BBC Micro B+ Capacitor Replacement Guide

This guide documents capacitor diagnosis and replacement for the BBC Micro Model B+ and B+128. The B+ has two distinct capacitor populations: the Astec linear power-supply module (mains-side), and the main board (low-voltage decoupling). Mains-side work is significantly more dangerous and is described first.

Safety Warning

The B+ PSU has lethal mains-side capacitance that retains charge after disconnection. Before any service work:

1. Power off and unplug the mains lead.
2. Wait at least 30 seconds.
3. Discharge the primary bulk electrolytic (C9) through a 1 kΩ / 5 W resistor.
4. Verify all PSU outputs read 0 V with a multimeter.

The RIFA-branded X2 mains-suppression caps (C1, C2) are the single most common preventive-replacement target on B+ machines. They fail open and vent acrid white smoke under continued use — at which point they cannot be recovered, only replaced.

Astec Power Supply Capacitors

The B+ uses an Astec linear PSU module identical to that fitted to late-issue Model Bs. Refer to the PSU schematic for component placement and reference designators.

B+ PSU capacitor list (Astec linear PSU)

Ref	Value	Voltage	Type	Function	Notes
C1	0.1 µF (100 nF)	250 V AC (X2 class)	Mains-suppression film	Live–neutral RFI suppression	Replace if RIFA-branded (preventive). Use modern X2 0.1 µF 275 V AC.
C2	0.01 µF (10 nF)	250 V AC (X2 class)	Mains-suppression film	Live–earth / neutral–earth RFI suppression	Replace if RIFA-branded (preventive). Use modern Y2 4.7 nF or X2 10 nF 275 V AC depending on board issue.
C9	220 µF	200 V (or 250 V)	Aluminium electrolytic, 105 °C	Primary bulk smoothing after bridge	Replace if any sign of bulging, leakage, age. This part causes intermittent cold-start failures on ageing PSUs.
C10	4700 µF	16 V	Low-ESR aluminium electrolytic, 105 °C	+5 V secondary bulk	Replace as preventive measure; failure causes voltage sag under disc load.
C11	470 µF	25 V	Aluminium electrolytic, 105 °C	+12 V secondary bulk	Replace as preventive measure.
C12	220 µF	16 V	Aluminium electrolytic, 105 °C	−5 V secondary smoothing	Replace as preventive measure.
C13–C16	0.1 µF	50 V	Ceramic / polyester	Regulator bypass × multiple	Ceramic; rarely fail. Inspect only.

Values above are taken from the Acorn BBC Microcomputer Service Manual, October 1985, Section 2 PSU schematic and have been cross-checked against Stardot recapping threads.

PSU Recap Procedure

1. Discharge the PSU as above.
2. Remove the four screws holding the Astec module in its shielding can.
3. Unplug the four-pin output Molex from the main board.
4. Lift the PSU PCB out of the can.
5. Photograph the board from both sides at high resolution before desoldering anything.
6. Mark each electrolytic's polarity (positive lead) before desoldering — even if the silkscreen is clear, faded markings are common.
7. Desolder C9, C10, C11, C12 with solder wick at no more than 350 °C, 5–7 seconds per cycle.
8. Replace C1, C2 with modern X2 / Y2 parts (Kemet R46/R41 series, EPCOS B32921/B32932 series, or Vishay BFC233 series — never another RIFA-branded part).
9. Fit electrolytic replacements matching the silkscreen polarity.
10. Solder both leads; inspect for clean fillets; trim flush.
11. Verify continuity across the new caps and to ground before re-fitting.

Main Board Capacitors

The B+ main board has ≈40 electrolytics of mixed values plus a substantial population of ceramic and polyester decoupling caps. The electrolytics are the primary recap targets; ceramics rarely fail and are not normally replaced.

