IBM PCjr Capacitor Replacement Guide: Difference between revisions

Latest revision as of 08:59, 27 September 2026

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:^[1]

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.^[1]
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.^[1]

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.^[1] 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.^[2]

**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.^[1] 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.^[2]

Protection behaviour

Knowing what the protection actually does is what turns a symptom into a diagnosis.^[1]

**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.^[1] 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.^[3]

**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.^[4] 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.^[3] 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:^[5]

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.^[6]
Short circuit — the capacitor pulls its rail down and overloads the supply, which will not then run.^[5] On a PCjr that means the Power Board's over-current protection holding the machine off until the fault is removed.^[1]

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.^[5] 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.^[5]

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.^[2]

Disconnect the external transformer at the wall and at the system unit.
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.
Inspect the system board tantalums. Most failures leave no mark, so a clean inspection proves nothing.
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.
Where a short is found, remove the capacitor to confirm. Other components on the same rail can produce a false short.
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.^[1]
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.^[2]

Removal and Replacement

Photograph or mark the polarity of the original before removing it.
Add fresh solder and flux to both pads to wet the old joint.
Heat one pad, lever that side of the capacitor up, then heat the other and lift the part clear.
Clear the holes with wick.
Fit the replacement with the + on the capacitor matching the + on the silkscreen.
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.^[4] 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.^[7]

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:

Unplug the transformer from the wall and from the system unit.
Open the system unit and remove the Power Board.
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.
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.
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.^[3]
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.^[1]

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.^[3] 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)".^[1]
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.^[2]
Ripple on the rails showing up as noise on the display points back at the Power Board electrolytics.

References

↑ ^1.00 ^1.01 ^1.02 ^1.03 ^1.04 ^1.05 ^1.06 ^1.07 ^1.08 ^1.09 ^1.10 IBM, IBM PCjr Technical Reference, "System Power Supply" and its Operating Characteristics, Over-Voltage/Over-Current Protection and Connector Specifications subsections. Source for the two-stage 33 W description, the transformer's short-circuit-protected extra-low ac output, the internal Power Board that converts that ac to three dc levels, the three dc rails and their regulation tolerances and load currents, the +5 V minimum load required for −6 V to be present, the input requirements, the transformer connector pinout, and the input and output protection tables. Hosted on this wiki as IBM PCjr Technical Reference (PDF).
↑ ^2.0 ^2.1 ^2.2 ^2.3 ^2.4 Mike Brutman, IBM PCjr Hardware, "The Power Supply" section. Source for the split supply description, the short and long Power Board variants rated at 33 W and 45 W, the roughly 400 mA of +5 V left for sidecars on an internally fully configured machine, the behaviour of overloaded sidecar configurations, the existence of power sidecars using the same external transformer, and the point that the system has no fan of its own while a diskette-equipped machine has a fan on the drive.
↑ ^3.0 ^3.1 ^3.2 ^3.3 IBM PCjr, Console5 Tech Wiki. Secondary source. A community-maintained capacitor list with per-designator values for the PCjr main board, the 33 W Power Board, the IR receiver board, the 64 KB memory card and the diskette controller, together with annotated cap-map images and system board schematics. The list is compiled from the boards themselves rather than from IBM documentation.
↑ ^4.0 ^4.1 Tantalum capacitors on IBM 51xx motherboards/cards, minuszerodegrees.net, together with its 2-legged and 3-legged pages. Source for the 10 µF/16 V value and its "106 16V" markings, the three-hole footprint and its two wiring variants, the negative-positive-negative polarity of the 3-legged variant, and the Kemet/Mouser replacement part numbers. These pages state their scope as the IBM 5150, 5155 and 5160 — not the PCjr.
↑ ^5.0 ^5.1 ^5.2 ^5.3 Commonly Failing Electronic Components (in vintage computers), minuszerodegrees.net. Source for the open-circuit and short-circuit failure modes of tantalum capacitors, the overloading of the supply by a short, the observation that a failure usually occurs on application of power or within about thirty seconds, the difference in failure rate between machines left unpowered for years and machines used regularly, and the fact that failures are sometimes dramatic.
↑ Vintage Computers — Filter Capacitors, minuszerodegrees.net. Explains the filtering role, notes that capacitors serving dynamic RAM are crucial while others may not be, and advises fitting a new capacitor whenever you cannot judge which case applies.
↑ SNCTOL diagram and its 5150 variant, minuszerodegrees.net. Both carry the instruction "Solder new capacitor to old legs" and show the cut-and-graft sequence; both note the method is unavailable where the capacitor sits hard against the PCB. The diagrams are drawn for IBM 51xx boards.

