Acorn
Interactive Flowchart Acorn Atom & Acorn Electron
Acorn Electron Issue 4/6 & Acorn Atom Issue 5 — Ferranti PLCC-68 ULA & 6502 mainboards
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 Acorn Electron takes 19V AC from its external transformer brick and generates +5V and -5V internally using a massive 4700 uF reservoir capacitor (C1) and a transistor regulator stage.
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
Clean the 68-pin PLCC ULA socket on every Acorn Electron
Intermittent crashes and garbage video on the Electron are 90% caused by oxidation on the J-lead pins of the Ferranti ULA socket.
Is This Your Board?
- Acorn Electron mainboard has the large square 68-pin PLCC Ferranti ULA in the centre and two large radial/axial filter cans near the 19V AC power jack.
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.
Acorn Electron Mainboard (Issue 4 / Issue 6)
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C1 |
4700 uF | 25 V | Radial / Axial | 19V AC bridge rectifier reservoir | Verified | |
C2 |
220 uF | 25 V | Radial | -5V negative rail filter | Verified | |
C3 |
100 uF | 16 V | Radial | +5V regulated rail filter | Verified | |
C9 |
10 uF | 25 V | Radial | Power-on /RESET delay | Verified | |
C12 |
47 uF | 16 V | Radial | Speaker audio coupling | Verified | |
C21 |
10 uF | 25 V | Radial | Cassette data slicer filter | Verified |
Acorn Electron Mainboard (Issue 4 / Issue 6) — shopping list
- 1 x 4700 uF 25 V Radial / Axial
- 1 x 220 uF 25 V Radial
- 1 x 100 uF 16 V Radial
- 2 x 10 uF 25 V Radial
- 1 x 47 uF 16 V Radial
Recap Tips
- When replacing C1 (4700 uF 25V), choose a low-profile 105 C radial or axial can so the keyboard assembly sits flush.
Related Symptoms
- Loud 50Hz hum from internal speaker & wavy video Dried-out 4700 uF reservoir C1 in the Electron power stage.
Sources & References
- Acorn Electron Service Manual Official Acorn Electron parts list.