Acorn Archimedes A305

Acorn Archimedes A305

Archimedes 300-series system. The A305 and A310 share the same case and main board.
Specifications
Manufacturer	Acorn Computers Ltd
Type	Personal computer (three-box desktop)
Released	June 1987
Intro price	£799 ex VAT (base), £999 ex VAT with colour monitor
CPU	ARM2 at 4/8 MHz (24 MHz master oscillator)
Memory	512 KB RAM, upgradeable to 1 MB (16 × 4464 DRAM fitted, 16 sockets empty)
Storage	3.5" floppy drive, 1 MB unformatted (800 KB ADFS)
Display	Analogue RGB and mono composite; up to 640×256 in 256 colours, 640×512 on a multisync
Sound	8 channels, stereo (VIDC1a)
Dimensions	362 mm wide × 406 mm deep × 97 mm high (main unit, excluding feet)
OS / Firmware	Arthur
Predecessor	BBC Master series
Successor	Acorn Archimedes A3000
Model no.	AKB10

The Acorn Archimedes A305 is the entry model of Acorn's original Archimedes range, launched with the A310 in June 1987.

The A305 and A310 are the same machine: the same 276,000 main board, case and chip set, with half the DRAM fitted in the A305. Acorn's service manual describes the A305 as fitted with 0.5 MB of DRAM and upgradeable "to model 310 specification by the addition of a further 0.5 Mbyte of DRAM".

Prices

Acorn's retail price list gives these prices:

Archimedes 305 prices

Product code	Configuration	RRP ex VAT	RRP inc VAT
AKB10	A305, no monitor	£799.00	£916.60
AKB10/AKF01	A305 with mono monitor	£849.00	£974.10
AKB10/AKF11	A305 with colour monitor	£999.00	£1,146.60

The A310 cost £76 more ex VAT in each configuration, and the AKA51 0.5 MB upgrade cost £89 ex VAT, so upgrading an A305 later cost more than buying an A310.

Chip set

The A305 uses Acorn's four-chip ARM set. Positions are from Acorn's component-level parts list.

Archimedes 300 series main ICs

Device	Position	Description
ARM (2 µm)	IC43	32-bit RISC processor, 26-bit address bus
MEMC	IC45	Memory controller: address translation, DMA, processor clocks
VIDC1a	IC17	Video and sound controller
IOC	IC21	I/O controller: podule bus, keyboard, timers, interrupts
PAL 16L8A (programmed)	IC44	Glue logic
1772	IC47	Floppy disc controller
65C51	IC9	Serial port
PCF8583	IC16	Real-time clock and 240 bytes of non-volatile RAM
26LS30, 26LS32	IC7, IC6	RS423 line driver and receiver
LM386	IC68	Internal speaker amplifier

A 24 MHz master oscillator drives the system, and the ARM2 runs at 4 or 8 MHz. Acorn quotes system performance as "typically 4 MIPS".

Memory

Memory is sixteen 4464 DRAMs (64K × 4, 120 ns, Acorn part 0704,105) at IC69–IC76 and IC78–IC85. The sixteen 18-pin sockets at IC51–IC58 and IC60–IC67 are empty on an A305. The AKA51 upgrade is a second set of sixteen 4464s for those sockets, taking the machine to the A310's 1 MB. Third-party boards later took 300-series machines to 4 MB; two of Chris Whytehead's restored A310s carry IFEL 4 MB boards with MEMC1a memory controllers.

ROM is 512 KB as standard in four 32-pin sockets, IC24–IC27, which also take 128 KB or 256 KB sets. Links LK2, LK6 and LK12 select the ROM device type, and Acorn's basic checks include confirming them for the ROMs fitted.

Display

Arthur provides eighteen standard screen modes plus three that need a multisync monitor. In every mode except teletext the colours come from a palette of 4096.

Arthur screen modes (300 series service manual p. 8)

Mode	Pixel resolution	Logical colours	Text
0	640 × 256	2	80 × 32
1	320 × 256	4	40 × 32
2	160 × 256	16	20 × 32
3	text only	2	80 × 25
4	320 × 256	2	80 × 25
5	160 × 256	4	20 × 32
6	text only	2	40 × 25
7	teletext	teletext	40 × 25
8	640 × 256	4	80 × 32
9	320 × 256	16	40 × 32
10	160 × 256	256	20 × 32
11	text only	–	80 × 25
12	640 × 256	16	80 × 32
13	320 × 256	256	40 × 32
14	text only	16	80 × 25
15	640 × 256	256	80 × 32
16	text only	16	132 × 32
17	text only	16	132 × 25
18	640 × 512	2	80 × 64
19	640 × 512	4	80 × 64
20	640 × 512	16	80 × 64

Modes 18 to 20 need a multisync monitor. Acorn offered the machine with no monitor, a mono monitor, a standard-resolution colour monitor or a multisync. There is no colour composite or UHF output.

