IBM 5120 Capacitor Replacement Guide

The IBM 5120 Computing System — which IBM's own service documentation calls the 5110 Model X3X or "5110-3" — has the most interesting power supply in the family, and the one the rest of this wiki's 51xx pages get wrong most often. It is not a linear supply. It is two supplies in one enclosure:

• The larger PC board (A2) is a small, high power, high frequency transistor switching regulator (TSR) developing five DC voltages for the computer itself. The logic manual labels this board "5110 Power Supply Board A2" — it is the IBM 5110's regulator board, reused.
• The smaller PC board (A1) is a ferroresonant supply developing three DC voltages for the internal diskette units.

That second supply is why this page exists separately from the 5100 and 5110 pages, because it brings with it the one capacitor IBM names as a functional component anywhere in the 51xx family: resonant capacitor C4.

IBM published no capacitance values for the 5120. Neither the maintenance manual nor the logic manual draws the inside of either supply board, and there is no 5120 parts catalogue on this wiki. This page therefore carries no capacitor table.

IBM 5120 Computing System. Both diskette drives, the display and the logic all run from one power supply enclosure containing two independent supplies. (Image: Wikimedia Commons, CC BY 3.0)

Safety

• Mains. The AC box carries live mains to the line filter module, terminal blocks TB1 and TB2, circuit breaker CB1 and the power on/off switch, and on to the two diskette drives through connectors P4 and P5. Unplug at the wall before the covers come off.
• Stored charge. Both supply boards have rectified-mains sections, and the ferroresonant board's resonant capacitor is a substantial part that will hold a charge. Discharge through a resistor and confirm with a meter.
• The resonant capacitor is under mains-level AC stress in normal operation. Treat it with the same respect as any mains-side component, not as a signal-level part.
• The display. Follow the CRT Discharge Procedure before working near the tube, and discharge a second time after a few minutes — the coatings on a CRT form a capacitor that can recover a charge after the first discharge.

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

Inside the power supply enclosure

Section 208 of the maintenance manual gives the physical layout. These are the parts IBM names:

IBM 5120 power supply — identified components

Item	Part
AC box	Power on/off switch, terminal blocks TB1 and TB2, a sealed line filter module, circuit breaker CB1, and the AC line cord (item 207)
T1	Transformer
C4	Resonant capacitor
PC board A1	Ferroresonant supply. Carries fuses F2 (+5 Vdc), F3 (−5 Vdc) and F4 (+24 Vdc), connector J1 on the rear of the card, and connectors J2 and J3
PC board A2	TSR regulator board. Carries the AC1 and AC2 inputs and the DC output terminals E1 to E14
CB1	Circuit breaker
P4, P5	Diskette AC connectors

Note that the 5120 has a circuit breaker, not a fuse holder, in its AC box — unlike the 5100 and 5110. A 5120 that trips CB1 is telling you something; resetting it repeatedly without finding out what is a good way to turn a repairable fault into a fire.

What a resonant capacitor is, and why it is not a recap candidate

A ferroresonant supply regulates without any active control loop. Part of the transformer core is driven into saturation, and a capacitor in a tank circuit with a resonating winding sustains the oscillation that holds the output constant. C4 is that capacitor.

Three consequences for anyone working on a 5120:

1. It is an AC-rated capacitor, not an electrolytic. It has no polarity, and fitting an electrolytic in its place would destroy it within seconds.
2. Its value is chosen for the resonance. Substituting "something close" detunes the supply and moves the output voltages. A replacement must match the original's capacitance and AC voltage rating.
3. It runs hot and hard. In a supply that is otherwise entirely passive, C4 is the component under the most continuous stress, and it is the most plausible single cause of a 5120 whose diskette rails are low or whose supply hums, buzzes or runs hot.

Because IBM never published its value, the only way to obtain it is to read the markings on the part fitted to the machine in front of you. Photograph it before removal.

Fuses tell you which supply has failed

IBM: the ferroresonant board "is protected against excessive overcurrent by a fuse on each of the three dc voltage outputs". Those are F2, F3 and F4 on board A1. This makes diagnosis unusually clean for a machine of this age:

• A blown F2, F3 or F4 points at the diskette side — a drive, the diskette control card, or a short on that rail. It does not implicate the computer's own supply.
• No fuse blown but the machine is dead points at board A2, the TSR regulator, which has no output fuses and protects itself by shutting down instead.