B+ main board electrolytic capacitor list (representative)

Value	Voltage	Type	Position / Function
10 µF	16 V	Radial aluminium electrolytic, 105 °C	+5 V local bypass × multiple
22 µF	16 V	Axial aluminium electrolytic, 105 °C	DRAM array bypass × multiple
47 µF	16 V	Radial aluminium electrolytic, 105 °C	Video DAC supply filtering
100 µF	16 V	Axial aluminium electrolytic, 105 °C	ROM bank bypass × multiple
220 µF	16 V	Radial aluminium electrolytic, 105 °C	ULA supply local filtering × 2–3
470 µF	16 V	Radial aluminium electrolytic, 105 °C	Main rail decoupling near PSU connector
1 µF	50 V	Axial aluminium electrolytic, 105 °C	Analogue / ADC bypass × 2
4.7 µF	25 V	Tantalum bead	Analogue input network (joystick port)
0.47 µF	50 V	Polyester film	Sync separator network on composite output
100 µF	25 V	Radial aluminium electrolytic, 105 °C	Cassette amplifier supply bypass

The most failure-prone parts on the B+ main board are the axial 22 µF and 100 µF bypass caps near the DRAM array, because their leads were dipped in flux that wicks under the seal over time, eventually allowing electrolyte to evaporate. Symptoms are flaky DRAM behaviour when the machine is fully warmed up.

Main Board Recap Procedure

1. Power off, unplug, discharge PSU.
2. Remove the four case screws and lift the top assembly off.
3. Disconnect the keyboard ribbon, speaker connector, and any floppy / printer leads.
4. Photograph both sides of the main board at high resolution.
5. Tag each electrolytic's polarity and the associated reference designator with masking tape if necessary.
6. Desolder each cap one at a time with solder wick at no more than 350 °C, 5 seconds per cycle to avoid lifting the through-hole plating.
7. Fit modern radial replacements matched for value, voltage, ESR, polarity and lead spacing. Mind the case-height clearance under the keyboard.
8. Solder both leads; inspect for clean fillets; trim flush.
9. Verify no shorts across the main rails (DMM diode-test mode is sufficient for triage).

Recommended Modern Replacements

• X2 / Y2 mains-suppression: Kemet R46/R41 series, EPCOS B32921/B32932, Vishay BFC233. Never RIFA.
• Bulk electrolytic: Panasonic FR / FM / FC series; Nichicon HE / HZ series (2007+ date codes); Rubycon ZLH / ZLJ / YXJ series.
• Small-signal electrolytic: Panasonic FC, Nichicon UPM, United Chemi-Con KZH.
• All replacements should be 105 °C rated and matched to or higher than the original voltage rating.
• Verify lead spacing — many original axials need to be replaced with radial parts on dropper leads, which works but should be done neatly with heat-shrink-insulated lead extensions.

Post-Recap Verification

1. Power on with a bench load on +5 V (2.5 Ω / 25 W is a good test load).
2. Verify all rails before connecting the main board.
3. Reconnect main board; verify the banner displays on first power-on.
4. Run *FX 0 self-test and *HELP to verify all ROMs and BASIC are present.
5. Insert a known-good DFS disc; verify CAT lists it correctly.
6. Test under sustained warm operation (1 h+) for the disc-load voltage-sag symptom.

When Not to Recap

If the B+ powers on cleanly, beeps, displays a perfect banner, runs BASIC II, reads and writes discs cleanly, and the PSU's RIFA caps have already been replaced or are confirmed not RIFA-branded, then the caps are within tolerance.

Always recap if:

• RIFA-branded C1 or C2 present — preventive replacement, no exceptions.
• Visible cap failure on the PSU or main board (bulging, leakage, crusty residue).
• PSU smoke, fishy odour, or audible whine.
• Random reboots when warm but stable when cold (suggests C9 PSU primary bulk ageing).
• DRAM errors only when warm (suggests local DRAM bypass cap ageing).
• B+128 reports reduced memory when warm (suggests sideways RAM bypass cap ageing).

Related Pages

• BBC Micro Model B+
• BBC Micro B+ General Maintenance
• BBC Micro B+ Troubleshooting Guide
• BBC Micro Model A/B Capacitor Replacement Guide
• Capacitor Failure Symptoms