Acorn Electron Plus 3
| Acorn Electron Plus 3 | |
|---|---|
 |
|
| The L-shaped Plus 3, with the Electron connector on the left and the drive in the right-hand arm (photo: Chris Whytehead, CC BY-SA 3.0) | |
| Manufacturer | Acorn Computers |
| Type | Floppy disc expansion unit |
| Release date | 1984 |
| Price | £229[1] |
| Interface | Electron expansion connector, with a pass-through connector for the Plus 1 |
| Compatible | Acorn Electron |
| Model | ALA13 |
| Power | Own mains adaptor (21 V AC), which also powers the Electron |
| Storage | 320 KB per disc (single-sided drive) or 640 KB (double-sided) |
| Media | 3.5-inch floppy discs, ADFS format |
The Acorn Electron Plus 3 is Acorn's floppy disc unit for the Acorn Electron. It is an L-shaped box that bolts onto the back of the Electron and wraps round its right-hand side, holding a 3.5-inch drive, a WD1770 disc controller, the Advanced Disc Filing System (ADFS) in ROM and a power supply. A socket at the back takes a second drive, and the Electron's expansion connector is repeated so that a Plus 1 can be fitted behind it.[2][3] Acorn's sales code was ALA13.[3]
Wikipedia gives a late-1984 launch at £229, and notes that the Plus 3 was also made with a double-sided drive.[1] The user guide describes both single-sided (320 KB) and double-sided (640 KB) versions; the only difference in use is the format command.[2]
Hardware
[edit | edit source]
| Part | Detail |
|---|---|
| Disc controller | Western Digital WD1770[3] |
| Filing system | Acorn ADFS in ROM (version 1.0)[1] |
| Drive | 3.5-inch, single- or double-sided; Chinon F353 in Chris Whytehead's unit[2][3] |
| Disc format | 80 tracks, 16 sectors of 256 bytes per side: 327,680 bytes (320 KB) single-sided[2] |
| External drive socket | For a second 3.5- or 5.25-inch drive with a Shugart interface and its own power supply[2] |
| Rear expansion connector | Repeats the Electron connector for a Plus 1 or similar unit[2][3] |
| Power | 21 V AC from the Plus 3 adaptor into a socket on the left of the unit; the Electron then runs from the Plus 3[3] |
The ADFS claims workspace from &E00, raising PAGE to &1D00, which stops some cassette software from running.[1] In screen modes 0 to 3 the screen blanks to the background colour while the drive is in use, because the Electron cannot refresh the display and keep up with the disc at the same time. Interrupts are off during every disc access, so the cursor stops flashing, sound stops and TIME loses time with heavy disc use.[2]
Chris Whytehead's photograph of the opened unit shows the power supply at the left end of the board: a regulator on a heatsink, a toroidal choke and a group of radial electrolytic capacitors.[3] Acorn's user guide has no circuit or parts list, and the Acorn Electron Service Manual covers the Electron alone, so there is no Acorn component data for the supply.[2]
Power adaptor
[edit | edit source]The Plus 3 comes with its own black mains adaptor, which plugs into the Plus 3 instead of the Electron and powers the Electron, the Plus 3 and a Plus 1. Acorn warns that the Electron's original cream adaptor cannot supply the combined load and that its thermal fuse will probably blow beyond repair.[2] Wikipedia gives the two adaptors as:[1]
| Adaptor | Part no. | Input | Output | Use |
|---|---|---|---|---|
| Original Electron (cream) | 0201,113 | 220/240 V AC, 50 Hz | 19 V AC, 0.737 A, 14 W | Electron, Electron with Plus 1 |
| Plus 3 (black, made by STC) | 0865,010 | 240 V AC, 50 Hz, 50 W | 21 V AC, 1.75 A, 36.75 W | Electron with Plus 3, with or without Plus 1 |
Warning: the adaptor carries mains voltage. Check its output on the AC range under load before blaming the Plus 3 or the Electron.
Fitting
[edit | edit source]- Switch off and unplug everything. If a Plus 1 is fitted, remove it first.
- Turn the Electron upside down and remove the plastic strip from the expansion connector.
- Turn the Plus 3 upside down and make sure the threaded ends of its two bolts are flush with the case. Push its edge connector socket onto the Electron's connector; the notch makes it impossible to fit upside down.
- Screw the bolts into the brass bushes either side of the connector, finger-tight with a coin or screwdriver. A slight wiggle may be needed to line up both bolts. Do not overtighten.
