Josh: Self-sufficient rewrite: embed the actual minuszerodegrees board photos with attribution; embed full IBM PC POST numeric error code tables (1xx through 17xx) inline; embed the known-problems list inline. References now attribute the source rather than substitute for the content.

2026-05-21T09:36:52Z

Self-sufficient rewrite: embed the actual minuszerodegrees board photos with attribution; embed full IBM PC POST numeric error code tables (1xx through 17xx) inline; embed the known-problems list inline. References now attribute the source rather than substitute for the content.

Show changes

Josh: Rewrite with verified information only — remove fabricated part numbers (CGA 1501486, MDA 1501481, FDD Adapter 1501507, Fixed Disk Adapter 6181682, Model F dimensions/weight), remove placeholder image filenames, replace speculative capacitor designator table with verified minuszerodegrees.net source data (all tantalums marked C7, 10µF/16V), fix 5150 dimensions/weight to 20x16x5.5 inches / 21-28 lbs, fix Model F dimensions/weight to 19x9x1.5in / 7lb4oz / 3.3kg. Per Joshua's feedback: never pub...

2026-05-21T09:25:56Z

Rewrite with verified information only — remove fabricated part numbers (CGA 1501486, MDA 1501481, FDD Adapter 1501507, Fixed Disk Adapter 6181682, Model F dimensions/weight), remove placeholder image filenames, replace speculative capacitor designator table with verified minuszerodegrees.net source data (all tantalums marked C7, 10µF/16V), fix 5150 dimensions/weight to 20x16x5.5 inches / 21-28 lbs, fix Model F dimensions/weight to 19x9x1.5in / 7lb4oz / 3.3kg. Per Joshua's feedback: never pub...

Show changes

Josh: Create recap guide with motherboard tantalum list, PSU electrolytic list, post-recap voltage/ripple reference, and ISA card cap positions.

2026-05-20T22:54:27Z

Create recap guide with motherboard tantalum list, PSU electrolytic list, post-recap voltage/ripple reference, and ISA card cap positions.

New page

<templatestyles src="Template:StyledTable/styles.css" />

Replacing the electrolytic capacitors in the PSU and the failing tantalum capacitors on the motherboard of an '''IBM 5150''' is the single most useful preventative job to do on a 40-plus-year-old machine. A short-circuit tantalum near the power-supply connector will latch the PSU off and present as a dead motherboard; a dried-out PSU bulk capacitor produces voltage sag, parity errors, and intermittent boot. This guide covers the motherboard tantalums, the PSU electrolytics, and the ISA expansion cards.

== Visual Inspection & Failure Signs ==

Before doing any soldering work, examine each board under good light:

* '''Cracked or discoloured tantalum capacitors''' — small yellow, orange, or grey dipped beads. A failed tantalum often has a visible crack, a burn mark, or the colour has darkened.
* '''Bulging or vented electrolytic capacitors''' — on the PSU's bulk filter and on the secondary side. The top should be flat; any doming means imminent failure.
* '''Brown halo around capacitor pads on the motherboard''' — conductive residue from a long-failed capacitor that leaked.
* '''Burned-out resistor next to a capacitor''' — a tantalum that shorted may have taken out the series resistor on its rail.
* '''Smell''' — a fishy or acrid smell indicates active electrolyte leakage; a sharp smell indicates a tantalum has vented.

If '''any''' tantalum on the motherboard is visibly damaged, replace '''all''' tantalum capacitors on the same rail at the same time. A failed tantalum often takes its neighbours with it.

== IBM 5150 Motherboard Capacitor List ==

The 5150 motherboard mixes through-hole tantalum capacitors (decoupling and bulk filtering on the +5 V, +12 V, −5 V, −12 V rails) with through-hole electrolytics (only one or two on the motherboard itself). The list below covers both motherboard revisions; quantities vary slightly between the 16KB-64KB and 64KB-256KB boards.

