Acorn Cambridge Workstation Maintenance Guide

The ACW opened for service: side trays lowered and the VDU cradle raised on its scissor stays. QTS marks the quarter-turn screws, RC the rear clips (ACW 443 Service Manual, figure 2.3.2 c)

Routine care of the Acorn Cambridge Workstation (ACW) covers opening the case safely, the power supply and its wiring, the fans, the Winchester and floppy drives, the monitor and the keyboard. The machines are now around 40 years old. The procedures and values come from Acorn's ACW 443 Service Manual (0420,001, issue 1, January 1987) unless stated otherwise; printed page numbers are cited. The ACW has no battery-backed clock or CMOS memory; its clock is reset by a hard reset or by switching off.

Safety

Warning: the integral monitor generates high voltages that can remain in its circuits for several hours after the mains is disconnected.

• Disconnect the ACW and every peripheral from the mains before lifting the upper case.
• Work on a dry surface isolated from earth, away from pipes, radiators and other earthed metal.
• Never leave the machine uncased where someone untrained can reach it.
• Before touching anything near the cradle, discharge the CRT as Acorn describes for CRT removal: unplug the EHT connector on the left of the tube without touching its centre clip, earth the centre clip to the chassis to discharge the tripler, then discharge the final anode with an insulated long-shafted screwdriver resting on the chassis. See CRT Discharge Procedure.
• Replacing the tube is an implosion hazard; Acorn says it should only be done as a bench repair, with a second person to help.

Siting and airflow

The ACW draws cooling air through an opening in its tilt-and-swivel stand. Acorn's installation notes ask for a rigid worksurface with room to tilt and swivel the unit, no cloth or mat under the base that could block the air intake, at least 150 mm of clear space above the machine, uncovered ventilation slots, and a mains socket not shared with motors or thermostatically switched equipment. Keep strong magnetic fields such as loudspeakers away from the floppy drive and the disc storage.

There are two fans. The upper fan at the rear is attached to the upper case and is unplugged when the case comes off. The lower fan sits on the plastic base assembly with the loudspeaker and the contrast and volume controls, and cools the Winchester; it runs from the red and blue pair, a nominal 12 V. The final assembly parts list includes an 80 mm fan filter assembly (item 145). After reassembly, check that the fan works; Acorn makes this the last step.

Opening the case

Removing the upper case

The upper case and its fixed screen bezel lift off as one. Two lugs at the front, hidden when the case is fitted, and two plastic clips at the rear hold it; each rear clip is covered by a metal safety clip.

1. Remove any floppy disc and set the floppy drive lever horizontal.
2. Remove the two Phillips screws holding the rear metal safety clips, and keep the clips and screws.
3. Unhook the two plastic case clips: pull the bottom of each clip away from the case, then unhook its top.
4. Pull the bottom edge of the black bezel away from the lower edge of the disc drives so that it rests on the drive fronts just below the floppy lever. Do not try to separate the bezel from the upper case; it is fixed.
5. Separate the case at the rear and raise the upper case vertically, sliding the bezel gently over the floppy lever.
6. Ease the rear fan past the rear cross member (it can be seen through the screen opening), then unplug the fan lead at the rear by squeezing the clip on its plug.
7. Keep the upper case level and lift it away.

Lowering the side trays and raising the cradle

1. Undo the two quarter-turn screws of a side tray with a large flat-bladed screwdriver; a quarter turn anticlockwise lets them spring out.
2. Lift the tray a few inches, then turn it down on its lower hinge into the horizontal position. The right-hand tray (seen from the front) holds the 32016 board; the left-hand tray holds the Winchester interface boards and the optional mouse board. The trays come out completely if their hinge pins are eased out of the chassis.
3. For the base, lower both trays, loosen the two remaining quarter-turn screws, and lift the rear of the cradle steadily until the two scissor stays unfold. Swing them past their fully open position so they lock.
4. To go further, for work on the I/O processor board, the Winchester or the floppy drive, put a block of sponge about 150 mm thick in front of the machine for the CRT to rest on, then remove the self-locking nut and nylon washers from the middle of each stay with a 6 mm spanner and let the cradle swing over.

Reassembly

1. Check that no ribbon or power cable will be trapped, then push the scissor stays and let the cradle close. Do not force it.
2. Push each lower quarter-turn screw into its locating hole and engage the slot over the mating bar before turning it; never turn a quarter-turn screw that is not correctly mated.
3. Refit the side trays vertically and lock their quarter-turn screws.
4. Lower the upper case with the fan lead outside the rear upper bracket, reconnect the fan, and ease the fan under the bracket. Do not trap the 40-way ribbon cable fixed to the outside of the right-hand tray between the top and bottom cases.
5. Tilt the case forward, ease the bezel over the floppy lever, mate the front edge, then close the rear while flexing the sides so they clip into the lower case. Clip in the rear connector panel, refit the plastic clips and the metal safety clips.
6. Switch on and check that the fan works.

Power supply and wiring

Faston polarity

The DC supply reaches the boards on Faston connectors, and the positive and 0 V posts are identical. Check every Faston for polarity before applying power after any work. The wire colours are red +5 V, black 0 V, purple −5 V and orange +12 V; the red and blue pair feeds the lower fan.

ACW supply limits (Acorn)

Rail	Limits	Where Acorn gives it
+5 V	4.9 to 5.2 V	Power supply specification, p. 6; Winchester checks, p. 117
+12 V	11.4 to 12.6 V	Same
+5 V at the I/O board	5 V ±0.1 V	I/O board power requirements, p. 36
+12 V and −5 V at the I/O board	±10 %	Same
+5 V at the 32016 board	Not less than 5.00 V at both pairs of Faston connectors	32016 fault finding, p. 100

Sources: ACW 443 Service Manual, pages as given.

