Editing Rainbow 100 Capacitor Replacement Guide

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[[File:Rainbow 100 system module layout (DEC Figure 4-2).png|thumb|right|340px|PC100-A system module layout. The RX50 controller plugs into J9 at bottom right and the power supply cable into J8 at right (DEC, ''Rainbow 100 Technical Manual'', Figure 4-2)]]
Capacitor values for the '''[[Rainbow 100]]''' (PC100-A) from DEC's PC100 field maintenance print set, MP-01491-00.<ref name="mp1491" /> The system module and the RX50 controller carry only six aluminium electrolytics between them. Most of the ageing electrolytics in a Rainbow are in the H7842-A power supply and the VR201 monitor, which the Rainbow shares with the [[DECmate II]]; their values are on [[DECmate II Power Supply Service]] and [[DECmate III Capacitor Replacement Guide]], and are summarised below. For the PC100-B board, see [[Rainbow 100+ Capacitor Replacement Guide]].

Warning: the H7842 rectifies the mains to about 325 V DC. DEC's instruction is to switch off, wait five minutes before removing the supply cover, and check with a voltmeter that no high voltage remains.<ref name="tm71" /> The VR201 holds over 12,000 V on its CRT with the power off; see [[CRT Discharge Procedure]].<ref name="tm113" />

== System module (5415486) ==
The PC100 mother board parts list, revision J, released January 1985, gives these non-ceramic capacitors. Everything else on the board is ceramic or mica.<ref name="mp11" />
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+ '''PC100-A system module electrolytic and tantalum capacitors'''
! Reference !! Value !! Rating !! Type !! DEC part
|-
| C11, C23, C24, C25 || 10 µF || 35 V || Aluminium electrolytic || 1017472-00
|-
| C62 || 3.9 µF || 10 V || Solid tantalum || 1000064-00
|}
<ref name="mp11" />

== RX50 controller (5415482) ==
The controller module plugs into the system module at J9.<ref name="tm44" /> Its parts list, revision C:<ref name="mp30" />
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''RX50 controller electrolytic and tantalum capacitors'''
! Reference !! Value !! Rating !! Type !! DEC part
|-
| C4, C21 || 10 µF || 35 V || Aluminium electrolytic || 1017472-00
|-
| C2 || 6.8 µF || 35 V || Solid tantalum || 1005306-00
|}
<ref name="mp30" />

== H7842-A power supply ==
Neither Rainbow print set includes the H7842's own parts list. Tony Duell's hand-drawn schematic gives component values without voltage ratings: two 820 µF mains reservoirs in series across the voltage doubler, 2200 µF on the start-up supply, 3200 µF on the +12 V output and 5600 µF on the +5 V output ahead of the chokes, and smaller parts on the −12 V winding and regulator.<ref name="duell" /> A list quoted on the cctech mailing list gives ratings for the same set, but shows two 3200 µF parts where Duell has 3200 µF and 5600 µF.<ref name="cctech" /> The full table, and Rob Jarratt's repair record for this supply, are on [[DECmate II Power Supply Service]]. Read the value and rating printed on each capacitor before ordering.

== VR201 monitor ==
DEC's VR201 print set, MP-01410, lists 21 aluminium electrolytics on the monitor board 54-15500, including C6, 10 µF 250 V; the full table is on [[DECmate III Capacitor Replacement Guide]].<ref name="vr201" /> Rob Jarratt, restoring the VR201 from his Rainbow 100A, found bulging or high-ESR parts behind a shimmering picture and replaced C24, C30, C41, C303, C39 and C404.<ref name="rob3" /> On DEC's list those are four 1000 µF 16 V parts (C24, C30, C41, C303) and two 100 µF 25 V parts (C39, C404).<ref name="vr201" />

== Procedure ==
# Switch off, unplug the system unit and disconnect the monitor. For the supply, wait five minutes and measure across the reservoir capacitors before touching anything.<ref name="tm71" />
# Remove the system module and RX50 controller as described on [[Rainbow 100 General Maintenance]].
# Photograph each board and mark the polarity of every electrolytic before removing it.
# Replace like for like in capacitance, at the original voltage rating or higher. Keep tantalums as tantalums, or fit a suitably rated low-ESR part.
# Refit the boards, reconnect the 13-pin power cable to the supply first, and power up with the monitor connected.
# Check that the self-test ends at the Main System Menu with no message and that the extended self-test (S key) passes.

