Service manual

Action if PSU fails
There are three ways in which the power supply unit
(PSU) may fail:
• complete failure
• individual rail failure
• thermal shutdown
The complete failure of the PSU is indicated by the
following symptoms:
• no cooling fan noise
• no green LED on the front panel
• no caps lock LED response when caps lock key
depressed
First check that the fuse in the mains plug has not blown.
To do this, remove the fuse from the plug and check with
a continuity tester. If it has blown then replace only with
a fuse of the same rating (for UK machines this will be
5 Amp). If the new fuse is blown when you switch the
unit on, then return the PSU to an Acorn Authorised
Central Service Workshop (CSW).
If there is an Individual rail failure within the PSU,
various functions of the computer will fail as follows:
• 5V feeds the main PCB, floppy disc and hard disc.
• -5V feeds the backplane expansion connector on pin
A31.
• +12V feeds the video connector (SK1) on pin 12 (
when driving a SCART TV) and the backplane
expansion socket on pins B31 and B32.
Ensure that the PSU faston power connectors are
securely attached to the main PCB and that the flying
connectors are firmly attached to the drives. if this does
not correct the fault, return the PSU to an Acorn
Authorised CSW for repair.
If the PSU functions for a while before failing, it is likely
that a controlled thermal shutdown has taken place.
This is caused by either a failure of the cooling fan or a
failure of a component that is internal to the PSU.
Switch on and listen for the fan and check that the fan
aperture is not congested or blocked with any debris.
Should the fan fail to run or the PSU continue to shut
down then return the PSU to an Acorn Authorised CSW
for repair.
Action if main PCB fails
Although the main PCB is a replacement only item, there
are some checks that you can make before sending a
faulty PCB to an Acorn Authorised CSW.
Remove the PCB from the main unit, using the
procedure given in Part 2 - Disassembly and assembly
and inspect the board for any of the following:
• ROMs not in correct sockets or pins lifted from
sockets.
• ROM size and type links incorrectly set. See Appendix
F - Main PCB links, plugs & sockets for links LK10,
LK11, LK12 and LK14.
• Incorrect location and orientation of any other
socketed devices.
• DRAM size links incorrectly set. See Appendix F -
Main PCB links, plugs & sockets for links LK23 and
LK24.
• Any loose debris lying on the PCB.
If these checks do not disclose the fault, return the PCB
to an Acorn Authorised CSW for repair.
Action if upgrades fail
RAM Card
To check the RAM card, disassemble the computer
as described in Part 2 - Disassembly and assembly
and check the following:
• The RAM card plug is securely placed in the main
PCB socket.
• The links on the main PCB are set correctly - see
Appendix F - Main PCB links, plugs & sockets for
LK23 and LK24.
• The ZIP ICs are correctly installed in their sockets on
the expansion card.
• The combined amount of DRAM on the RAM card and
the memory size of the main computer is correct:
• 2MB on the main PCB and 1MB or 3MB on the
RAM card is illegal
• 1MB on the main PCB and 2MB on the RAM card
is illegal
• 4MB on the main PCB and any RAM card is illegal
If these checks do not isolate the fault, substitute the
RAM card for a known working RAM card (checking
LK23 and LK24 as you do). Depending on the outcome
of this test, send either the RAM card or the main PCB
to an Acorn Authorised CSW for repair.
Backplane
To test the backplane, you need to run a number of
expansion cards from it. (See the documentation
supplied with the individual Acorn expansion cards for
details). Once you have seen a fault occur on more
than one expansion card, disassemble the unit and
remove the backplane as described in Part 2 -
Disassembly and assembly. Check that none of the
pins in the plug housing are damaged or bent. Also
inspect the expansion card sockets for damage or
blockage.
If you can detect the fault, refit the backplane making
sure that it is installed securely into the main PCB
socket. Retest the backplane using a known working
expansion card.
If you cannot isolate the fault, substitute a known
working backplane for the existing backplane and
retest. Depending on the result of this test, return either
the backplane or the main PCB to an Acorn Authorised
CSW.
Part 1- Fault diagnosis and repair Issue 1, November 1991 1-3