The analogue RGB outputs each drive a 75 Ω line at 0 V for black and 0.7 V for white. Sync into 75 Ω is below 0.2 V low and above 0.3 V high; unterminated, high is above 2.4 V.

Expansion

The 300 series has no expansion slots as shipped. The AKA01 backplane and fan upgrade (£39 ex VAT) plugs into the 64-way DIN 41612 connector on the main board and gives two podule positions; Acorn says the fan must be fitted with the backplane. One of Chris Whytehead's A310s has an IFEL four-slot backplane instead.

The backplane supply must not supply more than 20 W in total. Coprocessor podules, which need the main system data bus, are not supported on the 300 series. With an Econet module fitted, only a half-width podule, or one shaped to clear the module such as the I/O podule, fits in the lower position.

Acorn's podules for the range were the AKA05 ROM podule, the AKA10 I/O podule (BBC Model B and Master analogue port, user port and 1 MHz bus) and the AKA16 MIDI podule; the AKA15 adds MIDI to an I/O podule.

The internal options were the AKD50 second floppy drive (£125) and the AKD52 20 MB hard disc with its podule (£499, 300 series only, needing the backplane and fan). Only one of the two can be fitted.

Backup batteries

The 300 series uses two AA alkaline cells. Acorn's specification reads: "Two LR06 (AA size) 1.5 V Manganese Alkaline cells fitted inside computer main unit. Batteries require replacement once a year." The parts list gives them as Acorn part 0817,005, "Battery, Alkaline 1V5 2AH 'AA'", quantity 2, in battery holder assembly 0176,009. The holder is riveted to the base metalwork beside the fan position and wired to PL11 on the main board.

The cells keep the PCF8583 clock and its 240 bytes of configuration RAM running. With the power off, IC16 pin 8 should read about 2.8 V; if it does not, check PL11, D3 and the cells, each of which should read above 1.4 V. Alkaline cells left in place for decades leak potassium hydroxide along the leads; see Battery Explosion, Capacitor or Corrosion Damage and Battery Refurbishment. If the holder is replaced, rivet it through the two holes originally used: only two of its four holes hold it rigid.

Power and environment

The power supply connects to the main board on four tags: PL5 +12 V, PL6 0 V, PL7 +5 V and PL8 −5 V. Acorn says a clicking supply indicates a short between two of its outputs or a faulty supply.

Mains input is 198–264 V AC at 47–63 Hz, rated 0.4 A without a monitor and 0.9 A with one. The rear IEC outlet is unswitched: it is live whenever the inlet is powered, whatever the position of the PSU switch, and is rated 3 A continuous and 80 A surge. The operating range is 5–35 °C at 10–95 % relative humidity, non-condensing.

The PSU has a double-pole switch and fuses in both live and neutral. Acorn's manuals treat it as a service-replacement-only item and give no circuit for it. After refitting or replacing a PSU, Acorn requires an earth continuity test at 25 A from the mains plug earth pin to the base metalwork, the rear panels and the top cover.

Acorn production and field changes

The 300-series service manual lists these production changes and known faults:

• Mono video output fault on some issue 2 boards. Check for insulation tape beneath the RGB connector mounting bracket; without it the mono video track can short to earth.
• Sound output failure. Inserting a jack into the audio socket SKT1 briefly shorts the output transistors Q9 and Q11 to 0 V. Early boards have BC239s, which may fail; later boards have 2N3904s, which tolerate the short. The two types are fitted differently.
• Serial port, issue 1 boards only: IC7 pin 3 and IC15 pin 10 are not inserted in the board and are joined by a wire link on the top side, and a second link joins IC7 pins 1 and 4 on the underside.
• Speaker hum (FCO E008). The fix is a 100 µF capacitor of 10 V or more between IC68 pins 7 (positive) and 4, as close to the IC as possible and glued or waxed down. Machines 27-AKB10-1000001 to 27-AKB10-1001277 and 27-AKB15-1000001 to 27-AKB15-1001752 left production without it. Noise on BREAK or ESCAPE on early machines is a software fault cured by Arthur 1.20.
• RGB output levels (FCO E009). Acorn raised the RGB levels in production for multisync monitors and dropped sync on green. Where levels are too low for a monitor, change R20, R41 and R59 to 43.2 Ω 1 % 0.25 W and remove R39.
• Video noise (FCO E011). Rippling on screen, worst on colour monitors. Fit a 1N4148 across R67 next to VIDC (IC17), cathode towards Q13, and replace the decoupling capacitor between IC17 and Q12 with a 22 µF 6.3 V or higher axial electrolytic, positive end away from Q12.
• Link 12 selects the ROM device type. Issue 1 boards have no link; issue 2 boards have four pads or pins with shunts, tracked as issue 1. Reversed shunts can stop the machine, and changing the setting means cutting tracks.
• PCB mounting. Three arrangements were used: eight case pillars; finger grips with three pillars; finger grips with side guides and one central pillar. Take care not to snag the supports when removing a board.
• PSU. Sanken supplies have a screw head that can foul the rear-panel busbar, which may need filing to clear it.

