IBM 5100 Capacitor Replacement Guide

The IBM 5100 Portable Computer's DC supply is a switching supply. IBM describes it as "a small, high power, high frequency transistor switching regulator (TSR) supply" developing five DC voltages: +5, −5, +8.5, +12 and −12 V.

IBM's 5100 maintenance manual, Maintenance Analysis Procedures (MAPs) and parts catalogue give no capacitance values. The Power MAP treats the DC supply as one field-replaceable unit, the maintenance manual does not draw its circuit, and the parts catalogue lists the capacitors it does name by IBM part number only, with no capacitance, voltage or dielectric. There is therefore no capacitor table for this machine. What IBM did publish, and what a restorer can use, is below: the rails and tolerances, the protection trip points, the named mains-side parts and their part numbers.

The 5110 and 5120 guides cover what differs on those machines.

IBM 5100 Portable Computer (Wikimedia Commons)

Safety

• Mains: the AC box, under the display, carries mains to the line filter, fuse F1, the power on/off switch, the fan and the tape unit motor. Unplug the line cord before removing the covers.
• Stored charge: the supply takes mains from the AC box through plug J1, and IBM does not draw its circuit. Treat every large capacitor on the supply board as charged until measured, and discharge through a resistor.
• Display: the display and control panel take +5 V and +12 V from the Z3 cable, but the tube's high voltage is generated inside the display unit. IBM warns that "the display unit contains high voltages". Follow the CRT Discharge Procedure.

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

The supply and what each rail feeds

IBM 5100 DC rails and their loads (MIM p. 4-51)

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

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

Protection

The supply shuts down when the +12 V output exceeds +16 V, when the −5 V rail is less negative than −3 V, or when the current in the transformer primary is excessive. After any automatic shutdown, IBM says to leave the machine switched off for at least five seconds before powering up again.

A short on the DC side, such as a failed capacitor on a card, can also blow mains fuse F1. IBM's Power MAP checks for this by disconnecting the DC power cable at Y1: if F1 then holds, the fault is on the A1 board, a card, a cable or an attached device. The MAP then reconnects Y1 with every other cable to the A1 board unplugged. If F1 still blows or the power drops, it removes all the cards and refits them one at a time, powering down and up for each; otherwise it reconnects the cables one at a time while watching the rail that was out of tolerance. The last part refitted before the fault returns is the faulty one. Switch off before removing or refitting a read/write storage card (K2 to N4).

Expected measurements

Measure at the Y1 power connector on the A1 board:

IBM 5100 DC rail tolerances (MIM p. 2-52)

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

Unloaded, IBM's +5 V range is higher than the loaded one, and the +8.5, +12, −5 and −12 V ranges are lower in magnitude. Judge a supply tested off the machine against the unloaded column.

Y1 socket pin assignments

IBM 5100 Y1 power socket, A1 board pins (MIM p. 2-52)

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

Check the meter first

The display card provides a +6 V zener reference. Ground the meter at J2-D08 and measure at J2-S02. If it does not read exactly 6 V, correct each later reading with

actual voltage = 6 × measured voltage ÷ reference voltage

IBM's example: reference 5.8 V and +5 V rail reading 4.8 V gives an actual 4.97 V.

The capacitors IBM does identify

Only the AC box components carry designators, and there are two AC box styles.

IBM 5100 AC box components (MIM pp. 2-11, 2-12, 2-53, 2-54)

Designator	Part	Old style	New style
L1	Line filter	Yes	Yes
C1	AC capacitor	Yes	Yes
C2	Capacitor	Yes	Yes
C3, C4, C5	Capacitors	Yes	No
R1	Resistor assembly	Yes	On 220 V and 235 V machines only
F1	Fuse: 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	No
J1, J2, J3	Plugs to the power supply, fan and tape unit	Yes	Yes

IBM part numbers from the parts catalogue:

• Old-style AC box (figure 10): capacitors 1608347 (two) and 5252805, resistor 1608895, line filter 1860276, 5 A fuse 512137.
• New-style AC box (figure 11): capacitor assembly 1608119, filter 1608101, fuses 512137 (5 A, 100/115 V) and 855252 (3 A, 220/235 V).
• Tape drive (figure 8): capacitor 5252836 on the tape motor for 100/115 V 50/60 Hz machines; capacitor-resistor assemblies 2451254 (220 V 50 Hz) and 2451258 (235 V 50 Hz).
• Power supply assembly (figure 5): 5564570 for 100/115 V, 5564569 for 220/235 V.

These parts are on the mains side. A replacement must match the original's capacitance and have at least its AC voltage rating and safety class, which are printed on the part.

Getting real values for the rest

IBM's maintenance manual, MAPs and parts catalogue give no value for any capacitor inside the DC supply, the display unit or the tape electronics. Two methods work:

1. Read the parts. Photograph each board on both sides before removing anything, then record the designator, value, voltage, type, lead spacing and size for each position.
2. Measure. Out of circuit, capacitance and ESR show whether a part has drifted. A part that measures within tolerance does not need replacing.

Do not fit values taken from the IBM PC family (5150, 5155, 5160, 5170). Those are different machines with different supplies, and none of them has a +8.5 V rail.

When to recap

Replace capacitors on evidence:

• a capacitor that is vented, bulged, split or leaking;
• corrosion or residue around its pads;
• a rail outside the loaded tolerances above, or one that is stable cold and drifts warm;
• a supply that trips its overcurrent or undervoltage protection with no card fault found;
• a mains-side capacitor that is cracked or discoloured.

See Capacitor Failure Symptoms.

Replacement practice

• Fit a voltage rating equal to or higher than the original.
• Keep the capacitance the same as the original.
• Replace mains-side parts with parts of the same safety class.
• Check the lead form and hole spacing before ordering.
• Keep soldering times short on these boards to avoid lifting pads.

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, IBM 5120 Capacitor Replacement Guide
• Capacitor Failure Symptoms, CRT Discharge Procedure, Recommended Tools