== After the recap ==
Measure the supply rails at the system module power connector J8 with the machine running:<ref name="tm330" />
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+ '''J8 power connector and DEC's output tolerances'''
! Pins !! Rail !! DEC nominal and tolerance !! Acceptable range
|-
| 7, 8, 9 || +5 V || +5.1 V ±6% || 4.79–5.41 V
|-
| 5, 6 || +12 V || +12.2 V ±6% || 11.47–12.93 V
|-
| 4 || −12 V || −12 V ±7% || −11.16 to −12.84 V
|-
| 10–13 || Ground || – || –
|}
<ref name="tm330" /><ref name="tm78" />

DEC's chapter 1 summary gives the +12 V output as +12.1 V.<ref name="tm110" /> See [[Capacitor Failure Symptoms]] for what failed parts look like.

== Related pages ==
* [[Rainbow 100]]
* [[Rainbow 100 General Maintenance]]
* [[Rainbow 100 Troubleshooting Guide]]
* [[Rainbow 100+ Capacitor Replacement Guide]]
* [[DECmate II Power Supply Service]]
* [[DECmate III Capacitor Replacement Guide]]
* [[Capacitor Failure Symptoms]]

== References ==
<references>
<ref name="mp1491">Digital Equipment Corporation, PC100 field maintenance print set, MP-01491-00, 1982 ([[:File:Rainbow 100 PC100-A Print Set (MP-01491-00).pdf]]).</ref>
<ref name="mp11">DEC, print set MP-01491-00, PC100 mother board parts list 5415486-0-DBP, revision J, sheet A1 (PDF p. 11). C11, C23–C25 10 µF 35 V aluminium electrolytic; C62 3.9 µF 10 V solid tantalum.</ref>
<ref name="mp30">DEC, print set MP-01491-00, PC100 RX50 controller parts list 5415482-0-DBP, revision C, sheet A1 (PDF p. 30). C4, C21 10 µF 35 V aluminium electrolytic; C2 6.8 µF 35 V solid tantalum.</ref>
<ref name="tm44">Digital Equipment Corporation, ''Rainbow 100 Technical Manual'', EK-PC100-TM-001, May 1984, p. 4-4, Figure 4-2 ([[:File:Rainbow 100 Technical Manual (EK-PC100-TM-001).pdf]]).</ref>
<ref name="tm71">DEC, ''Rainbow 100 Technical Manual'', EK-PC100-TM-001, pp. 7-1 to 7-2. Power supply warnings: five-minute wait and voltmeter check.</ref>
<ref name="tm78">DEC, ''Rainbow 100 Technical Manual'', EK-PC100-TM-001, pp. 7-8 to 7-9, section 7.5.2. Output specification.</ref>
<ref name="tm110">DEC, ''Rainbow 100 Technical Manual'', EK-PC100-TM-001, p. 1-10. Summary power specification.</ref>
<ref name="tm113">DEC, ''Rainbow 100 Technical Manual'', EK-PC100-TM-001, p. 11-3. VR201 high-voltage warning.</ref>
<ref name="tm330">DEC, ''Rainbow 100 Technical Manual'', EK-PC100-TM-001, p. 3-30, Table 3-11, Power Supply Connector (J8) Signals.</ref>
<ref name="duell">Tony Duell, H7842 power supply schematic, sheets 1 to 3 ([[:File:DEC H7842 Power Supply Schematic (Tony Duell).pdf]]).</ref>
<ref name="cctech">Alan Perry, "The continuing story of a Rainbow 100 (Was: Rainbow 100 PSU capacitor list)", cctech mailing list, 12 March 2019, [https://www.classiccmp.org/pipermail/cctech/2019-March/037949.html classiccmp.org].</ref>
<ref name="vr201">Digital Equipment Corporation, VR201 monitor print set, MP-01410, May 1982, parts list 5415500 ([[:File:VR201 Monitor Print Set (MP01410).pdf]]).</ref>
<ref name="rob3">Rob Jarratt, "Rainbow 100A Restoration", ''Rob's Old Computers'', 29 July 2025, [https://robs-old-computers.com/2025/07/29/rainbow-100a-restoration/ robs-old-computers.com]. VR201 capacitors replaced.</ref>
</references>

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