IBM PC (5150) Capacitor Replacement Guide
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.
| 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.
| 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):
| 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:
| Card | Capacitor type | Failure symptom |
|---|---|---|
| MDA | 1-2 × 22 µF tantalum on the +5 V rail near the DB-9 | Garbled text; missing video; smoke |
| CGA | 2-3 × 10/22 µF tantalum near the colour PROM | No video; one colour missing; smoke |
| FDD adapter | 1-2 × 22 µF tantalum near the floppy connector | No floppy detect; controller failure 6xx |
| 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)