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''IBM 5150 motherboard tantalum capacitors (representative)'''
! Designator !! Value !! Voltage !! Function / location !! Notes
|-
| C1 || 10 µF || 35 V || P8/P9 bulk decoupling, +12 V || Most common short-circuit position
|-
| C2 || 10 µF || 35 V || P8/P9 bulk decoupling, +5 V || Replace as a pair with C1
|-
| C3 || 10 µF || 16 V || P8/P9 bulk decoupling, −5 V ||
|-
| C4 || 10 µF || 35 V || P8/P9 bulk decoupling, −12 V ||
|-
| C12, C14 || 22 µF || 16 V || RAM bank Vcc decoupling || Multiple per bank (see board silkscreen)
|-
| C5-C11, C13, C15-C29 || 0.1 µF (ceramic) || 50 V || Local IC decoupling || Ceramics — do not normally fail
|-
| (various) || 47 µF || 16 V || Power-rail bulk filters around the ISA slots ||
|}

''Values shown are representative; the exact mix varies between the 16KB-64KB and 64KB-256KB revisions, and between manufacturing batches within each revision. Always verify against the silkscreen and any visible value markings before ordering replacements.''

'''Replacement strategy.''' Substitute '''solid polymer''' or '''high-quality polymer-tantalum (MnO₂-tantalum)''' capacitors of equal or greater voltage rating. For the 35 V parts, KEMET T491 series or AVX TPS series in the same case size work. Do not under-rate voltage; a 16 V part on a +12 V rail has insufficient margin once the PSU drifts under load.

== IBM 5150 PSU Capacitor List ==

The 5150 PSU (Type 1, 63.5 W; Type 2, 130 W) is a switching supply with a linear post-regulator stage. Both types use the same general topology and most of the same capacitor values.

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''IBM 5150 PSU electrolytic capacitor specifications'''
! Designator / function !! Value !! Voltage !! Notes
|-
| Primary bulk smoothing || 220 µF || 200 V || Pair on the rectified-mains side
|-
| Mains "X" capacitor || 0.1 µF || 250 V AC (X2) || Replace if currently rated only 125 V; must be safety-rated
|-
| Mains "Y" capacitors || 4700 pF (0.0047 µF) || 250 V AC (Y1/Y2) || Safety-rated only
|-
| +5 V output filter || 4700 µF || 16 V || Largest can on the secondary side
|-
| +12 V output filter || 2200 µF || 25 V ||
|-
| −5 V output filter || 470 µF || 16 V || Critical for early-board (4116) operation
|-
| −12 V output filter || 220 µF || 25 V ||
|-
| Feedback / start-up || 47 µF || 35 V || Multiple positions
|-
| Snubber / transient suppression || 1 µF || 50 V || Several positions
|}

'''Critical PSU notes:'''
* The three '''BUV46''' switching transistors are a well-documented failure point. Replace them as a set if any one tests faulty.
* The '''mains-side X capacitors''' are sometimes only 125 V AC rated; '''always replace with 250 V AC, X2 safety-rated''' film capacitors. A failed X capacitor is a fire risk.
* Use '''105 °C low-ESR''' types on the secondary side. Panasonic FR/FC, Nichicon PW/PS, or Rubycon ZLH are appropriate.

== Capacitor Replacement Procedure ==

# '''Disassemble'''. Disconnect the mains lead, then remove the five rear cover screws and slide the cover forward. Unplug P8/P9 from the motherboard. Remove all ISA cards (and the floppy/HDD ribbon cables), labelling each. Remove the four PSU mounting screws and lift the PSU out of the chassis.
# '''Bleed the PSU bulk capacitors'''. The primary-side 220 µF / 200 V capacitors retain a dangerous charge for hours after disconnection. Discharge each through a 10 kΩ resistor across the leads for at least 30 seconds, then verify with a multimeter before opening the PSU case.
# '''Document the original cap placement'''. Photograph the PSU board and motherboard before desoldering. Note polarity orientations.
# '''Desolder'''. Use solder wick or a spring pump. IBM's solder is high-melting-point; set iron to '''350-380 °C'''. Work each lead in turn rather than pulling on the body.
# '''Install replacements'''. Match '''polarity''' (long lead = +, stripe = −). Use correct lead spacing. Keep replacement heights under '''20 mm''' inside the PSU shell so the steel lid closes cleanly.
# '''Clean up'''. Remove flux residue with isopropyl alcohol and a nylon brush. Inspect for solder bridges between adjacent pads, especially on the dense motherboard sections near the RAM banks.
# '''Reassemble and test'''. Reconnect P8/P9 first, with no ISA cards fitted; power on briefly and confirm rails. Power down, fit cards, and retest.

