Osborne Executive Capacitor Replacement Guide

Recapping the Osborne Executive is essential for ensuring reliable operation, clean power delivery, and long-term preservation of this classic portable CP/M computer. The original electrolytic capacitors, now over forty years old, are prone to drying out, leaking, or failing, which can cause erratic behaviour, power issues, and even damage to rare custom ICs. Proactive replacement of all electrolytic capacitors is strongly recommended for any Executive in regular use or on display.

Visual Inspection & Failure Signs

Before beginning, examine the Osborne Executive’s boards for these common capacitor failure indicators:

• Bulging or domed cans – especially on the power supply and mainboard filter capacitors.
• Leaking electrolyte – brown, green, or white crust at the base of capacitors or on the PCB.
• Corrosion or PCB staining – discolouration or green residue around capacitor leads.
• Unstable operation – random resets, video distortion, or disk errors may be traced to failing capacitors.

If any single capacitor shows signs of failure, it is best practice to replace all electrolytic capacitors in the system.

Osborne Executive Capacitor Lists

The Osborne Executive consists of three main assemblies requiring recapping: the power supply, the main logic board, and the CRT video board. Always cross-check values with your specific revision, as some parts may differ between early and late units.

Power Supply Board

Osborne Executive Power Supply Electrolytic Capacitors

Ref. Designator	Capacitance	Voltage	Notes
C1	220 µF	250 V	Mains filter (high voltage, safety rated)
C2	10 µF	50 V	Start-up (bootstrap)
C3	2200 µF	25 V	+12 V output filter
C4	2200 µF	16 V	+5 V output filter
C5	470 µF	25 V	+12 V secondary filter
C6	470 µF	16 V	+5 V secondary filter
C7	1 µF	50 V	Control circuit decoupling
C8	1 µF	50 V	Control circuit decoupling

Note: Some power supply revisions may use 3300 µF for C3/C4. Always match the installed value or consult the silkscreen.

Main Logic Board

Osborne Executive Mainboard Electrolytic Capacitors

Ref. Designator	Capacitance	Voltage	Function
C101	10 µF	16 V	Reset timing
C102, C103, C104, C105	1 µF	50 V	Local decoupling (various ICs)
C106, C107	22 µF	16 V	Audio and video path coupling
C108	100 µF	16 V	Bulk decoupling
C109	47 µF	16 V	Floppy interface filter
C110, C111, C112	10 µF	16 V	Miscellaneous decoupling

SMD electrolytics are not present on original Osborne Executive boards; all are through-hole types.

CRT Video Board

Osborne Executive CRT Board Electrolytic Capacitors

Ref. Designator	Capacitance	Voltage	Function
VC1	10 µF	160 V	Vertical deflection coupling
VC2, VC3	22 µF	50 V	Video signal coupling
VC4	100 µF	16 V	Video output filter
VC5	1 µF	50 V	Sync decoupling

Always verify capacitor polarity on the CRT board—incorrect installation may cause display failure or damage.

Recapping Procedure

1. Unplug and discharge: Disconnect the Executive from mains power. Discharge the CRT (see CRT Discharge Procedure) and power supply filter capacitors before working.
2. Disassemble: Remove the rear case screws, gently lift the top cover, and disconnect the keyboard and CRT cables.
3. Remove boards: Unscrew and carefully extract the power supply, mainboard, and CRT board as needed.
4. Desolder capacitors: Use solder wick or a pump. Take care not to lift pads—Osborne PCBs are robust but can be damaged by excessive heat.
5. Install replacements: Fit new capacitors, matching polarity and lead spacing. Use 105 °C, low-ESR types for all replacements.
6. Clean up: Remove flux residue with IPA and inspect for solder bridges or cold joints.
7. Reassemble and test: Refit all boards, reconnect cables, and power up. Monitor for any unusual sounds, smells, or behaviour.

Recommended Tools & Parts

• Temperature-controlled soldering iron (60–80 W, 2 mm chisel tip)
• Desoldering pump and solder wick
• Multimeter (with continuity and ESR test if possible)
• Isopropyl alcohol (IPA) and antistatic brush
• High-quality 105 °C, low-ESR radial electrolytic capacitors (e.g., Nichicon PW/PS, Panasonic FR/FC, Rubycon YXF/ZLH)
• Kapton tape for insulating capacitors near metal shields
• Protective eyewear and insulated gloves

Post-Recap Voltage & Ripple Checks

After recapping, verify key voltage rails at the mainboard edge connector or test points:

Osborne Executive Power Supply Output Targets

Rail	Expected Voltage	Max Ripple (p-p)
+5 V	4.90 – 5.10 V	< 50 mV
+12 V	11.8 – 12.5 V	< 100 mV
-12 V	-11.5 – -12.5 V	< 100 mV

Excessive ripple or out-of-range voltages after recapping may indicate a failing regulator or transformer. Shut down immediately if voltages are abnormal.

Extra Tips

• Replace all electrolytics at once – partial recaps are not recommended due to similar ageing across all parts.
• Check for PCB damage – clean up any leaked electrolyte thoroughly, as it can corrode traces.
• Use correct voltage ratings – never install a capacitor rated for lower voltage than the original.
• Observe polarity carefully – especially on the CRT and power supply boards.
• Store and operate in a dry environment – moisture accelerates capacitor ageing.
• Power up gently – use a variac or current-limited bench supply for the first power-on after recapping.

Related Pages

• Osborne Executive Troubleshooting Guide
• CRT Discharge Procedure