Acorn computer A3000 Benutzerhandbuch

Servicing Module

Archimedes

A3000

Servicing

Contents

This module describes module-level servicing on Archimedes and A3000 computers. It

covers routine maintenance of the computer and its peripherals, and fault-finding

techniques for when the machine breaks down.

Opening the Machines

In order to carry out anything but the simplest of maintenance and repair work, it is

necessary to remove the lids of machines. The two styles of machine have different

methods which need to be used to gain entry. These are covered in the relevant service

manuals,

but the essential information is also given here, along with a few things which

aren't covered in

the manuals.

Archimedes

The Archimedes is generally the easiest of the two machines to break into. Its case is

comprised of two parts: a tray on which the main PCB and keyboard sit, and a lid. The lid slides off

the tray to the rear.

In order to free the lid, undo the three screws along the top at the back of the machine, and the two

screws (found down small holes) at the front of the machine on the left and right side panels. The

theory goes that the lid will now slide off backwards.

This rarely happens. Instead, the lid will slide back half an inch and then the sides will jam. Either

push downwards firmly in the centre of the lid towards the front of the machine (this causes the

sides to splay outwards) and then push back, or lever the sides out directly and push back.

A3000

The A3000 also comes in two pieces, a lid and a tray. Here, the lid should just lift off the

tray

beneath. Begin by removing the two metal clamps which secure the prongs from the

lid at the

back of the machine, and undo the screw in the centre of the bottom of the

machine. Leave

the four which secure the floppy drive alone!

Push the prongs from the lid at the back of the machine inwards and upwards, and the back of the

lid should become free. The front, however, will still be held in by three more bits of plastic under

the front of the machine. Push these in turn towards the rear, lifting the front

of the lid up as you

go. Be careful that the keyboard doesn't drop out as you remove the

last one.

Routine Maintenance

Routine maintenance for Archimedes/A3000 machines is limited to a few simple operations,

which

are detailed in the service manuals for the appropriate machines. Again, the

essential

information is covered here.

Batteries

The Archimedes machines contain a pair of alkaline AA dry cells which are used to provide

power to

the real-time clock chip when the computer is turned off. These should be changed every

year, although they may last much longer than a year, to help prevent

damage to the

machine from old batteries leaking.

The batteries are found inside the machine on the left at the front. It is quite safe to change them

with the power turned on; changing them with the power off is also possible as a capacitor on the

board can supply the RTC chip for at least 15 minutes without the batteries being present.

The A3000 is fitted with a nickel-cadmium (NiCad) battery which is recharged when the machine is

turned on and hence doesn't need replacing.

Mice

As the mice move, they pick up dust from whatever surface they are moving on and ingest it. For

reliable mouse operation, they should be cleaned at the same time as the batteries are changed,

and more frequently if subjected to heavy use.

Cleaning the mouse is a simple task, involving alcohol and a cotton-wool bud. Remove the ball,

which is secured by a screw in older mice and by a twist ring in newer ones, and clean

it. Clean the

rollers inside the mouse on which the ball sits. If the mouse is really dirty

inside, then remove

its lid (undo the four screws at the corners underneath) and clean thoroughly.

Fan Filters

Archimedes machines are fitted with fans, whose filters should be changed yearly to

prevent their clogging up with dust, which blocks the airflow through the case.

Module Level Servicing

Module level servicing involves fixing computers by swapping modules (PCBs, PSUs, etc.)

rather than replacing defective components. This has a number of benefits for the

technician:

The fault-finding process is very much simpler and quicker than at component

level. The Archimedes machine, for instance, consists of seven modules, as

opposed to several hundred components.

The equipment required to service the machine is much simpler and cheaper.

There is no need to have an oscilloscope; for most jobs, a screwdriver and a

voltmeter are all that is required.

Tumround time can be significantly reduced. Keeping several hundred spares

in stock (as may be required for component level servicing) is usually

impractical and hence spares must be ordered for each machine which is to be

fixed. It is much more practical to keep a stock of modules, meaning that

machines can be turned around very much faster.