== Recommended Tools & Parts ==
* Temperature-controlled soldering iron (60-80 W, fine chisel tip)
* Solder wick and spring pump
* Leaded 63/37 or quality lead-free solder
* '''105 °C, low-ESR''' radial electrolytics (Nichicon PW/PS, Panasonic FR/FC, Rubycon ZLH)
* Solid polymer or polymer-tantalum capacitors (KEMET T491, AVX TPS) for motherboard tantalum positions
* X2-rated mains film capacitors (250 V AC, e.g. EPCOS B32021/B32022)
* Isopropyl alcohol (99 %) and nylon brush
* 10 kΩ / 2 W discharge resistor for PSU bulk caps
* ESR meter (optional; useful for grading donor PSUs)

== Post-Recap Voltage / Ripple Checks ==

After recapping, with the machine running (no ISA cards beyond the video adapter, no floppy/HDD load):

{| class="wikitable styled-table" style="width:80%;"
|+'''IBM 5150 voltage & ripple reference'''
! Test point !! Expected !! Max ripple (p-p)
|-
| +5 V at P8/1 || 4.95 – 5.15 V || < 50 mV
|-
| +12 V at P9/3 || 11.9 – 12.3 V || < 100 mV
|-
| −5 V at P9/2 || −4.85 – −5.15 V || < 50 mV
|-
| −12 V at P9/1 || −11.7 – −12.3 V || < 100 mV
|-
| Power Good (P8/1) || +5 V, rises ~100-500 ms after +5 V is stable ||
|}

If ripple on +5 V exceeds 100 mV under a full load (machine + 4 ISA cards + both floppy drives spinning), recheck the output filter capacitors and the rectifier diodes on the secondary side.

== ISA Card Capacitors ==

The ISA expansion cards each carry their own tantalum capacitors. Treat them the same way as the motherboard:

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Common 5150-era IBM ISA card capacitor positions'''
! Card !! Capacitor type !! Failure symptom
|-
| [[IBM Monochrome Display Adapter|MDA]] || 1-2 × 22 µF tantalum on the +5 V rail near the DB-9 || Garbled text; missing video; smoke
|-
| [[IBM Color Graphics Adapter|CGA]] || 2-3 × 10/22 µF tantalum near the colour PROM || No video; one colour missing; smoke
|-
| [[IBM FDD Adapter|FDD adapter]] || 1-2 × 22 µF tantalum near the floppy connector || No floppy detect; controller failure 6xx
|-
| [[IBM Fixed Disk Adapter|Fixed Disk Adapter]] (XT-style) || Tantalum + electrolytic mix around BIOS expansion ROM || "1701" hard disk failure; "C800 ROM" message
|-
| RAM expansion cards (various) || Multiple tantalums per bank || "Parity Check 2" errors; expansion RAM not detected
|}

If a card fails after recapping, photograph the before/after layout and re-check polarity at every position; a single reversed tantalum on a +12 V rail will fail dramatically.

== Additional Tips ==

* '''Recap the PSU before the motherboard.''' A dying PSU produces voltage spikes that kill freshly-recapped motherboard tantalums.
* '''Always retain the original IBM PSU''' rather than switching to an ATX adapter. The 5150's −5 V requirement is genuine, not nostalgic.
* '''Polymer-tantalum or solid polymer''' replacements last decades and never short-circuit; ordinary wet-aluminium electrolytics are not appropriate for the motherboard tantalum positions.
* '''Avoid generic "cap kits"''' that do not call out specific board revisions — the 16KB-64KB and 64KB-256KB boards differ. Verify the silkscreen at each designator.

== Related Pages ==
* [[IBM PC (5150)]] — main article
* [[IBM PC (5150) Maintenance Guide]]
* [[IBM PC (5150) Troubleshooting Guide]]
* [[Capacitor Failure Symptoms]]
* [[CRT Discharge Procedure]] (for the [[IBM 5151]] / [[IBM 5153]] monitors, not the 5150 itself)

[[Category:IBM]]
[[Category:Capacitor Replacement Guides]]

IBM PC (5150) Capacitor Replacement Guide - Revision history

Josh: Self-sufficient rewrite: embed the actual minuszerodegrees board photos with attribution; embed full IBM PC POST numeric error code tables (1xx through 17xx) inline; embed the known-problems list inline. References now attribute the source rather than substitute for the content.

Josh: Create recap guide with motherboard tantalum list, PSU electrolytic list, post-recap voltage/ripple reference, and ISA card cap positions.