IBM PCjr Capacitor Replacement Guide

IBM PCjr (4860) system board

The IBM PCjr's electrolytic and tantalum capacitors are on the system board, the Power Board inside the system unit, the infrared receiver board, the 64KB Memory and Display Expansion and the diskette-drive adapter. IBM's service manual works at field-replaceable-unit (FRU) level and lists no component values, but the logic diagrams in Appendix B of IBM's Technical Reference print them.

How the PCjr is powered

IBM's supply is a 33 W, two-stage design. The first stage is an external transformer with a single-fuse-protected, extra-low-voltage AC output. The second stage is the Power Board, a card mounted vertically in the system board, which converts that AC into +5 V, +12 V and −6 V. IBM's connector specification gives the transformer output as "Pin 1 – 17 Vac, Pin 2 – GND, Pin 3 – 17 Vac". The ageing electrolytics of the supply are therefore on the Power Board, not in the external transformer.

IBM's schematic of the Power Board shows the transformer input on J4, a bridge of four 3 A rectifiers (CR7 to CR10) charging C13, switch S1 and a 5 A fuse F1, a +12 V regulator (Q1, on heatsink HS1), and a switching +5 V stage built round controller Z2, Darlington Q2 and inductor L1, with C4 on its output. Brutman reports a short 33 W and a long 45 W Power Board. IBM's drawings come from the November 1983 Technical Reference and do not say which board they show.

IBM PCjr DC outputs (Technical Reference p. 2-136)

Rail	Minimum load	Maximum load	Regulation	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

The −6 V output is present only when +5 V carries at least 1.5 A, so a lightly loaded Power Board on the bench may show no −6 V without being faulty. The +5 V over-current protection trips at 3.9 ± 0.25 A and resets once the fault is removed and the power has been off for at least 5 seconds. The +12 V protection trips at 2.2 ± 0.9 A; IBM's table does not mark it as resettable. Over-voltage protection on both rails is by fuse F1. A shorted capacitor on the +5 V or +12 V rail therefore holds the machine off until the short is removed.

Power Board capacitor list

IBM's component placement drawing of the PCjr Power Board (Technical Reference p. B-24). C13 is the large round can on the left; C1 to C4 are the small radial capacitors on the right.

IBM PCjr Power Board capacitors (IBM schematic, Technical Reference p. B-23)

Ref	Value	Voltage	Drawn as	Console5 list
C1	22 µF	25 V	Polarised	1000 µF 16 V
C2	100 µF	16 V	Polarised	100 µF 16 V
C3	22 µF	25 V	Polarised	22 µF 25 V
C4	1000 µF	12 V	Polarised	1000 µF 12 V, low ESR
C13	15,000 µF (printed "15,00 UF")	25 V	Large can	15000 µF 25 V
C5	1000 pF	Not printed	Non-polarised	Not listed
C6	1000 pF	50 V	Non-polarised	Not listed
C8	47 pF	50 V	Non-polarised	Not listed
C9	220 pF	25 V	Non-polarised	Not listed
C10	0.1 µF	25 V	Non-polarised	Not listed
C11	0.1 µF	25 V	Non-polarised	Not listed
C12	0.10 µF	100 V	Non-polarised	Not listed
C14	0.047 µF	100 V	Non-polarised	Not listed
C15	0.047 µF	100 V	Non-polarised	Not listed
C16	0.1 µF	100 V	Non-polarised	Not listed

There is no C7 on the drawing. The Console5 column is a community list read from boards, given for comparison. It agrees with IBM on C2, C3, C4 and C13 but gives C1 as 1000 µF 16 V where IBM's schematic prints 22 µF 25 V. Read the markings on the part fitted to your board before ordering. IBM prints 12 V for C4; a replacement rated 16 V or more meets that.

C13 is the reservoir on the rectified transformer output. Console5 notes that the original has three legs with very wide spacing, that modern replacements are smaller two-legged parts, and that the board already has solder points for the newer spacing, needing only a slight change of orientation. Its cap map marks one hole as the only positive point, with the others usable for negative.

