IBM 5100 Capacitor Replacement Guide

The IBM 5100 Portable Computer does not have a linear power supply. IBM's own maintenance manual describes the DC supply board as "a small, high power, high frequency transistor switching regulator (TSR) supply", developing five DC voltages — +5 V, −5 V, +8.5 V, +12 V and −12 V. That matters before anything is unsoldered: the failure modes, the ripple frequency and the expected component types in a 1975 switching supply are not those of a mains-transformer-and-series-pass design.

IBM published no capacitance values for this machine. The maintenance manual treats the DC supply as one field-replaceable unit and never draws its circuit; the parts catalogue lists capacitors by IBM part number only, with no capacitance, no voltage rating and no dielectric type. This page therefore carries no capacitor table. What it carries instead is everything IBM did publish that a restorer can use: the rails and their tolerances, the protection trip points, the designators and ratings of the parts that are identified, and the method for recovering the values that are not.

This page is the reference for the whole 51xx family. The 5110 and 5120 pages cover only what differs on those machines.

IBM 5100 Portable Computer. The DC supply, the AC box and the tape drive are all reachable once the covers are off; the display is fed low-voltage DC from the logic board. (Image: Wikimedia Commons)

Safety

Three separate hazards, and they are not all obvious.

• Mains. The AC box carries live mains to the line filter, the terminal block, the fuse holder, the power on/off switch, the fan and the tape unit motor. Unplug the machine at the wall before the covers come off; the machine's own switch does not isolate everything in the box.
• Stored charge in the DC supply. It is a switching regulator, so it has a rectified-mains reservoir on its primary side. Treat every large capacitor on that board as charged until a meter says otherwise, and discharge through a resistor rather than a screwdriver.
• The display. The display and control panel are fed +5 V and +12 V over the Z3 cable and have no mains connection of their own, but the tube's extra-high-tension supply is generated inside the display assembly. The anode and the high-voltage circuit around it hold a charge after power-off. Follow the CRT Discharge Procedure before working anywhere near the tube, and discharge a second time a few minutes later — the aquadag coatings on a CRT behave as a capacitor and can recover a charge after the first discharge.

See also Battery Explosion, Capacitor or Corrosion Damage and Recommended Tools.

The supply, and what each rail feeds

IBM's five rails and their loads:

IBM 5100 DC supply — rails and loads

Rail	What it feeds
+5 Vdc	Basic logic voltage
−5 Vdc	Tape control card, storage cards, BSCA cards, common ROS
+8.5 Vdc	Storage cards, keyboard, display adapter, printer adapter, BSCA cards, all ROS cards
+12 Vdc	Display unit, tape select magnets, tape LEDs, BSCA cards, 5114 R1 relay
−12 Vdc	Tape unit, 5114 R1 relay, BSCA cards, asynchronous communications / serial I/O card

The +8.5 V rail is the family signature and the one most often missed: nothing in the later IBM PC family has it, and a supply that comes up on +5 V and +12 V but not +8.5 V will leave the machine dead with a perfectly healthy-looking logic rail.

IBM states that the DC outputs may vary from +10% to −9% of the rated voltage before they affect system operation.

Built-in protection, and what it means when the machine shuts down

The supply has overvoltage, undervoltage and overcurrent protection:

• Overvoltage shuts the supply down when the +12 Vdc output exceeds +16 Vdc.
• Undervoltage shuts the supply down when the −5 Vdc rail is less negative than −3 Vdc.
• Overcurrent shuts the supply down when the current in the primary of the transformer is excessive.

After any automatic shutdown, leave the machine switched off for at least five seconds before switching it on again. That is IBM's instruction, not a rule of thumb; a faster cycle can leave the supply latched off and send you chasing a fault that is not there.

A shorted decoupling capacitor on a card will present as an overcurrent trip rather than as a blown fuse. Pulling cards one at a time and retrying, with a five-second wait between attempts, is the way to find which board is pulling the supply down.

Expected measurements

These are the go/no-go numbers. Measure at the Y1 power connector on the A1 board.

IBM 5100 — DC rail tolerances (IBM figures)

Voltage	Loaded	Unloaded (Y1 disconnected)
+5 Vdc	4.6 to 5.5	5.5 to 6.5
+8.5 Vdc	7.9 to 9.35	7.4 to 9.0
+12 Vdc	11.0 to 13.2	9.8 to 12.2
−5 Vdc	−4.6 to −5.5	−3.7 to −4.7
−12 Vdc	−11.0 to −13.2	−9.0 to −11.5

Note that the unloaded figures are not simply "a bit high". The +5 V rail rises out of its loaded band when unloaded, while +12 V, −5 V and −12 V all fall. Bench-testing the supply with Y1 disconnected and condemning it because +12 V reads 10.5 V is a mistake this table prevents.

