IBM PCjr Capacitor Replacement Guide

IBM PCjr (4860) motherboard

This guide covers capacitor work on the IBM PCjr (model 4860) — the system board, the sidecars, and the machine's unusual two-stage power supply.

Read the power supply section before opening anything. The PCjr's power arrangement is not like the 5150's, the 5160's or any other machine in the IBM PC family, and treating it as though it were leads to both wrong diagnoses and unnecessary risk.

How the PCjr is powered

The PCjr uses what IBM calls a two-stage supply, and what it does is split in an unusual way:

• Stage one is the external "brick" — and it is a transformer, nothing more. It steps mains down to a short-circuit-protected, extra-low ac voltage. IBM's connector specification for its output reads "Pin 1 – 17 Vac, Pin 2 – GND, Pin 3 – 17 Vac". There is no rectifier, no switching regulator and no high-voltage bulk capacitor inside it. Its mains cord is 3.08 m (10.16 ft) long.
• Stage two is the Power Board inside the system unit. This is the printed-circuit board that rectifies and regulates the transformer's ac output into the machine's three dc rails. It connects directly to the system board, and carries the diskette drive power connector and the drive fan connector on its front edge, plus the machine's power switch on its rear.

Two consequences follow immediately:

• The electrolytic capacitors that age in a PCjr are on the Power Board inside the system unit, not in the brick.
• The over-current protection that trips when a capacitor shorts is on the Power Board, not in the brick. It is not a "fold-back" that makes the brick click on and off.

The rails are +5 V, +12 V and −6 V — not the −5 V and −12 V of the 51xx family. The whole supply is rated at 33 W. The Power Board itself came in two lengths over the machine's production life, the short one rated at 33 W and the long one at 45 W.

IBM PCjr dc outputs, per IBM

Rail	Load current, min	Load current, max	Regulation tolerance	Acceptable range
+5 V	1.5 A	3.6 A	±5 %	+4.75 to +5.25 V
+12 V	0.04 A	1.2 A	±5 %	+11.40 to +12.60 V
−6 V	0.0 A	0.025 A	±16 %	−5.04 to −6.96 V

There must be a minimum load of 1.5 A on +5 V for the −6 V rail to be present at all. A missing −6 V on a lightly loaded bench setup is therefore not necessarily a fault.

A fully configured machine — internal modem, 64 KB memory expansion and diskette drive, but no sidecars — leaves roughly 400 mA of +5 V for sidecars. That is not much, and a heavy or numerous sidecar load produces flaky operation rather than a clean failure. A "power" sidecar was sold to add capacity, and it uses the same external transformer.

IBM PCjr with the top removed. The Power Board — the second stage of the supply, and the board that carries the electrolytic capacitors — is inside the system unit, not in the external transformer.

Protection behaviour

Knowing what the protection actually does is what turns a symptom into a diagnosis.

IBM PCjr power protection

Where	Type	Threshold / behaviour
Transformer input	Non-resettable fuse, plus thermal/over-current	A blown transformer fuse is not user-resettable.
Power Board, +5 V over-voltage	Fuse	Trips at 6.3 ± 0.7 Vdc
Power Board, +12 V over-voltage	Fuse	Trips at 14.4 ± 1.4 Vdc
Power Board, +5 V over-current	Resettable	Trips at about 3.9 A
Power Board, +12 V over-current	Resettable	Trips at 2.2 ± 0.9 A

The over-current protection is resettable by removing the fault condition for at least five seconds and then re-applying power. That is the behaviour to expect from a shorted capacitor: the machine will not run, and it will keep not running until the short is removed. Over-voltage protection, by contrast, is by fuse — a Power Board that has been through an over-voltage event may need more than the fault cleared.

Motherboard capacitor list

IBM published no component-level capacitor list for the PCjr: the only capacitor entries in the text of the PCjr Technical Reference are glossary definitions.