- Refit the Plus 1, if any, to the back of the Plus 3 in the same way.
- Connect the new adaptor to the Plus 3, not the Electron.[2]
At power-up the screen should read "Acorn Electron", "Acorn ADFS" and "BASIC".[2]
Faults and problems
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| "Acorn ADFS" missing from the start-up banner | Bad contact at the edge connector | Disconnect, remove the Plus 3, clean the Electron's edge connector fingers with a non-abrasive pencil eraser and a soft cloth, refit[2] |
| "Acorn ADFS" missing after running certain programs | ADFS workspace from &E00 corrupted; the ADFS has disabled itself | Type *FX200,2 then CTRL BREAK, or switch off and on. "Bad FS map" also clears with CTRL BREAK[2]
|
| Blank screen with the Plus 3 fitted | Video lead not reconnected; edge connector contact; Plus 3 fault | Check the lead, clean the connector, try the Electron without the Plus 3[2] |
| Nothing at all after fitting, adaptor dead | Old cream adaptor used and its thermal fuse blown | Use the Plus 3 adaptor; replace the old adaptor[2] |
| Repeated "Disc error" on the internal drive | Unformatted or damaged disc | Format a new disc and copy what can be read[2] |
| "Disc error" on an external 5.25-inch drive | Disc inserted the wrong way, or a BBC Micro DFS disc, which ADFS cannot read | Check the disc[2] |
| "Not found" or "Bad command" on SHIFT BREAK | Boot option set with no matching !BOOT file | Clear it with *OPT4,0[2]
|
| Tape game will not run with the Plus 3 fitted | ADFS workspace raises PAGE | Start the ADFS from the Welcome disc with CTRL A BREAK, type *NOADFS, wait for the drive light to go out and press BREAK; switch off and on to restore[2][1]
|
| Internal drive has no power; drive power connector damaged | Original AMP 800-930 4-pin connector | Wikipedia's repair note gives the Molex 5264 50-37-5043 Mini-SPOX as a substitute[1] |
Acorn advises against fitting and removing the Plus 3 repeatedly.[2] For the drive see Floppy Drive Repair; for the Electron see the Acorn Electron Troubleshooting Guide. For general signs of capacitor failure in the supply see Capacitor Failure Symptoms.
Documentation
[edit | edit source]- The Electron Plus 3 User Guide (Acorn, 0441,000 issue 1, October 1984): installation, ADFS reference, error messages, disc format and fault finding.[2]
Related pages
[edit | edit source]- Acorn Electron
- Acorn Electron Plus 1
- Acorn Electron Maintenance Guide
- Acorn Electron Capacitor Replacement Guide
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 "Acorn Electron", Wikipedia (retrieved 2026-10-02), section "Acorn Plus 3": launched late 1984 at £229; WD1770 and ADFS; second drive connector and repeated edge connector; PAGE raised to &1D00; screen blanking and TIME errors in modes 0 to 3; the black STC power supply (part 0865,010, 240 V AC 50 W in, 21 V AC 1.75 A 36.75 W out) replacing the cream Electron adaptor (0201,113, 19 V AC 0.737 A 14 W); repair note on the internal drive power connector (AMP 800-930, replaceable with Molex 5264 50-37-5043 Mini-SPOX). Section "Acorn Electron" history: ADFS v1.0 ROM.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 Acorn Computers, The Electron Plus 3 User Guide, part no. 0441,000, issue 1, October 1984. Hosted on this wiki: File:Acorn Electron Plus 3 User Guide.pdf. Introduction (320K or 640K per disc) p. 1; installation with and without a Plus 1, mains adaptor warning and external drives pp. 3 to 5; single- and double-sided versions and recommended discs p. 6; formatting (80 tracks, 16 sectors of 256 bytes) pp. 6 to 7; ADFS in modes 0 to 3 p. 14; disabling the ADFS (*NOADFS) p. 15; error messages pp. 95 to 101; technical information pp. 102 to 105; appendix A fault finding pp. 106 to 108.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Whytehead, Chris. "Acorn ALA13 Electron Plus 3", Chris's Acorns, The Centre for Computing History (retrieved 2026-10-02). WD1770 and ADFS ROM; connector for a second drive and for a Plus 1 or Slogger ROMBox at the back, under plastic covers; 21 V AC power socket on the left side, the Electron running from the Plus 3 supply; inside, the power supply, expansion bus, ADFS and WD1770 and a Chinon F353 drive.