Acorn says the 32016 board's supply must be checked before and after any service, and if it reads below 5.00 V the power supply must be adjusted. Doing so can lift the related +12 V slightly above 12 V; Acorn says this does not matter because that output only drives the disc drive stepper motors.

Removing the power supply

The supply is an 85 W unit, Acorn part 0831,104, which Acorn treats as a field replacement item; the manual gives no component data for it.

1. Remove the upper case.
2. Unplug the 11-wire Molex DC connector and the 2-wire Molex 240 V connector.
3. Support the supply and remove the four countersunk Phillips screws that hold it to the cradle.
4. Avoid skin contact with the heat-conducting compound between the supply and the chassis.
5. On refitting, apply new compound liberally over the whole surface of the supply where it meets the VDU cradle.

Warning: the supply is mains-powered. Treat its primary capacitors as charged until you have measured them.

Fuses

The moulded mains plug takes a 5 A BS 1362 fuse. Acorn's final assembly parts list gives the rear-panel fuse as a 3.15 A 20 x 5 mm HBC fuse, part 0815,223. The rear panel of Chris Whytehead's ACW is marked T2A/250V. Fit the rating marked on your machine and find out why a fuse blew before replacing it.

Discs

Parking the Winchester heads

Park the heads before switching off and before moving the machine. Moving or knocking a running ACW can damage the drive or lose data.

Parking commands (Acorn)

Prompt	Commands
Panos (->)	Logoff ADFS: then .Quit, then switch off
Pandora (*)	*ADFS then *BYE, then switch off
BASIC (>)	*ADFS then *BYE, then switch off
Unknown	CTRL and BREAK together to reach a prompt, then as above

Anything typed after parking, especially BREAK or *ADFS, makes the drive unpark the heads again.

Moving the machine

Keep the ACW upright, pack it in its original polystyrene if you have it, refit the cardboard insert in the floppy drive, and keep the machine's Service Card with it. The Service Card holds the defect list for the fitted Winchester.

Drive handling

Acorn treats both drives as field replacement items. The Winchester is fixed by four 6-32 UNC 3/8 inch screws; keep them apart from the others because they are non-standard. The floppy drive is fixed by four M3 Phillips screws. Mitsubishi floppy drives have an adaptor plate that moves to the replacement drive. Remove either drive through the front aperture with the cradle partly lowered, supporting the drive from the front. The Winchester's electronics and the Adaptec and host adapter boards are static-sensitive. Acorn forbids opening the drive itself. For drive cleaning and mechanical care, see Hard Drive Maintenance and Repair and Floppy Drive Repair.

Compacting the Winchester

Acorn recommends compacting the Winchester from time to time as part of normal housekeeping, and more often when it is nearly full:

1. Switch off the Tube with the rocker switch on the back of the keyboard (the Tube LED goes out).
2. Type *BASIC, then *SHADOW 1, *ADFS and *DIR :0.
3. Type *FREE and check that used and free space are not nearly equal.
4. Type *COMPACT 30 50. Do not interrupt it: a BREAK or power cut can lose or corrupt the file being moved.
5. Type *MAP, and repeat the compaction until the list of free spaces is down to between six and a dozen entries, usually two or three runs.

Back up important files to floppy or an Econet file server before you start.

Monitor

The monitor is a medium-resolution Microvitec unit. Acorn refers all servicing to Microvitec's service manual GP0019AA0 and gives its own adjustment notes for reference only.

• The only user control is contrast, the left-hand knob at the front under the Winchester position.
• HT preset VR4 is set at the factory to 124 V with a dark picture. Acorn calls this a critical safety adjustment that must not be altered.
• The other factory presets are line frequency VR218, field frequency VR307, line phase VR220, width L202 (use a non-metallic tool; it is close to the tripler and line output transformer), height VR306, field linearity VR312, field shift VR321, east-west correction VR328 and focus, on the end of the tripler.
• After any work on the monitor, check that the lead from the CRT earth to the CRT PCB is connected. Acorn warns that the monitor can be damaged without it.

The specification is 625 lines at 50 fields interlaced, line frequency 15.625 kHz, EHT about 24 kV, 0.64 mm dot pitch and automatic degaussing at switch-on.

Keyboard

The ACW keyboard is similar to the BBC Master keyboard apart from its layout, with a reset button and the Tube rocker switch on the back. Acorn's notes on keyboard faults: a single key that will not work reliably has a worn switch or a track broken by excess force, so replace the key or repair the track; a group of dead keys points first to the connectors, then the keyboard cable, then the internal cable between the rear panel and the I/O processor board.

Capacitors

The polarised capacitors on the I/O processor board, the 32016 board and the host adapter are listed, with Acorn part numbers, on the Acorn Cambridge Workstation Capacitor Guide. See also Capacitor Failure Symptoms.

Before switching on after work

1. Check that every Faston is on its correct post and the right way round.
2. Check that no ribbon cable is trapped by the cradle, stays or case.
3. Check that the CRT earth lead is connected if the monitor was disturbed.
4. Measure the rails against the table above with all connectors in place.
5. Check that the fans run.

Related pages

• Acorn Cambridge Workstation
• Acorn Cambridge Workstation Troubleshooting Guide
• Acorn Cambridge Workstation Capacitor Guide
• Acorn Cambridge Workstation Documentation
• CRT Discharge Procedure
• Recommended Tools