Acorn
Interactive Flowchart BBC Micro & Master 128
Model B (Issue 4 / Issue 7), Master 128 & Astec Switched-Mode PSU — Acorn 6502A mainboards & Astec AA10050 PSU
Complete Diagnostic Flowchart: Capacitive Load, ESR & Power Rail Ripple Diagnostic Tree (Universal Retro Mainboards (C64, Amiga, ZX Spectrum, Game Gear, Consoles))
STEP 1: Measure 100Hz/120Hz Peak-to-Peak AC Ripple on the +5V / +12V Bulk Filter Cap
[node-1]
Set your oscilloscope to AC Coupling, 200 mV/div, and 2.5 ms/div (20 MHz bandwidth limit ON). Probe directly across the main bulk filter capacitor (e.g. C19 2200µF / C88 1000µF on C64, or C303/C304 on Amiga) under full system load. Measure the peak-to-peak sawtooth ripple amplitude (mVpp).
Expected Reading: < 50 mVpp clean regulated rail (post-regulator) or < 350 mVpp on raw unregulated bulk DC before the 7805/7812.
STEP 2: Check Linear Regulator (7805 / 7812) Input-to-Output Dropout Margin
[node-2]
Switch oscilloscope or multimeter to DC Volts and measure Pin 1 (Input) of the 7805 (+5V) or 7812 (+12V) regulator at the bottom of the ripple trough. Verify that Input Voltage remains at least 2.0V above the regulated output (e.g. ≥7.2V into a 7805, ≥14.2V into a 7812).
Expected Reading: 7805 Input ≥ 7.2V DC; 7812 Input ≥ 14.2V DC. Below this dropout threshold, ripple punches straight through to the logic rail.
STEP 3: Check Local IC Decoupling & Audio/Video AC Coupling Capacitors
[node-3]
Probe audio output coupling caps (e.g. C91 100µF on C64 SID Pin 27, or C324/C334 22µF SMD on Amiga Paula) — compare upstream vs. downstream signal amplitude. Inspect SMD electrolytic pads (Amiga 600/1200, Game Gear, DMG-01) under magnification for fishy odor, dull grey solder corrosion, or lifted copper traces.
Expected Reading: Full audio/video swing across the coupling capacitor with zero DC leakage on the output side and clean, shiny solder pads.
[ISOLATED FAULT: RES-BULK-CAP] Fault Isolated: High-ESR Bulk Filter Capacitor (>100 mVpp Rail Ripple)
VERDICT
Excessive 100/120Hz sawtooth ripple on the DC rail confirms that the primary reservoir/filter electrolytics have dried out and lost effective capacitance.
[ISOLATED FAULT: RES-REG-DROPOUT] Fault Isolated: Linear Regulator Dropout / Bridge Rectifier Diode Open
VERDICT
The pre-regulator DC rail is sagging below the minimum 2V dropout headroom required by the 7805/7812 regulator.
[ISOLATED FAULT: RES-COUPLING-CAP] Fault Isolated: Open/Leaky Audio or Video Coupling Capacitor & Trace Corrosion
VERDICT
Signal attenuation, muffled audio, or DC offset hum across the coupling stage indicates dried-out or leaking coupling capacitors.
[ISOLATED FAULT: RES-RAILS-HEALTHY] Verification Passed: Power Rails, Ripple & Coupling Capacitors Within Spec
VERDICT
All measured DC supply rails, AC ripple levels (<50 mVpp), and coupling stages pass bench specification. If performing a preventive recap, use the verified board BOM below.
Summary
The #1 mandatory recap on every BBC Micro and Master 128 is inside the Astec switched-mode power supply: two RIFA PME271M metallised paper X2 suppression capacitors (0.1 uF and 0.01 uF) crack and explode in acrid white smoke, and the 220 uF startup electrolytic (C9) dries out so the PSU fails to tick/start.
Interactive Bench Oscilloscope — Verify Pre- & Post-Recap Waveforms
Open Full Signal Visualizer →Click any test point below to compare a healthy recapped rail or clock against high-ESR electrolytic capacitor ripple and droop:
Safety & Warnings
Mains 240V AC & 340V DC primary reservoir inside the Astec PSU!