Y1 socket pin assignments

IBM 5100 — Y1 power socket, A1 board pins

A1 board pin	Line	A1 board pin	Line
A1D11, A1E11, B1A11, B1B11	+5 Vdc	A1D13, A1E13, B1A13, B1B13	+5 Vdc
B1C11, B1D11, B1E11, C1A11, C1B11	Ground	B1C13, B1D13, B1E13, C1A13	Ground
C1C11	+8.5 Vdc	C1B13	Not used
C1D11	+12 Vdc	C1C13	+8.5 Vdc
C1E11	−5 Vdc	C1D13	+12 Vdc
		C1E13	−12 Vdc

Check the meter before you trust it

The 5100 provides its own voltage reference so that a service meter can be checked against it. Ground the meter at J2-D08 and measure at J2-S02: a zener diode provides a +6 Vdc reference. If the meter does not read exactly +6 V, correct every subsequent reading by

actual voltage = 6 × measured voltage ÷ reference voltage

IBM's own worked example: with the reference reading 5.8 V and the +5 V rail reading 4.8 V, the rail is actually 4.97 V. On a fifty-year-old machine being assessed with a cheap meter, this is worth doing first.

The capacitors IBM does identify

Only the mains-side parts in the AC box are given designators, and there are two different AC boxes.

IBM 5100 AC box — identified components

Designator	Part	Old style	New style
L1	Line filter — IBM part 1860276 on the old-style box, 1608101 on the new-style box	Yes	Yes
C1	Capacitor, AC	Yes	Yes
C2, C3, C4, C5	Capacitor	Yes	C2 only
R1	Resistor assembly	Yes	—
F1	Fuse holder — 5 A 125 V on 100 V and 115 V machines, 3 A 250 V on 220 V and 235 V machines	Yes	Yes
SW1	Power on/off switch	Yes	Yes
TB1	Terminal block	Yes	—
J1, J2, J3	Connector assemblies to the power supply, the fan and the tape unit	Yes	Yes

The parts catalogue gives IBM numbers for the AC box capacitors — 1608347 (two off) and 5252805 on the old-style box, and the capacitor assembly 1608119 on the new-style box — and no electrical values for any of them.

Separately, the tape drive carries a motor run capacitor — IBM part 5252836 on 100/115 V 50/60 Hz machines, with capacitor/resistor assemblies 2451254 (220 V) and 2451258 (235 V) in place of it on 50 Hz world-trade machines. This is a mains-rated motor capacitor, not a smoothing part, and it must be replaced like for like.

These mains-side parts are the ones worth replacing on sight. Suppression and motor capacitors of this era degrade, and when they fail they fail across the mains. Any replacement must be a correctly safety-rated part of the same class and the same voltage rating — a general-purpose film capacitor is not a substitute for a mains-rated one, whatever its capacitance says.

Getting real values for the rest

No document on this wiki, and none the wiki has been able to locate, gives a capacitance for any part inside the 5100's DC supply, its display assembly or its tape drive electronics. There are two sound routes:

1. Read the parts. Every electrolytic and film capacitor in the machine has its value and voltage printed on it. Photograph each board from both sides at high resolution before touching anything, then record designator, value, voltage, dielectric, lead pitch and can diameter for each position. This is the only route IBM's documentation supports, and it is how a real parts list for this machine will eventually be written.
2. Measure, don't assume. Out of circuit, capacitance and ESR measurements tell you whether a part has drifted. A 1975 electrolytic that still measures within tolerance and shows sane ESR does not have to be replaced to satisfy a rule.

If you transcribe a board, publish it — with photographs, board revision and machine serial — and this page can carry a sourced table instead of an apology.

What not to do

Do not fit values copied from a later IBM machine. The IBM PC family (5150, 5155, 5160, 5170) is a completely different product line that happens to share a "51xx" numbering style; its boards, rails and capacitor complement have nothing to do with the 5100. In particular the +8.5 V rail does not exist on any PC-family board, and the 5100's supply is not the PC's 130 W unit.

When to recap

There is no case for a blanket recap of a machine that works. Replace on evidence:

• Visible failure — a vented, bulged, split or leaking capacitor anywhere.
• Corrosion or residue around a capacitor's pads.
• A rail outside the loaded tolerances above, or unstable when warm and stable when cold.
• The supply tripping its overcurrent or undervoltage protection with no card fault found.
• Any mains-side suppression or motor capacitor that is cracked, bulged, discoloured, or that produces a fishy smell in use. That smell is the impregnation venting, and it is a fire risk; replace immediately with a correctly rated part.

Replacement practice

• Voltage rating equal to or higher than the original. Never lower.
• Capacitance equal to the original. Increasing bulk capacitance loads the rectifier harder at switch-on.
• Temperature rating of 105 °C where a through-hole electrolytic is being fitted, even if the original was 85 °C.
• Mains-rated parts must be replaced with mains-rated parts of the correct class.
• Lead form matters on this machine: many originals are axial and modern stock is mostly radial. Check the hole pitch before ordering.
• Keep desoldering cycles short and the iron no hotter than needed. These are 1970s boards and the pads lift.

See Capacitor Failure Symptoms for how a failing capacitor presents at the symptom level.

Related Pages

• IBM 5100 · IBM 5100 Maintenance Guide · IBM 5100 Troubleshooting Guide
• IBM 5100 Maintenance Information Manual · IBM 5100 Portable Computer Parts Catalog
• IBM 5110 Capacitor Replacement Guide — same DC supply, different AC box, plus the 5114 diskette unit
• IBM 5120 Capacitor Replacement Guide — same regulator board behind a ferroresonant front end
• Capacitor Failure Symptoms · CRT Discharge Procedure · Recommended Tools