Related Pages

[tr-1] 1.00 ^1.01 ^1.02 ^1.03 ^1.04 ^1.05 ^1.06 ^1.07 ^1.08 ^1.09 ^1.10 IBM, IBM PCjr Technical Reference, "System Power Supply" and its Operating Characteristics, Over-Voltage/Over-Current Protection and Connector Specifications subsections. Source for the two-stage 33 W description, the transformer's short-circuit-protected extra-low ac output, the internal Power Board that converts that ac to three dc levels, the three dc rails and their regulation tolerances and load currents, the +5 V minimum load required for −6 V to be present, the input requirements, the transformer connector pinout, and the input and output protection tables. Hosted on this wiki as IBM PCjr Technical Reference (PDF).

[brutman-2] 2.0 ^2.1 ^2.2 ^2.3 ^2.4 Mike Brutman, IBM PCjr Hardware, "The Power Supply" section. Source for the split supply description, the short and long Power Board variants rated at 33 W and 45 W, the roughly 400 mA of +5 V left for sidecars on an internally fully configured machine, the behaviour of overloaded sidecar configurations, the existence of power sidecars using the same external transformer, and the point that the system has no fan of its own while a diskette-equipped machine has a fan on the drive.

[console5-3] 3.0 ^3.1 ^3.2 ^3.3 IBM PCjr, Console5 Tech Wiki. Secondary source. A community-maintained capacitor list with per-designator values for the PCjr main board, the 33 W Power Board, the IR receiver board, the 64 KB memory card and the diskette controller, together with annotated cap-map images and system board schematics. The list is compiled from the boards themselves rather than from IBM documentation.

[mztant-4] 4.0 ^4.1 Tantalum capacitors on IBM 51xx motherboards/cards, minuszerodegrees.net, together with its 2-legged and 3-legged pages. Source for the 10 µF/16 V value and its "106 16V" markings, the three-hole footprint and its two wiring variants, the negative-positive-negative polarity of the 3-legged variant, and the Kemet/Mouser replacement part numbers. These pages state their scope as the IBM 5150, 5155 and 5160 — not the PCjr.

[mzfail-5] 5.0 ^5.1 ^5.2 ^5.3 Commonly Failing Electronic Components (in vintage computers), minuszerodegrees.net. Source for the open-circuit and short-circuit failure modes of tantalum capacitors, the overloading of the supply by a short, the observation that a failure usually occurs on application of power or within about thirty seconds, the difference in failure rate between machines left unpowered for years and machines used regularly, and the fact that failures are sometimes dramatic.

[mzfilter-6] Vintage Computers — Filter Capacitors, minuszerodegrees.net. Explains the filtering role, notes that capacitors serving dynamic RAM are crucial while others may not be, and advises fitting a new capacitor whenever you cannot judge which case applies.

[mzsol-7] SNCTOL diagram and its 5150 variant, minuszerodegrees.net. Both carry the instruction "Solder new capacitor to old legs" and show the cut-and-graft sequence; both note the method is unavailable where the capacitor sits hard against the PCB. The diagrams are drawn for IBM 51xx boards.