The TSR board: rails, tolerances and the 60-second rule

IBM 5120 — PC board A2 (TSR) rails and loads

Rail	What it feeds
+5 Vdc	Basic logic voltage
−5 Vdc	Storage cards, BSCA cards, common ROS
+8.5 Vdc	Storage cards, keyboard, display adapter, printer adapter, BSCA cards, all ROS cards
+12 Vdc	BSCA cards, load resistor RL1, and the 5114 R1 relay
−12 Vdc	5114 R1 relay, BSCA cards, asynchronous communications / serial I/O card

The DC outputs may vary from +10% to −9% of the rated voltage before they affect system operation. A +6 Vdc controlled reference is provided on the display card (G2) for checking a service meter before critical measurements.

The A2 board has the same protection as the 5100's and 5110's:

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

The restart rule is different on this machine. Where the 5100 and 5110 manuals say five seconds, the 5120 manual says that after an automatic shutdown the system "should be powered down with the on/off switch for at least 60 seconds before it is powered up again". Cycling a 5120 faster than that will make a healthy supply look dead and will waste an afternoon.

Expected measurements

Measure at the Y1 power connector on the A1 board, or at terminals E1 to E14 on supply board A2.

IBM 5120 — 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

The unloaded column is not simply "a bit high": +5 V rises above its loaded band with Y1 disconnected while +12 V, −5 V and −12 V all fall. Bench-testing the supply off-load and condemning it because +12 V reads 10.5 V is a mistake this table prevents.

Terminals on board A2

E1, E2 and E3 are +5 V; E4, E5 and E6 are ground; E7 and E8 are +8.5 V; E9 and E10 are +12 V; E11 and E12 are −12 V; E13 and E14 are −5 V — the same arrangement as the 5110, because it is the same board.

Things that are not faults

• A 5120 that hums. Ferroresonant transformers are noisy by nature. A hum that has always been there is not evidence of a failing capacitor; a hum that has changed character, or that comes with heat or low rails, is.
• +12 V reading low with the logic disconnected. See the tolerance table. The +12 V rail also feeds load resistor RL1, a deliberate minimum load; a supply measured with that path broken will not behave.
• A short delay before the machine responds after a protection shutdown. That is the 60-second rule, not a fault.

Getting real values

There is no published capacitance for any part of this machine, and the maintenance manual's scan has no text layer, so nothing can be searched out of it — pages have to be read by eye. The routes that do work:

1. Read the parts. Photograph both supply boards from both sides at high resolution, then record designator, value, voltage, dielectric, lead pitch and can size for every capacitor. Start with C4: its markings are the single most valuable missing figure for this machine.
2. Measure, do not assume. Out of circuit, capacitance and ESR tell you whether a part has drifted. On the ferroresonant side, an oscilloscope on the diskette rails will show whether the smoothing is still working.
3. Do not copy values from another machine. Nothing in the IBM PC family (5150, 5155, 5160, 5170) is related to this design, and the IBM System/23 Datamaster is a different product line again.

If you transcribe either board, publish it with photographs and the machine's serial, and this page can carry a sourced table instead of an explanation of why it cannot.

When to recap

• Any visibly failed, bulged, vented or leaking capacitor on either board.
• A rail outside the loaded tolerances above.
• Visible ripple on the diskette rails, or diskette errors that appear only once the machine is warm.
• A supply that has started to hum or buzz differently, or that runs noticeably hotter than it used to.
• CB1 tripping, or F2, F3 or F4 blowing — find the cause first; a blown fuse is a symptom, not a maintenance item.
• Any mains-side capacitor that is cracked, discoloured or smells of fish in use. Replace it immediately with a correctly safety-rated part of the same class.

Replacement practice, lead-form cautions and soldering advice are on the 5100 page. See Capacitor Failure Symptoms for how a failing capacitor presents at the symptom level.

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 — the family reference: rails, tolerances, protection, method
• IBM 5110 Capacitor Replacement Guide — same regulator board; the 5114's ferroresonant supply is documented there in more detail
• Capacitor Failure Symptoms · CRT Discharge Procedure · Recommended Tools