IBM 5120 Capacitor Replacement Guide

The IBM 5120 Computing System, whose computer unit IBM calls the 5110 Model 3, has two supplies in one power supply enclosure.

• The larger PC board (A2) is a transistor switching regulator producing +5, −5, +8.5, +12 and −12 V for the computer. IBM's logic manual labels it "5110 Power Supply Board A2".
• The smaller PC board (A1) is a ferroresonant supply producing +5, −5 and +24 V for the internal diskette drives.

IBM's 5120 maintenance manual and logic manual give no capacitance values, and neither draws the circuit inside either supply board. There is therefore no capacitor table here. IBM did publish rail tolerances, ripple limits, fuse ratings and the location of the supply's named parts, including resonant capacitor C4, and a removal procedure for the capacitor on each diskette drive's AC motor.

IBM 5120 Computing System (Wikimedia Commons, CC BY 3.0)

Safety

• The AC box holds the power on/off switch, terminal blocks TB1 and TB2, the line filter, circuit breaker CB1 and the line cord. The diskette drives take mains through connectors P4 and P5. IBM warns that AC line voltage is present in the power supply when the power switch is off, so unplug the line cord before opening the machine.
• Both supply boards hold charge after power-off. Discharge through a resistor and confirm with a meter.
• The display tube's high voltage is generated inside the display unit. Follow the CRT Discharge Procedure.

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

Inside the power supply enclosure

IBM's location drawing (item 208) names these parts:

IBM 5120 power supply components (MIM pp. 2-11 and 2-12)

Item	Part
AC box	Power on/off switch, terminal blocks TB1 and TB2, line filter, circuit breaker CB1, line cord
PC board A1	Ferroresonant supply for the diskette drives; carries fuses F2, F3 and F4
PC board A2	Switching regulator for the computer; AC inputs AC1 and AC2
C4	Resonant capacitor
T1	Transformer
P4, P5	Diskette AC connectors

IBM's DC voltage distribution drawing (item 274, p. 2-84) labels the boards the other way round, with the five-rail supply as A1 and the diskette supply as A2. The labels in the table follow the location drawing (p. 2-12), the fuse procedure (p. 2-80) and logic sheet 480B, which agree that A1 carries the diskette fuses and A2 is the switching regulator.

Fuses F2, F3 and F4 are on card PSA1 at the front of the power supply; remove the keyboard (item 251) to reach them. Logic sheet 480B rates them F2 +5 V 3 A, F3 −5 V 1 A and F4 +24 V 3 A. IBM describes the ferroresonant board as "protected against excessive overcurrent by a fuse on each of the three dc voltage outputs", while the switching board protects itself by shutting down. A blown F2, F3 or F4 therefore points to the diskette side of the machine.

IBM names C4 as the resonant capacitor of the ferroresonant supply and gives no value or rating for it. Read the markings on the part before removing it; any replacement must match its capacitance and AC voltage rating. Logic sheet 480A draws a capacitor, without designator or value, across the transformer winding that also feeds 240 V AC to the display unit at P6 through 0.3 A fuse F1.

Diskette drive motor capacitors

Each internal diskette drive is turned by an AC motor wired to a capacitor clamped beside it (item 230). IBM gives no value or rating. Its procedure is to unplug the machine, remove the insulator caps, discharge the capacitor (IBM warns that high voltage may be present at its terminals), disconnect the motor leads and loosen the clamp. Refit it with its terminals facing the pulley side of the diskette unit, motor leads 2 and 3 on the upper terminal and lead 1 on the lower, then refit the insulator caps. Discharge it through a resistor, and read its capacitance and AC voltage rating from the can before ordering a replacement.

The switching board: rails, tolerances and protection

IBM 5120 PC board A2 rails and loads (MIM p. 3-66)

Rail	Loads
+5 V	Basic logic voltage
−5 V	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	BSCA cards, load resistor RL1 and the 5114 R1 relay
−12 V	5114 R1 relay, BSCA cards and asynchronous communications/serial I/O card

The supply shuts down when +12 V exceeds +16 V, when −5 V is less negative than −3 V, or on excessive current in the transformer primary. After an automatic shutdown, IBM says to leave the machine off for at least 60 seconds before powering up again; the 5100 and 5110 manuals give five seconds. The outputs may vary from +10 % to −9 % before they affect operation, and a +6 V reference on the display card (G2) is provided for checking a meter.

Expected measurements

IBM's voltage specification for the 5110 Model 3 includes peak-to-peak ripple:

IBM 5120 voltage specifications (MIM p. 2-84)

Voltage	With load	No load (Y1 removed)	Ripple (V p-p)
+5 V	4.6 to 5.5	5.5 to 6.5	0.1
−5 V	−4.6 to −5.5	−3.7 to −4.7	0.1
+8.5 V	7.9 to 9.35	7.4 to 9.0	0.17
+12 V	11.0 to 13.2	9.8 to 12.2	0.24
−12 V	−11.0 to −13.2	−9.0 to −11.5	0.24
+24 V	22.0 to 26.4	–	–

The notes to the table say the supply shuts itself down if all loads are removed, and that a terminator must be fitted at the I/O interface port when no I/O device is attached. The same loaded and unloaded tolerances appear with the Y1 socket pin assignments (item 272).

Measure ripple with an oscilloscope on a running machine.

Terminals on board A2

Logic sheet 480B labels the supply "5110 Power Supply Board A2" and draws its output terminals as E1 to E3 (+5 V), E4 to E6 (ground), E7 and E8 (+8.5 V), E9 and E10 (+12 V), E11 and E12 (−12 V) and E13 and E14 (−5 V), with the mains inputs AC1 and AC2 on E15 and E16. The layout is the same as on the 5110 supply; see the 5110 guide.

Things that are not faults

• A supply that will not come up with all loads disconnected. IBM says it shuts down in that condition.
• A machine that does not restart immediately after a protection shutdown. Wait the 60 seconds IBM specifies.
• A machine with no I/O device and no terminator on the I/O interface port. IBM requires the terminator.

Getting real values

Read the values printed on the parts. Photograph both supply boards on both sides before removing anything, then record designator, value, voltage, type and lead spacing for each capacitor, starting with C4. Measure capacitance and ESR out of circuit rather than replacing on age alone. Do not use values from the IBM PC family (5150, 5155, 5160, 5170) or the IBM System/23 Datamaster, which are different designs.

When to recap

• A capacitor that is bulged, vented or leaking, on either board.
• A rail outside its loaded tolerance.
• Ripple above IBM's peak-to-peak limit.
• CB1 tripping, or F2, F3 or F4 blowing. Find the cause first.

Replacement practice is on the 5100 guide. See Capacitor Failure Symptoms.

Related pages

• IBM 5120, IBM 5120 Maintenance Guide, IBM 5120 Troubleshooting Guide
• IBM 5120 Maintenance Information Manual, IBM 5120 Computing System Logic Manual
• IBM 5100 Capacitor Replacement Guide, IBM 5110 Capacitor Replacement Guide
• Capacitor Failure Symptoms, CRT Discharge Procedure, Recommended Tools