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-{{Unverified data}}
+<templatestyles src="Template:StyledTable/styles.css" />
-This guide documents the procedure for diagnosing and replacing capacitors on the IBM PCjr (4860) motherboard, sidecars, and the external AC power brick.
+[[File:IBM PCjr motherboard.jpg|right|thumb|360px|IBM PCjr (4860) motherboard]]
-The PCjr uses the same family of '''10 µF / 16 V tantalum''' filter capacitors as the rest of the IBM 51xx range. Tantalums on the PCjr motherboard are less numerous than on the 5170 (the PCjr has fewer rails) but they fail in the same modes: '''short circuit''' (most common, pulls the external brick into fold-back) and '''open circuit''' (less common, no immediate symptom).
+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.
-This guide does not enumerate each tantalum's reference designator on the PCjr motherboard, because IBM did not publish a unified silkscreen reference and surviving PCjr boards show different revisions of the silkscreen. Treat every orange/yellow bead tantalum as a candidate for inspection if the PCjr will not start.
+'''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.
-== Failure Mode ==
+== How the PCjr is powered ==
-Tantalum capacitors on the PCjr are commonly marked "106 16V" or "10µF 16V" (10 µF, 16 V working voltage). They are '''polarised'''. They fail in two modes:
+The PCjr uses what IBM calls a '''two-stage''' supply, and what it does is split in an unusual way:<ref name="tr">IBM, ''IBM PCjr Technical Reference'', "System Power Supply" and its Operating Characteristics, Over-Voltage/Over-Current Protection and Connector Specifications subsections. Source for the two-stage 33 W description, the transformer's short-circuit-protected extra-low ac output, the internal Power Board that converts that ac to three dc levels, the three dc rails and their regulation tolerances and load currents, the +5 V minimum load required for &minus;6 V to be present, the input requirements, the transformer connector pinout, and the input and output protection tables. Hosted on this wiki as [[IBM PCjr Technical Reference]] ([[:File:IBM PCjr Technical Reference.pdf|PDF]]).</ref>
-* '''Open circuit''' &mdash; filtering is lost; the rail is still nominally correct, but decoupling is degraded. The system may still boot but may be sensitive to noise from sidecars.
+* '''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.<ref name="tr" />
-* '''Short circuit''' &mdash; the cap pulls its rail down. With the PCjr's external brick, this causes the brick to enter fold-back protection (the brick clicks off, the LED on the system unit goes out, the brick rests, then tries again in a cycle). Severe shorts can damage the brick.
+* '''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.<ref name="tr" />
-The PCjr also uses '''aluminium electrolytic''' capacitors in the AC power brick. These dry out over decades; symptoms include reduced or unstable DC output, ripple on the system unit's rails, and the brick failing to start when warm.
+Two consequences follow immediately:
-== Diagnostic Procedure (motherboard) ==
+* '''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.
-If the PCjr will not start (no audio at power-on, no fan because there is no fan, no system unit LED on the front of the case):
+The rails are '''+5 V, +12 V and &minus;6 V''' — not the &minus;5 V and &minus;12 V of the 51xx family.<ref name="tr" /> 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.<ref name="brutman">Mike Brutman, [https://www.brutman.com/PCjr/pcjr_hardware.html IBM PCjr Hardware], "The Power Supply" section. Source for the split supply description, the short and long Power Board variants rated at 33 W and 45 W, the roughly 400 mA of +5 V left for sidecars on an internally fully configured machine, the behaviour of overloaded sidecar configurations, the existence of power sidecars using the same external transformer, and the point that the system has no fan of its own while a diskette-equipped machine has a fan on the drive.</ref>
-# Disconnect the external AC brick.
+{| class="wikitable styled-table" style="width:100%;"
-# Open the system unit and remove all sidecars and both cartridges.
+|+'''IBM PCjr dc outputs, per IBM'''
-# Visually inspect each tantalum on the motherboard for damage (cracked body, black spot, ruptured top). Most failed tantalums show no visible damage.
+! Rail !! Load current, min !! Load current, max !! Regulation tolerance !! Acceptable range
-# With a multimeter on the diode test or 200 Ω range, probe across each tantalum '''in-circuit''' (negative probe to GND, positive probe to the +5 V or +12 V side). A good tantalum reads open or high resistance after a brief charge. A failed (shorted) tantalum reads close to 0 Ω.
+|-
-# Where a short is found, '''remove the capacitor''' to confirm. Other components on the same rail can give a false-short reading.
+| +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
+|-
+| &minus;6 V || 0.0 A || 0.025 A || ±16 % || &minus;5.04 to &minus;6.96 V
+|}
-If no tantalum on the motherboard is shorted, inspect each fitted sidecar individually: each sidecar has its own filter caps on its connector. A shorted cap on a sidecar will pull the system rails down even with the motherboard healthy.
