Editing IBM PC AT Capacitor Replacement Guide

This guide documents the procedure for diagnosing and replacing capacitors on the IBM PC AT (5170) motherboard, expansion cards and switching power supply.

The 5170 motherboard uses the same family of 10 µF / 16 V '''tantalum''' filter capacitors as the 5150 and 5160 motherboards. These tantalums are known to fail '''short-circuit''' after long periods unpowered, which results in the +5 V or +12 V rails being pulled down at power-on and the 192 W AT PSU refusing to start.

This guide does not enumerate each tantalum's reference designator on the Type 1 / Type 2 / Type 3 boards, because the layout differs between the three revisions and IBM did not publish a unified silkscreen reference for the AT. Treat every orange or yellow bead-style capacitor on the motherboard as a candidate for inspection.

== Failure Mode ==

Tantalum capacitors used by IBM on 51xx motherboards are commonly marked "106 16V" or "10µF 16V" and are rated 10 µF / 16 V. They are polarised (positive leg goes into the positive hole on the board). They fail in two modes:

* '''Open circuit''' &mdash; the cap stops filtering, but the rail still rises to its nominal voltage. The motherboard usually still boots, although decoupling noise tolerance is reduced. Detect with an in-circuit ESR meter or by removing the cap and measuring on the bench.
* '''Short circuit''' &mdash; the cap pulls its rail down. The PSU detects the short and goes into protection (fan may still turn, but no rails come up). If the short is not detected by the PSU, current can flow through the cap until it ruptures &mdash; sometimes spectacularly, with sparks or smoke. A short on +12 V is the most common AT-class symptom.

Some tantalum failures show '''no visible damage''' on the cap. Do not rely on visual inspection alone.

== Tools and Materials ==

* Temperature-controlled soldering iron with a chisel or fine tip.
* Solder wick or a desoldering pump.
* '''60/40 leaded solder''' or modern lead-free; flux paste improves wetting on 40-year-old joints.
* Replacement '''10 µF / 16 V tantalum''' capacitors (or low-ESR 16 V / 25 V ceramics, but match form factor) &mdash; two-legged for almost all 51xx positions. The early 51xx motherboards occasionally use a three-hole position for a two-legged tantalum; the middle hole is unused.
* Anti-static strap connected to chassis ground.
* Digital multimeter with continuity / diode test.
* A reference photo for polarity (see image gallery below).

== Diagnostic Procedure ==

The standard procedure for an AT that does not start (PSU fan spins but no +5 V on the bus) is:

# Remove the motherboard from the chassis.
# Disconnect the PSU and inspect the +5 V and +12 V tantalums on the motherboard with the board out of the chassis.
# Set the multimeter to '''diode test''' or '''200 Ω''' resistance range.
# Probe each tantalum '''in-circuit''' &mdash; black probe to the ground side, red probe to the rail side. A good tantalum reads as an open / high resistance after a brief charge pulse. A failed (shorted) tantalum reads close to 0 Ω.
# When you find a short, the cap '''must be removed''' to confirm; other components on that rail can mimic a short.

In severe cases the cap may show charring at the body or board around it. In other cases the only sign is a brief flash on the rail at power-on.

== Removal and Replacement ==

# Mark polarity of the original cap on the board with a paint pen or photograph.
# Apply fresh solder + flux to both pads to wet them.
# Heat one pad, gently lever the cap up on that side. Heat the other pad and lift the cap clear.
# Clean the holes with solder wick.
# Insert the new tantalum, matching polarity: the '''+''' marking on the cap to the '''+''' marking on the silkscreen.
# Solder both legs from the underside of the board. Inspect for a clean fillet.
# Trim the legs flush with the solder fillet.
# Power up the board on the bench (a spare PSU, no drives) and verify rails before reinstalling.

If the original cap is in a three-hole IBM footprint and you have a two-legged replacement, use the two outer holes (the polarity pair); leave the centre hole empty. The middle hole is for a three-legged tantalum that IBM used in some board generations and is not electrically required if the replacement is two-legged.