Other boards

Electrolytic and tantalum capacitors on the other PCjr boards

Board	Ref	Value	Source
System board	C2, C4, C8, C12, C13, C22, C23, C35, C38, C39, C45	10 µF, polarised (IBM prints no voltage; Console5 gives 16 V)	IBM sheets 1, 6, 10 and 13 (pp. B-3, B-8, B-12, B-15); Console5
Infrared receiver board	C1, C4, C6	10 µF, polarised (IBM prints no voltage; Console5 gives 50 V)	IBM p. B-42; Console5
64KB Memory and Display Expansion	C1	10 µF on +5 V (IBM); 16 V (Console5)	IBM sheet 3, p. B-27; Console5
Diskette-drive adapter	C1; C5	1 µF 50 V; 10 µF 16 V	Console5

On IBM's sheets C2, C4, C22, C38 and C45 are on +5 V, C39 on +12 V and C8 on −6 V (sheet 13, p. B-15); C12 and C13 are on the input and output of the LM340T regulator VR1 (sheet 6, p. B-8), C23 is on the reset input of the 8284A clock generator (sheet 1, p. B-3) and C35 is in the audio circuit (sheet 10, p. B-12). Console5's list has the same positions except that it gives C59 as a 10 µF 16 V part and leaves out C23. IBM's sheet 10 prints C59 as 680 pF, on the IR test data line, so the table follows IBM. The diskette-drive adapter values are Console5's alone. On IBM 51xx boards the 10 µF 16 V filter capacitors are tantalums, usually marked "106 16V" or "10µF 16V". minuszerodegrees.net's tantalum pages name the 5150, 5155 and 5160, not the PCjr.

Failure modes

Tantalum capacitors fail either open or short. A short overloads the supply so that it will not run. Failures most often happen on power-up or within about thirty seconds of it, and are more likely on a board left unpowered for years than on one used every few months. Some fail with sparks or burst open; many show no visible damage at all.

A failed tantalum on an IBM 51xx board. The small black hole is the only sign of failure. (Image: minuszerodegrees.net)

On the PCjr a short on +5 V or +12 V trips the Power Board's over-current protection and the machine stays dead. The Power Board electrolytics age by drying out, which raises ripple and lets the rails sag under load.

Finding a short

IBM's power fault procedure isolates the failing FRU by removal:

1. Disconnect everything outside the system unit except the transformer (IBM leaves its service plug fitted). If the machine now gives one beep, reconnect the devices one at a time until the fault returns.
2. Remove the 64KB Memory and Display Expansion and try again.
3. Disconnect the diskette-drive adapter, the diskette-drive power cable, the drive fan cable and the internal modem, and try again.
4. Unplug the transformer from the system unit and measure its output plug: pin 1 to pin 3 must read 15.0 to 30.0 V AC.
5. With the transformer connected and the drive cables unplugged, measure the Power Board's diskette-drive connector: pin 1 to pin 2, +11.4 to +12.6 V; pin 4 to pin 3, +4.7 to +5.3 V.
6. If the voltages are wrong, IBM replaces the Power Board. If they are right, IBM tries a new Power Board and, if there is still no beep, replaces the system board.

To go below FRU level, switch off, then measure resistance from each rail to ground on the suspect board. A reading near zero points to a shorted part on that rail; lift the capacitors on the rail one at a time to find it.

A machine that fails only with several sidecars fitted may be over its power budget rather than faulty. IBM gives 400 mA of +5 V as the spare capacity of a fully configured machine, and Brutman reports erratic operation from heavy sidecar loads.

Replacement

1. Unplug the transformer from the wall and from the system unit before working on the Power Board.
2. Before removing C13, discharge it through a resistor and check with a meter. It is a low-voltage part, but at 15,000 µF it can deliver a large current into a short.
3. Note the polarity of each part before removing it.
4. Fit parts of the same capacitance and at least the voltage rating on IBM's drawing, and observe the polarity marked on the board.
5. Where the old part's legs allow, minuszerodegrees.net's "solder new capacitor to old legs" method avoids reheating the pads: cut the old legs a few millimetres above the board and solder the new part to the stubs. The method cannot be used where the part sits flat on the board.

Polarity marking for tantalum capacitors on an IBM 5150 board: the hole marked + takes the positive leg. (Image: minuszerodegrees.net)

After recapping

• Advanced POST should end with one beep.
• Check the Power Board's diskette-drive connector against IBM's limits: pin 1 to pin 2, +11.4 to +12.6 V; pin 4 to pin 3, +4.7 to +5.3 V. The drive fan connector should read +11.4 to +12.6 V from pin 2 to pin 1 and from pin 2 to pin 3.
• Remember that −6 V needs at least 1.5 A on +5 V before it appears.

Related pages

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