The Console5 Tech Wiki publishes a designator-level list, derived from working on the boards rather than from an IBM document. It is a secondary source and is given here as such — check the markings on the parts in your own machine before ordering.

PCjr capacitors per the Console5 Tech Wiki (secondary source)

Board	Designators and values
Main board	C2, C4, C8, C12, C13, C22, C35, C38, C39, C45, C59 — all 10 µF / 16 V
Power Board (33 W)	C1 1000 µF / 16 V; C2 100 µF / 16 V; C3 22 µF / 25 V; C4 1000 µF, low ESR; C13 15000 µF / 25 V
IR receiver board	C1, C4, C6 — 10 µF / 50 V
64 KB memory card	C1 — 10 µF / 16 V
Diskette controller	C1 1 µF / 50 V; C5 10 µF / 16 V

Two observations about that list are worth drawing out.

The main board positions are not a tidy consecutive run, which is what real board numbering looks like. All eleven are the same 10 µF / 16 V value, consistent with the tantalum filter capacitors used across the IBM 51xx boards of the period. Note that minuszerodegrees.net's tantalum documentation names the 5150, 5155 and 5160; it does not cover the 4860, so the family resemblance here rests on the Console5 list and on the parts themselves, not on a source that names the PCjr.

The Power Board list contains nothing high-voltage. C13 at 15000 µF / 25 V is the large smoothing capacitor that follows the rectifier fed by the 17 Vac transformer, and it is the biggest thing on the board. Console5 notes that the original is a three-legged part with very wide lead spacing, that modern replacements are smaller two-legged parts, and that the board already carries solder points supporting the newer spacing — a slight change of orientation is all that is required. The "1000 µF / 12 V" rating listed for C4 is not a standard capacitor voltage; read the marking on your own part rather than ordering to that figure.

Failure Modes

The tantalum filter capacitors fail in two ways:

• Open circuit — filtering is lost but the rail still rises. The machine may still run and simply be more sensitive to noise. Capacitors serving dynamic RAM are the ones where this matters most.
• Short circuit — the capacitor pulls its rail down and overloads the supply, which will not then run. On a PCjr that means the Power Board's over-current protection holding the machine off until the fault is removed.

Failures happen overwhelmingly on application of power, or within about thirty seconds of it, and the rate is high for a machine left unpowered for years and low for one used every few months. They are sometimes dramatic, so eye protection on first power-up of an unknown board is sensible.

Many failed tantalums show no visible damage at all.

A failed tantalum capacitor on an IBM 51xx motherboard. The small black hole on the body is the only visual indication of failure — in many cases there is none. (Image: minuszerodegrees.net)

The aluminium electrolytics on the Power Board age differently: they dry out over decades, producing sagging or unstable rails, ripple, and a supply that works cold and misbehaves warm.

Diagnostic Procedure

The PCjr gives you less to go on than a 5150 or 5160 does. It has no fan of its own — a machine with a diskette drive has a fan on the drive, fitted to stop the diskette cooking rather than to cool the system — so "the fan spins but nothing happens" is not a symptom available here.

1. Disconnect the external transformer at the wall and at the system unit.
2. Open the system unit and remove all sidecars and both cartridges. A shorted capacitor on a sidecar produces exactly the same symptom as one on the system board.
3. Inspect the system board tantalums. Most failures leave no mark, so a clean inspection proves nothing.
4. With a multimeter on a low resistance or diode range, probe across each tantalum in circuit — negative probe to ground, positive probe to the rail side. Remember the rails here are +5 V, +12 V and −6 V. A good tantalum reads high after a brief charging pulse; a shorted one reads close to zero.
5. Where a short is found, remove the capacitor to confirm. Other components on the same rail can produce a false short.
6. After clearing a fault, leave the machine off for at least five seconds before re-applying power — that is what resets the Power Board's over-current protection.
7. If nothing on the system board is shorted, refit the sidecars one at a time. The one that reintroduces the failure has the fault.