== ISA Card Capacitors ==

The IBM EGA card, the IBM CGA card, the IBM MDA card, the IBM Asynchronous Communications Adapter, the IBM Serial/Parallel Adapter and the IBM Fixed Disk and Diskette Drive Adapter all carry tantalums of the same 10 µF / 16 V type. Any one of these on +5 V or +12 V can short and pull down the system rails at power-on. If the motherboard tests clean off-system but the system fails to start with the cards installed, pull each card one at a time and re-test.

== Power Supply Capacitor Replacement ==

The 5170 PSU is a 192 W switching unit. Its primary-side mains filter typically uses an '''X2-class line suppression capacitor'''. These capacitors (often made by RIFA in the original units) age and rupture; symptoms include smoke, a strong foul odour, and visible fluid on the cap body. Replace with a modern X2-class cap of equivalent capacitance and voltage class.

The secondary-side aluminium electrolytic capacitors in the PSU (typical values 1000 µF / 16 V to 220 µF / 25 V) dry out over decades. Symptoms include reduced or sagging rails, audible whine, and PSU failure under load. Replace with new low-ESR electrolytics of the same capacitance, equal or higher voltage rating, and equal or higher temperature rating (105 °C is a common upgrade from 85 °C).

'''Warning:''' The PSU's primary side carries mains voltage and the bulk capacitor (usually 220 µF / 200 V or higher) can hold a lethal charge after the unit is unplugged. Always discharge the bulk capacitor through a current-limited resistor (1 kΩ / 5 W to ground) before working on the primary side.

== Post-Recap Verification ==

After any capacitor work, do the following before fitting the board back to the system:

# Bench-test the board with a known-good PSU and no peripherals. Verify each rail at the +5 V test point, the +12 V test point and at the disk drive connector.
# Confirm the PSU is in regulation under light load.
# Install the motherboard back in the chassis and re-fit cards one at a time, testing after each.

If a particular ISA card causes the PSU to drop, the fault is on that card; either repair it or remove it from the slot for now.

== Polarity Reference ==

The polarity-marking convention on IBM 51xx motherboards is consistent across the 5150, 5160 and 5170: the '''+ marking on the silkscreen''' is the positive (rail) side; the '''bar / minus marking on the cap body''' is the negative side. Match these when fitting a replacement.

[[File:IBM 5150 tantalum polarity reference.jpg|center|thumb|720px|Polarity reference for IBM 51xx motherboard tantalum capacitors. The same convention applies to the 5170 motherboard. (Image: minuszerodegrees.net)]]

A failed tantalum often has '''no visible damage'''. The photo below shows the kind of subtle damage that does appear in roughly one case in four.

[[File:IBM 5150 failed tantalum visual example.jpg|right|thumb|300px|A failed tantalum capacitor on an IBM 51xx motherboard. The small black hole on the body is the only visual indication of failure &mdash; in most cases there is no visual sign at all. (Image: minuszerodegrees.net)]]

The recommended soldering pattern (SNCTOL &mdash; "solder needs cleaning to other leg") used by minuszerodegrees for through-hole desoldering on 51xx boards is shown below.

[[File:IBM 5150 SNCTOL soldering technique.png|center|thumb|600px|SNCTOL soldering technique for through-hole component removal on IBM 51xx boards. (Diagram: minuszerodegrees.net)]]

== Related Pages ==

* [[IBM PC AT (5170)]]
* [[IBM PC AT Maintenance Guide]]
* [[IBM PC AT Troubleshooting Guide]]
* [[IBM PC (5150) Capacitor Replacement Guide]]
* [[IBM PC XT Capacitor Replacement Guide]]
* [[Capacitor Failure Symptoms]]

== References ==

* [https://www.minuszerodegrees.net/failure/failure.htm Commonly Failing Electronic Components], minuszerodegrees.net.
* [https://www.minuszerodegrees.net/failure/IBM%2051xx%20motherboards%20-%203-legged%20tantalum%20capacitors.htm IBM 51xx motherboards &mdash; 3-legged tantalum capacitors], minuszerodegrees.net.
* IBM, ''Personal Computer AT &mdash; Hardware Maintenance and Service''.

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