Osborne 1 Capacitor Replacement Guide

Replacing the electrolytic capacitors (recapping) in your Osborne 1 is a vital preventative measure to ensure stable operation, reliable power delivery, and to protect the components from failure. The original capacitors in these early 1980s portable computers are now far past their intended lifespan, and failure can cause erratic behaviour, power instability, or even catastrophic damage to the motherboard and power supply.

Visual Inspection & Failure Signs

Before beginning, inspect all electrolytic capacitors for these common signs of ageing or failure:

Bulging or domed ends – Any swelling at the top or bottom of the can indicates internal gas build-up.
Leaking electrolyte – Brown, black, or white residue at the base or on the PCB is a clear sign of leakage.
Corrosion or green deposits – Especially around capacitor leads or pads, often accompanied by PCB staining.
Unusual odours – A fishy or acrid smell can indicate a failed capacitor.
Power instability or boot issues – Random resets, failure to boot, or display flicker can often be traced to failing capacitors.

If any capacitor shows signs of failure, it is strongly recommended to replace all electrolytic capacitors in the system.

Osborne 1 Capacitor Lists

The Osborne 1 contains electrolytic capacitors on both the main logic board and the internal power supply. Values and reference designators may vary slightly between early and later revisions (Osborne 1, Osborne 1A, and OCC1). Always verify against your specific board.

Main Logic Board Electrolytic Capacitors

**Osborne 1 Logic Board Electrolytic Capacitors**
Ref. Designator	Capacitance	Voltage	Notes / Function
C1, C2, C3, C4, C5, C6, C7, C8, C9, C10	1 µF	50 V	Decoupling (scattered across logic rails)
C11, C12, C13, C14	10 µF	16 V	Local decoupling (CPU, video, ROM)
C15, C16	22 µF	16 V	Video and audio section
C17, C18	47 µF	16 V	Floppy controller and I/O rails
C19	220 µF	16 V	Bulk 5 V decoupling (near power input)
C20	470 µF	16 V	Main 5 V filter (often near edge connector)

Some early boards may use axial capacitors; later boards use radial types. Always match lead spacing and orientation.

Power Supply Electrolytic Capacitors

The Osborne 1 uses an internal linear power supply. These capacitors are critical for safe and stable operation.

**Osborne 1 Power Supply Electrolytic Capacitors**
Ref. Designator	Capacitance	Voltage	Notes / Function
C101	10 000 µF	25 V	Main 5 V DC reservoir (large can)
C102	2 200 µF	25 V	+12 V DC filter
C103	470 µF	25 V	-12 V DC filter
C104	220 µF	25 V	-5 V DC filter (if fitted)
C105, C106	47 µF	25 V	Local regulation smoothing

If your Osborne 1 has a switching power supply (rare), values may differ—always check markings before ordering replacements.

Recapping Procedure

Unplug and discharge – Disconnect from mains and allow several minutes for capacitors to discharge. For safety, short large power supply capacitors with a resistor before handling.
Disassemble – Remove the rear case screws and gently lift off the case. Disconnect the keyboard and CRT cables, labelling if needed.
Remove logic and power supply boards – Take care with the CRT and high-voltage areas.
Desolder old capacitors – Use a temperature-controlled iron (350–375 °C) and desoldering braid or pump. Take care not to lift PCB pads, especially on older fibreglass boards.
Clean pads and inspect for damage – Remove any leaked electrolyte with isopropyl alcohol and a soft brush.
Install new capacitors – Observe polarity (long lead = positive). Match lead spacing and keep replacement cans as low-profile as possible to avoid mechanical interference.
Check work – Inspect for solder bridges, correct orientation, and clean joints.
Reassemble and test – Refit boards, reconnect all cables, and power up via a current-limited supply or with a fused mains lead.

Recommended Tools & Parts

Temperature-controlled soldering iron (60–80 W, 2 mm chisel tip)
Desoldering braid and/or pump
Isopropyl alcohol (99%) and antistatic brush
Multimeter with continuity and ESR testing
High-quality 105 °C, low-ESR radial electrolytic capacitors (e.g., Nichicon PW/PS, Panasonic FR/FC, Rubycon YXJ)
Kapton tape for insulating capacitors near metal chassis
Protective eyewear and ESD wrist strap

Post-Recap Voltage & Ripple Checks

After recapping, verify the output voltages at the power supply connector (with system running, CRT connected):

**Expected Osborne 1 Power Supply Rails**
Rail	Nominal Voltage	Acceptable Range	Max Ripple (p-p)
+5 V	5.00 V	4.85 – 5.15 V	< 50 mV
+12 V	12.00 V	11.5 – 12.5 V	< 120 mV
-12 V	-12.00 V	-11.5 – -12.5 V	< 120 mV
-5 V	-5.00 V	-4.75 – -5.25 V	< 100 mV

If ripple exceeds these values, recheck your soldering and ensure all capacitors are installed with correct polarity and value.

Extra Tips

Replace all electrolytics at once – Piecemeal replacement is not recommended due to similar age and stress.
Check for PCB damage – Leaked electrolyte can corrode traces; repair as needed before fitting new parts.
Use quality brands – Cheap capacitors may fail prematurely; always use reputable manufacturers.
Keep capacitor height low – Some areas have tight clearance due to the CRT and case.
Inspect connectors and wiring – Ageing insulation and oxidised connectors can also cause instability.
CRT safety – The Osborne 1’s CRT carries high voltage; avoid the anode cap and discharge if necessary.
Fuse check – Replace any blown fuses with the correct type and rating.