For some items, the cost of a new unit is sufficiently low compared to the cost of repairing

the unit that a defective part is simply thrown away. For others, such as an Archimedes

PCB with a cost (at the time of writing) of around £420, the defective part is worth repairing.

Acorn has a system of 'central repair' for such items, which takes in defective parts and

returns recoditioned ones.

For a complete treatment of procedures and conditions for servicing machines, refer to the

forthcoming Acorn document which will cover these issues (due in Feb. 91.)

The Modules

For the purposes of module level servicing, the machine is split into several modules.

Servicing a faulty machine simply involves determining which module is responsible for the

fault and exchanging it. A list of the modules follows, with each being treated in more detail

on subsequent pages.

Power supply: a component of all machines, with two variants: an enclosed

version for Archimedes which generates +/-5 and +12V, and an open version

for A3000 which generates +5V only.

Main PCB: four major variants: A3000, A305/310, A440, A4x0/1

Floppy disk drive: 3.5" 800k (standard density). Two variants: a Sony drive

fitted to A305/310 and A440 machines and a Citizen drive fitted to A3000 and

A4x0/1.

Keyboard: a 103-key IBM AT-style keyboard. Two variants: a membrane

keyboard for A3000 machines and a conductive rubber pad keyboard for

Archimedes.

Mouse: a 3-button mouse. Two variants: one for A305/310 and A440

machines, and another for A4x0/1 and A3000 machines

Monitor: a rebadged Philips CM8832 colour monitor is the standard colour

monitor supplied with Archimedes and A3000 systems. The machines can,

however, connect to a wide variety of different types of monitor.

Monitor lead: a 9-pin D to SCART lead. Two versions: an old, short, stiff lead

supplied with A305/A310 and A440 machines, and a new, longer, flexible lead

supplied with A3000 and A4x0/1 machines.

This list is not exhaustive - such items as Econet modules, backplanes, expansion cards

and hard disk drives may also be found inside machines. However, the list of modules

above provides a reasonable 'core' of modules which any machine may be expected to

have.

The Power Supply

The power supply supplies regulated, low voltage DC to the circuitry in the machines, and

hence is

an important part. It is also responsible for dealing sensibly with faults, such as short circuits,

so that machines do not catch fire every time a paper clib gets dropped inside. For this

reason, the power supply is not to be serviced in any way other than by

direct replacement.

This restriction also applies to component level service centres.

Power supplies have proven reliable during the machines' lifetimes so far. Whether this

state

will continue into the future or not remains unclear.

The power supplies are all switch-mode units. Rather than having a bulky and heavy mains

transformer, large filtering capacitors and an inefficient series regulator, they rectify mains directly,

and chop it at supersonic frequencies. In this way, the size of transformer and filter

capacitor

required can be dramatically reduced. In addition, by varying the mark-space

ratio of the

chopped mains, the output may be regulated without any need for a series regulator. Typical

efficiencies of 85% may be achieved, as opposed to 55-65% for a linear PSU.

Archimedes PSU

The Archimedes PSU is fully enclosed in a perforated metal case. It delivers +/-5 and +12V to the

system, thus being able directly to power a fan and a hard disk drive. It has an

IEC

mains outlet on

the back, which may be used to power a monitor. Note that this outlet is

not switched, and

therefore remains live unless the mains supply to the machine is

switched off.

The PSU is fixed to the bottom of the case by four screws. Removal of these will free the power

supply, but it is usually necessary to remove either the back panel of the machine or its fan before

the power supply will come free.

A3000 PSU

The A3000 PSU delivers a single rail of +5V. It is enclosed in a cardboard cover. It has no mains

outlet. To remove, free the cardboard cover at the PCB side of the PSU to reveal the

PSU itself.

There is a single fixing, in the centre of the PSU, which should be removed.

The PSU will just

lift out.

The Main PCB

The main PCB is the heart of the system. It carries processor, ROM, RAM, display

hardware, sound hardware and all of the peripheral interface circuitry which makes up the

computer. Each type of machine has its own PCB. There are some common faults and undesirable

features of the circuitry of the PCBs; these will be dealt with in the next few pages.