Unplug the mains lead and allow the primary reservoir capacitor inside the Astec PSU to discharge completely before removing the PSU PCB. Use only safety-certified 275V/310V AC Class X2 polypropylene capacitors for C1 and C2!
Is This Your Board?
- Open the gold/silver Astec PSU cage on the right rear corner of the BBC Micro case to locate C1 (0.1 uF X2), C2 (0.01 uF X2), and C9 (220 uF 25V).
- Mainboard Issue number (Issue 3, 4, 7) is printed in large copper text near the center of the PCB.
Verified
Check
Verified rows are agreed by two or more independent sources. Check rows are revision-dependent or single-sourced — confirm the value against your own board before ordering.
Astec Switched-Mode PSU (BBC Model B & Master 128)
Mandatory preventative service on every BBC Micro before applying mains power.
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C1 |
0.1 uF (100 nF) | 275 V AC | Class X2 Film (15mm pitch) | Mains live-neutral X2 filter | Replaces cracking amber RIFA PME271M smoke bomb! | Verified |
C2 |
0.01 uF (10 nF) | 275 V AC | Class X2 Film (10mm pitch) | Mains secondary X2 filter | Replaces smaller RIFA PME271M cap. | Verified |
C9 |
220 uF | 25 V | Radial (105 C Low-ESR) | Primary PWM startup rail | When C9 dries out, the PSU refuses to start from cold. | Verified |
C13 |
1000 uF | 10 V | Radial | +5V DC output filter | Verified | |
C14 |
1000 uF | 10 V | Radial | +5V DC secondary filter | Verified | |
C16 |
220 uF | 16 V | Radial | -5V DC output filter | Verified |
Astec Switched-Mode PSU (BBC Model B & Master 128) — shopping list
- 1 x 0.1 uF (100 nF) 275 V AC Class X2 Film (15mm pitch)
- 1 x 0.01 uF (10 nF) 275 V AC Class X2 Film (10mm pitch)
- 1 x 220 uF 25 V Radial (105 C Low-ESR)
- 2 x 1000 uF 10 V Radial
- 1 x 220 uF 16 V Radial
BBC Micro Model B Mainboard (Issue 4 / Issue 7)
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C1 |
220 uF | 10 V | Axial | +5V mainboard entry reservoir | Verified | |
C2 |
47 uF | 16 V | Axial | -5V rail decoupling | Verified | |
C7 |
10 uF | 25 V | Radial | NE555 /RESET timer | Verified | |
C15 |
10 uF | 25 V | Radial | SN76489 sound generator coupling | Verified | |
C19 |
100 uF | 10 V | Axial | LM386 speaker amplifier out | Verified | |
C36 |
47 uF | 16 V | Radial | Cassette motor / uPD7002 ADC ref | Verified |
BBC Micro Model B Mainboard (Issue 4 / Issue 7) — shopping list
- 1 x 220 uF 10 V Axial
- 1 x 47 uF 16 V Axial
- 2 x 10 uF 25 V Radial
- 1 x 100 uF 10 V Axial
- 1 x 47 uF 16 V Radial
Whole-Machine Shopping List
- 1 x 0.1 uF (100 nF) 275 V AC Class X2 Film (15mm pitch)
- 1 x 0.01 uF (10 nF) 275 V AC Class X2 Film (10mm pitch)
- 1 x 220 uF 25 V Radial (105 C Low-ESR)
- 2 x 1000 uF 10 V Radial
- 1 x 220 uF 16 V Radial
- 1 x 220 uF 10 V Axial
- 1 x 47 uF 16 V Axial
- 2 x 10 uF 25 V Radial
- 1 x 100 uF 10 V Axial
- 1 x 47 uF 16 V Radial
Recap Tips
- Always plug the 6-way output molex plug back onto the BBC Micro mainboard before powering on the Astec PSU - the switched-mode supply requires a minimum load on the +5V rail to regulate safely!
Related Symptoms
- White smoke & burning smell from rear right corner Classic RIFA C1/C2 X2 filter cap failure inside the Astec PSU (machine usually still works).
- Dead power / continuous high-pitched tick from PSU Dried-out C9 (220 uF) inside the Astec PSU or a shorted tantalum on the mainboard.
Sources & References
- BeebWiki - Astec Power Supply Repair Standard C1, C2 and C9 replacement values.