Bear in mind that a machine which will not start with several sidecars fitted but is fine with fewer may not have a fault at all — it may simply be past the roughly 400 mA of spare +5 V the supply has to give.

Removal and Replacement

1. Photograph or mark the polarity of the original before removing it.
2. Add fresh solder and flux to both pads to wet the old joint.
3. Heat one pad, lever that side of the capacitor up, then heat the other and lift the part clear.
4. Clear the holes with wick.
5. Fit the replacement with the + on the capacitor matching the + on the silkscreen.
6. Solder from the underside, inspect for a clean fillet, and trim the legs.

The like-for-like replacement for the motherboard positions is a 10 µF / 16 V tantalum, or a part of higher voltage rating; nothing below 16 V. Solid polymer tantalums of the same value are a reasonable substitute because they do not have the short-circuit failure mechanism — a choice about failure mode, not a claim about what IBM fitted.

Where a capacitor's legs allow it, the cut-and-graft technique minuszerodegrees.net names SNCTOL — "solder new capacitor to old legs" — avoids reheating the pads entirely: cut the old capacitor's legs a few millimetres above the board, lift the body away, and solder the new part to the stubs left in the holes.

Polarity reference for IBM 51xx motherboard tantalum capacitors. The same silkscreen convention — the hole marked + takes the positive leg — is what to follow on the PCjr, though the reference image itself is a 5150 board. (Image: minuszerodegrees.net)

Power Board Recap

The board to recap is the one inside the system unit, and the capacitor list above is the starting point for it.

This board is fed with low-voltage ac from the transformer, so its capacitors are low-voltage parts and there is no mains-derived high-voltage bulk capacitor on it. That materially changes the risk compared with recapping a 5150, 5160 or 5170 supply. It does not remove the need to unplug the machine at the wall first, and the transformer itself is a mains-connected assembly that should be left closed and unplugged while you work.

Procedure:

1. Unplug the transformer from the wall and from the system unit.
2. Open the system unit and remove the Power Board.
3. Discharge C13 — the large smoothing capacitor — through a resistor and confirm with a meter before handling the board. It is a low-voltage part, but 15000 µF holds a meaningful amount of charge.
4. Replace each electrolytic with a new low-ESR part of equal capacitance, equal or higher voltage rating, and equal or higher temperature rating. 105 °C in place of 85 °C is a reasonable upgrade.
5. For C13, note that the original is a wide-spaced three-legged part and a modern two-legged replacement fits the solder points already on the board with a slight change of orientation.
6. Refit and verify the three rails against the table above, remembering that −6 V will not appear without at least 1.5 A on +5 V.

Sidecar Capacitors

Each sidecar draws its power from the system unit's sidecar bus and carries its own filter capacitors. A shorted capacitor on a sidecar produces the same symptom as one on the system board.

Console5's list covers a few of the PCjr's attachments — the 64 KB memory card (C1, 10 µF / 16 V) and the diskette controller (C1 1 µF / 50 V, C5 10 µF / 16 V) among them. No comprehensive per-sidecar map is published, and this wiki has not measured one. Diagnose behaviourally: remove all sidecars and re-add them one at a time.

Post-Recap Verification

• The PCjr's POST ends with one short beep when it completes without error — the BIOS listing in IBM's Technical Reference labels that case "1 SHORT BEEP (NO ERROR)".
• Verify each rail against the table above, under load rather than open-circuit.
• If the machine starts but a particular sidecar still drops it, re-inspect that sidecar — and check the power budget before assuming a fault.
• Ripple on the rails showing up as noise on the display points back at the Power Board electrolytics.

Related Pages

• IBM PCjr
• IBM PCjr Maintenance Guide
• IBM PCjr Troubleshooting Guide
• IBM PC (5150) Capacitor Replacement Guide
• IBM PC XT Capacitor Replacement Guide
• Capacitor Failure Symptoms