The main board has four layers. Acorn says to desolder only with vacuum equipment, or to cut the old part's leads and solder the new part to the stubs.

Capacitors

The electrolytic and tantalum positions in Acorn's component-level parts list are:

Archimedes 300 series main board: tantalum and electrolytic capacitors

Designators	Type and value	Acorn part
C40 and the eighteen positions marked 'B'	Tantalum, 4.7 µF 10 V 20 %	0610,005
C14, C20, C35, C36, C52	Tantalum, 10 µF 10 V 20 %	0610,010
C37, C38, C39	Tantalum, 100 µF 16 V 20 %	0611,101
C50	Radial aluminium electrolytic, 16 V; printed "220p."	0635,221

The C50 value is printed as "220p." An aluminium electrolytic is not made in picofarads, and Acorn's part-number suffix carries the value code (0611,101 is 100 µF), so read it as 220 µF and confirm against the part on the board. The other capacitors are ceramic, polystyrene or polyester; the 39 positions marked 'A' and 18 marked 'Z' are 33/47 nF decouplers.

The circuit diagram puts C37 across +12 V (PL5), C38 across +5 V (PL7) and C39 across −5 V (PL8), each 100 µF 16 V. C37 is the usual cause of a board that will not start with +12 V connected; see Acorn Archimedes A310#Decoupling capacitors C37, C38 and C39.

Component-level fault finding

Acorn's component-level supplement gives these checks for the 300-series board:

Archimedes 300 series checks (component level supplement section 4)

Fault	Check
No video	+5 V on both ends of L1; if L1 is open, check C14 for a short. About 3.5 V on IC17 pin 43, otherwise check R67. 24 MHz on IC17 pin 19 through PL3a and its shunt. Video data on IC17 pins 39, 40 and 41.
Dead system	Change ARM (IC43), MEMC (IC45), IOC (IC21) and VIDC (IC17) in turn. 24 MHz at LK1 position c or d, then IC15 pins 2 and 3, then crystal XL2. Clocks on IC45 pin 67 and IC17 pin 19. RST at IC45 pin 44 and IC43 pin 9 not stuck high.
Wrong or missing colours	With a full white screen, IC17 pins 39, 40 and 41 carry the same signal.
No sound	+5 V on both ends of L3; if L3 is open, check C36. −5 V on IC13 pin 11. About 3 V on IC17 pin 12. A low-level signal on IC17 pins 13–16, about 1.5 V peak to peak at Q9 and Q11. For the speaker only, a 3 V signal at IC68 pin 5.
Serial port	−5 V on IC7 pin 8 and R12–R15; clock on IC9 pins 6 and 7 (else XL1); then IC6 and IC7.
Keyboard	+5 V on SK12 pin 4 and 0 V on pin 3; IC20, R176, R180; then IOC IC21.
Clock and settings lost	About 2.8 V on IC16 pin 8 with the power off; PL11, D3, cells above 1.4 V each. Clock signal at TP1; crystal XL3.

Measured performance

Acorn quotes "typically 4 MIPS" for the 300-series board. Chris Whytehead measured a restored A310, the same board with more memory, at 4.80 MIPS on ARMSi v4.0 with an ARM2 and MEMC1a under RISC OS 2, and 12.75 MIPS with a 25 MHz ARM3 upgrade; SICK gave 4,797 and 17,326 Dhrystones per second.

Maintenance and repair

There are no separate maintenance or troubleshooting pages for the 300 series. In order of priority:

• Remove the two AA cells and check the holder and leads for leakage (Battery Refurbishment).
• Measure the four supply rails at PL5–PL8 before chasing any other fault, and suspect C37 if +12 V is pulled down.
• Check links LK2, LK6 and LK12 after any ROM change.
• Keep total podule draw under 20 W.
• For the floppy drive, see Floppy Drive Repair; for tools, Recommended Tools.

Documentation

• Archimedes 300 Series Service Manual: module-level service manual, part 0476,140, with the circuit diagrams.
• Archimedes 300 Series Service Manual Component Level Supplement: part 0476,145, with parts lists and component-level fault finding.
• Acorn Archimedes Retail Price List 1987

See also

• Acorn Archimedes A310, the same machine with 1 MB fitted
• Acorn Archimedes A440, the 4 MB model of the same 1987 range
• Acorn Archimedes A3000