+'''There must be a minimum load of 1.5 A on +5 V for the &minus;6 V rail to be present at all.'''<ref name="tr" /> A missing &minus;6 V on a lightly loaded bench setup is therefore not necessarily a fault.
-== Removal and Replacement (motherboard or sidecar) ==
+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.<ref name="brutman" />
-# Mark the polarity of the failed cap on the board with a paint pen or photograph.
+[[File:IBM PCjr internal teardown.jpg|center|thumb|640px|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.]]
-# 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 cap with correct polarity (the '''+''' marking on the cap matches the '''+''' on the silkscreen).
-# Solder both legs from the underside. Inspect for a clean fillet.
-# Trim the legs flush.
-Modern '''10 µF / 16 V tantalum''' replacements are the like-for-like fit. Two-legged radial parts work in every PCjr footprint. As an alternative, '''10 µF / 25 V low-ESR ceramics''' (X5R or X7R) work as drop-ins for filtering duty.
+== Protection behaviour ==
-[[File:IBM 5150 tantalum polarity reference.jpg|center|thumb|640px|Polarity reference for IBM 51xx motherboard tantalum capacitors. The same convention applies to the PCjr motherboard. (Image: minuszerodegrees.net)]]
+Knowing what the protection actually does is what turns a symptom into a diagnosis.<ref name="tr" />
-A failed tantalum often shows '''no visible damage'''. Do not rely on visual inspection.
+{| class="wikitable styled-table" style="width:100%;"
+|+'''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
+|}
-[[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 over-current protection is '''resettable by removing the fault condition for at least five seconds and then re-applying power'''.<ref name="tr" /> 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.
-== AC Power Brick Recap ==
+== Motherboard capacitor list ==
-The PCjr's external AC brick contains a mains rectifier, a switching regulator, and a small number of secondary-side aluminium electrolytic capacitors. After thirty years many bricks supply marginal DC under load.
+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.
-'''Warning:''' The brick's primary side carries mains voltage and the bulk capacitor can hold a lethal charge after the brick is unplugged. Always discharge the bulk capacitor through a current-limited resistor (1 kΩ / 5 W to ground) before working on the primary side.
+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.<ref name="console5">[https://wiki.console5.com/wiki/IBM_PCjr IBM PCjr], Console5 Tech Wiki. '''Secondary source.''' A community-maintained capacitor list with per-designator values for the PCjr main board, the 33 W Power Board, the IR receiver board, the 64 KB memory card and the diskette controller, together with annotated cap-map images and system board schematics. The list is compiled from the boards themselves rather than from IBM documentation.</ref>
+{| class="wikitable styled-table" style="width:100%;"
+|+'''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.<ref name="mztant">[https://www.minuszerodegrees.net/failure/IBM%2051xx%20motherboards%20-%20tantalum%20capacitors.htm Tantalum capacitors on IBM 51xx motherboards/cards], minuszerodegrees.net, together with its [https://www.minuszerodegrees.net/failure/IBM%2051xx%20motherboards%20-%202-legged%20tantalum%20capacitors.htm 2-legged] and [https://www.minuszerodegrees.net/failure/IBM%2051xx%20motherboards%20-%203-legged%20tantalum%20capacitors.htm 3-legged] pages. Source for the 10 µF/16 V value and its "106 16V" markings, the three-hole footprint and its two wiring variants, the negative-positive-negative polarity of the 3-legged variant, and the Kemet/Mouser replacement part numbers. These pages state their scope as the IBM 5150, 5155 and 5160 — not the PCjr.</ref> 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.<ref name="console5" /> 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:<ref name="mzfail">[https://www.minuszerodegrees.net/failure/failure.htm Commonly Failing Electronic Components (in vintage computers)], minuszerodegrees.net. Source for the open-circuit and short-circuit failure modes of tantalum capacitors, the overloading of the supply by a short, the observation that a failure usually occurs on application of power or within about thirty seconds, the difference in failure rate between machines left unpowered for years and machines used regularly, and the fact that failures are sometimes dramatic.</ref>
+* '''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.<ref name="mzfilter">[https://www.minuszerodegrees.net/electronics/filter_capacitors.htm Vintage Computers — Filter Capacitors], minuszerodegrees.net. Explains the filtering role, notes that capacitors serving dynamic RAM are crucial while others may not be, and advises fitting a new capacitor whenever you cannot judge which case applies.</ref>
+* '''Short circuit''' — the capacitor pulls its rail down and overloads the supply, which will not then run.<ref name="mzfail" /> On a PCjr that means the Power Board's over-current protection holding the machine off until the fault is removed.<ref name="tr" />
+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.<ref name="mzfail" /> 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.<ref name="mzfail" />
+[[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 — 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.<ref name="brutman" />