A305/A310

These systems were supplied with no hard disk controller fitted, and either 0.5 or 1 MB of

RAM on

the circuit board. 0.5MB systems are upgradable to 1 MB systems simply by

plugging in

another 0.5 MB of RAM as 16x4464 dynamic RAM chips into the sockets on the board. The machine

is not designed to have a hard disk controller added by soldering in the relevant chips; instead, a

suitable interface card should be used.

A440

These systems have a hard disk controller fitted, and the memory area on the PCB is

reworked to give 4MB with 32 1x1Mbit DRAM chips fitted. In addition, the clock circuitry is

somewhat different to that on the A305/A310 board, allowing the machine to drive 64kHz

monochrome workstation monitors.

A3000

The A3000 PCB is the first one from Acorn to use SMT (surface mount technology.) The four

custom chips are soldered directly to the top side of the board, rather than being

socketed in

conventional PTH (pin through hole) sockets. All of the other components on the board, with the

exception of some passives (resistors and capacitors) are mounted

conventionally. There is no

HDC fitted, and only 1 MB of RAM, although there is an

expansion connector which allows this

to be expanded to 4MB.

A4x0/1

These machines have a reworked version of the old A440 PCB, with the primary change being

made to the memory system, which now accepts 1,2 or 4MB in 4x256k ZIP

packages.

Common PCB problems

There are a number of problems with the circuitry of the machines which can lead to

problems, especially in the peripheral area. These are detailed below, with details of

causes

and possible remedies.

Serial Port

There are a number of problems with the serial interface circuitry in the machines.

Baud rates. Many machines nowadays (including cheap Taiwanese PC

clones) are capable of running their serial ports at 38.4kbaud and even

115kbaud. The Archimedes/A3000 is limited to 19,200 baud, which proves to be a

limiting factor if the machine is used with the latest types of 9,600 baud modems with

data compression.

Split baud rates. For many applications, the serial port is run with transmit and

receive baud rates set identically. If they are to be different, then only the transmit

baud rate will be set accurately. This is not a problem, provided that

the receive

rate is low enough that the small error in its frequency is

insignificant. As a

guide, 1200 baud will work fine, but 9600 will not.

Line levels (not A3000.) The serial port on Archimedes drives the line with +/5V

levels, to the RS423 specification. This will drive most RS232 peripherals, although

the RS232 spec. calls for levels of +/-9V or greater. Some RS232 peripherals,

however, will not work correctly with the Archimedes' levels.

Failure to comply to standard (A305/A310/A440 only.) The early machines

were

fitted with a serial port chip which failed to implement the RS423 protocol

correctly,

which caused data to be corrupted. RISC OS contains a software

patch which will

cure the problem; for those with Arthur 1.2, the RS423 Line

Driver patch is a

module available from Acorn. This is not a problem with

A3000 and A4x0/1

machines.

Buffer failure. There have been reports that the buffers which buffer RS423 in and

out of the machine are prone to failure.

Common PCB Problems (II)

Parallel Port

The parallel port is a Centronics-compatible unidirectional parallel printer interface. It is designed as

printer interface, but has been put to other uses, such as providing 8 output

lines from the

machine and as a connection for dongles (hardware keys for software

protection.)

Failure to drive Integrex Colourjet printers (A305/A310/A440 only.) The timing on the

parallel port is slightly wrong for these printers. There is an FCO for the cure, which

involves fitting one extra chip and can be carried out by any component level service

centre.

Failure to drive early Qume Crystalprint Publishers. A fault in the Qume's

firmware is to blame; Qume supply an alternative ROM which should be fitted in the

printer.

Analogue RGB

The Analogue RGB output is susceptible to static damage (A305/A310/A440 only.) This results

in the early demise of VIDC, and is a component level repair. A3000 and A4x0/1

have static

protection diodes added which cure the problem.

Sound

A305/A310/A440 machines may produce excess background noise through their internal

speaker.

There is an FCO describing the fix for this, which involves adding a single

capacitor to

increase the power supply rejection of the power amplifier chip used.

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