 12, "Real-Time Clock" ([[:File:DECstation 3100 KN01 System Module Schematic (54-19463-01, Oct 1988).pdf]]). Power connector decoupling and the "cap", "bulk" and "mongo" capacitor counts.</ref>
<ref name="fs3">Digital Equipment Corporation, ''DECstation 3100 Desktop Workstation Functional Specification'', revision 1.3, 28 August 1990, section 3, p. 2, Table 3-1 ([[:File:DECstation 3100 Desktop Workstation Functional Specification (Rev 1.3, 1990).pdf]]).</ref>
<ref name="og7">Digital Equipment Corporation, ''DECstation 2100/3100 Operator's Guide'', EK-302AB-OG-002, September 1989, pp. 7-2 to 7-3 ([[:File:DECstation 2100 3100 Operator's Guide (EK-302AB-OG-002).pdf]]). Self-test display.</ref>
</references>

[[Category:Digital Equipment Corporation]]
[[Category:Capacitor Replacement Guides]]