+# '''Disconnect the external transformer at the wall and at the system unit.'''
+# 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.
+# Inspect the system board tantalums. Most failures leave no mark, so a clean inspection proves nothing.
+# 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 &minus;6 V'''. A good tantalum reads high after a brief charging pulse; a shorted one reads close to zero.
+# Where a short is found, '''remove the capacitor to confirm.''' Other components on the same rail can produce a false short.
+# 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.<ref name="tr" />
+# 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.<ref name="brutman" />
+== Removal and Replacement ==
+# Photograph or mark the polarity of the original before removing it.
+# Add fresh solder and flux to both pads to wet the old joint.
+# Heat one pad, lever that side of the capacitor up, then heat the other and lift the part clear.
+# Clear the holes with wick.
+# Fit the replacement with the '''+''' on the capacitor matching the '''+''' on the silkscreen.
+# 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.<ref name="mztant" /> 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.<ref name="mzsol">[https://www.minuszerodegrees.net/soldering/snctol.png SNCTOL diagram] and [https://www.minuszerodegrees.net/soldering/snctol_5150.png its 5150 variant], minuszerodegrees.net. Both carry the instruction "Solder new capacitor to old legs" and show the cut-and-graft sequence; both note the method is unavailable where the capacitor sits hard against the PCB. The diagrams are drawn for IBM 51xx boards.</ref>
+[[File:IBM 5150 tantalum polarity reference.jpg|center|thumb|640px|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:
-# Unplug the brick from the wall and from the system unit.
-# Wait at least 30 seconds for the bulk capacitor to bleed through the internal resistor; verify with a multimeter on the bulk capacitor terminals.
+# Unplug the transformer from the wall and from the system unit.
-# Open the brick. Many IBM bricks are ultrasonically welded; cut the seam carefully and plan to glue or screw it back together.
+# Open the system unit and remove the Power Board.
-# Identify the secondary-side electrolytics (the larger caps closer to the DC output connector). Replace each with a new low-ESR aluminium electrolytic of equal capacitance, equal or higher voltage, and equal or higher temperature rating (105 °C preferred over 85 °C).
+# 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.
-# If the primary-side line suppression cap (X2 class) shows any cracking, bulging or fluid leakage, replace it with a new X2 cap of equal capacitance and voltage class.
+# 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.
-# Reassemble the brick. Verify DC output with a multimeter under no load and then under a 1 A resistive dummy load.
+# 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.<ref name="console5" />
-# Re-test with the system unit.
+# Refit and verify the three rails against the table above, remembering that &minus;6 V will not appear without at least 1.5 A on +5 V.<ref name="tr" />
 == Sidecar Capacitors ==
-Each PCjr sidecar contains its own filter capacitors on the inlet from the system unit's sidecar bus. These are the same 10 µF / 16 V tantalums as on the motherboard. A shorted cap on a sidecar produces the same symptom as a shorted cap on the motherboard: the brick goes into fold-back and the system will not start.
+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.
-To diagnose, remove all sidecars and re-add them one at a time. The faulty sidecar is the one that reintroduces the power-up failure.
+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.<ref name="console5" /> 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 ==
-* With the system unit closed and the AC brick fitted, the system should power up to the IBM logo / BIOS POST and emit a single short beep.
+* 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)".<ref name="tr" />
-* If the system POSTs but a fitted sidecar still drops it, re-inspect that sidecar's caps.
+* Verify each rail against the table above, under load rather than open-circuit.
-* If the system POSTs and runs but the IBM 4863 display shows ripple or noise, the brick electrolytics may still need replacement.
+* If the machine starts but a particular sidecar still drops it, re-inspect that sidecar — and check the power budget before assuming a fault.<ref name="brutman" />
+* Ripple on the rails showing up as noise on the display points back at the Power Board electrolytics.
+== References ==
+<references />
 == Related Pages ==
@@ Line 80: / Line 169: @@
 * [[IBM PC XT Capacitor Replacement Guide]]
 * [[Capacitor Failure Symptoms]]
-== References ==
-* [https://www.minuszerodegrees.net/failure/failure.htm Commonly Failing Electronic Components], minuszerodegrees.net. Reference for tantalum and electrolytic failure modes on IBM 51xx hardware.
-* IBM, ''IBM PCjr Technical Reference''. Reference for motherboard chip layout, sidecar pinout and power requirements.
-* [https://www.brutman.com/PCjr/pcjr_hardware.html IBM PCjr Hardware], Mike Brutman. Reference for sidecar power behaviour.
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