BBC Micro Model A/B Capacitor Replacement Guide

Recapping a BBC Micro Model A or B is essential for ensuring stable power delivery, protecting irreplaceable ICs, and preventing damage from capacitor leakage. After four decades, the original electrolytic capacitors in these machines are often degraded or failing, causing unreliable operation, video instability, audio noise, or even catastrophic PCB corrosion. Proactive replacement preserves your BBC Micro for years to come.

Visual Inspection & Failure Signs

Before starting, examine all electrolytic capacitors for:

• Bulging or domed tops – Indicates internal pressure and imminent failure.
• Leaking electrolyte – Brown, yellow, or green residue at the base or on the PCB, especially near the power supply section.
• Corroded or stained PCB – Discoloured or lifted tracks around capacitors, particularly C9 and C1.
• Unusual odours – A fishy or acrid smell can signal capacitor leakage.

If any capacitor shows signs of failure, replace all electrolytics on the mainboard and power supply. Even apparently “good” capacitors may be out of tolerance or about to fail.

BBC Micro Capacitor Lists

The BBC Micro Model A and B use a mixture of through-hole aluminium electrolytic capacitors, both on the mainboard and in the internal power supply unit (PSU). All values below are for original Issue 3–7 boards and standard Astec AA11040C PSU; always cross-check your board’s silkscreen and PSU label.

Mainboard Electrolytic Capacitors

BBC Micro Mainboard Electrolytic Capacitors

Ref	Capacitance	Voltage	Notes / Function
C1	22 µF	25 V	Video circuit decoupling
C2	1 µF	63 V	Video output coupling
C3	10 µF	16 V	Audio path (DC block)
C4	10 µF	16 V	Audio path (DC block)
C5	10 µF	16 V	Audio path (DC block)
C6	10 µF	16 V	Audio path (DC block)
C7	10 µF	16 V	Audio path (DC block)
C8	10 µF	16 V	Audio path (DC block)
C9	220 µF	16 V	Main 5 V rail decoupling (critical)
C10	10 µF	16 V	Audio path (DC block)
C11	10 µF	16 V	Audio path (DC block)
C12	10 µF	16 V	Audio path (DC block)
C13	10 µF	16 V	Audio path (DC block)
C14	10 µF	16 V	Audio path (DC block)
C15	10 µF	16 V	Audio path (DC block)
C16	10 µF	16 V	Audio path (DC block)
C17	10 µF	16 V	Audio path (DC block)
C18	10 µF	16 V	Audio path (DC block)
C19	10 µF	16 V	Audio path (DC block)
C20	10 µF	16 V	Audio path (DC block)

Some later Issue 7 boards use 25 V rated parts for all 10 µF capacitors. Use 25 V or higher if in doubt.

Power Supply Unit (Astec AA11040C) Electrolytic Capacitors

BBC Micro PSU Electrolytic Capacitors

Ref (Astec)	Capacitance	Voltage	Notes / Function
C1	220 µF	400 V	Mains filter (primary, high voltage)
C2	2200 µF	16 V	+5 V output smoothing
C3	2200 µF	16 V	+5 V output smoothing
C4	470 µF	25 V	+12 V output smoothing
C5	470 µF	25 V	-5 V output smoothing
C6	10 µF	50 V	Control circuit decoupling
C7	1 µF	50 V	Control circuit decoupling

Some PSUs use slightly different numbering or values (e.g., 330 µF for C1 in rare variants). Always check the markings on your PSU before ordering replacements.

Recapping Procedure

1. Unplug the BBC Micro and allow it to sit for at least 10 minutes.
2. Disassemble – Remove the case screws, lift off the lid, and disconnect the keyboard ribbon and speaker wires.
3. Remove the mainboard – Unscrew and carefully lift out the mainboard.
4. Discharge the PSU capacitors – If recapping the PSU, ensure all capacitors are fully discharged before working. Use a resistor (e.g., 10 kΩ, 2 W) across the terminals of large cans.
5. Desolder old capacitors – Use a temperature-controlled iron (350–375 °C), desoldering braid, or a pump. Take care not to lift PCB pads, especially on the mainboard.
6. Clean any leaked electrolyte – Neutralise and clean with isopropyl alcohol and a soft brush. For stubborn corrosion, use a fibreglass pen.
7. Install new capacitors – Match polarity (long lead = positive), capacitance, and voltage (use equal or higher voltage). Observe correct orientation for all electrolytics.
8. Trim leads and inspect joints – Ensure all solder joints are shiny and free of bridges.
9. Reassemble and test – Refit the mainboard and keyboard, reconnect all cables, and reassemble the case.

Recommended Tools & Parts

• Temperature-controlled soldering iron (60–80 W, 2 mm chisel tip)
• Desoldering braid and/or pump
• Isopropyl alcohol (IPA) and nylon brush
• High-quality 105 °C, low-ESR capacitors (e.g., Panasonic FC/FR, Nichicon PW/PS, Rubycon YXF)
• Multimeter (for continuity and voltage checks)
• Fibreglass pen (for corrosion removal)
• Kapton tape (to insulate tall capacitors if needed)
• Protective eyewear and ESD precautions

Post-Recap Voltage & Ripple Checks

After recapping, check the main voltage rails at the user port or directly on the mainboard:

Expected Rails – BBC Micro (no expansion fitted)

Test Point	Nominal Voltage	Acceptable Range	Max Ripple (p-p)
+5 V (ICs, user port pin 1)	5.00 V	4.85 – 5.15 V	< 50 mV
+12 V (user port pin 6)	12.00 V	11.5 – 12.5 V	< 100 mV
-5 V (user port pin 8)	-5.00 V	-4.75 – -5.25 V	< 100 mV

If ripple exceeds these values, re-check PSU capacitor installation and soldering.

Additional Tips

• Replace all electrolytics together – If one has failed, the rest are not far behind.
• Use higher voltage ratings if space allows – Modern capacitors are smaller; 25 V or 35 V parts often fit where 16 V were used.
• Mind PSU safety – The mains filter capacitor (C1) holds charge and can be dangerous. Discharge with care.
• Check for PCB corrosion – Clean thoroughly before fitting new parts, especially around C1 and C9.
• Avoid cheap generic capacitors – Use reputable brands for reliability.
• Retain original lead spacing – Modern capacitors may have narrower leads; bend gently to fit.
• Power up with a current-limited bench supply or fused mains lead after recapping the PSU.

Related Pages

• BBC Micro Model A/B Troubleshooting Guide