Acorn BBC Master 128, Service Manual

Page: 37 / 87

Master Series Service Manual
7.1 Switch on
If the suspect microcomputer has a second or co-processor fitted then
disconnect it. Connect the suspect microcomputer to a UHF TV and an
RGB monitor (connecting both a TV and a monitor will show up any fault
in the connecting leads, or the polarity of CSYNC). Connect the mains
supply and switch on both the monitors and the computer. One of the
following will happen.
7.1.1 There is noise on the monitor screens (no signal from computer).
There is no power-on beep sound and the keyboard power indicator does
not light.
Results: the power supply is dead.
Follow the sequence of checks shown below.
1 Check the power supply (see 7.2). Try replacing the power supply
anyway as the fuse may have blown.
7.1.2 The power supply is working (the keyboard indicator is lit), but
nothing else happens.
Results: There is a board fault.
Follow the sequence of checks shown below.
1 Check that all ICs have power and ground on the correct pins at the
correct voltage. Make good any dry joints.
2 Feel whether any of the ICs are hot. Check for shorted PCB tracks.
Watch for shorts from -5V to the IO controller causing it to latch-up.
Clear shorted tracks and then replace any affected ICs.
Note: ICs which get abnormally hot due to a fault, but which work
after the fault has been cured, should still be replaced. Their long
term reliablility may well be impaired.
3 Check the 16MHz oscillator output IC43 pin 2, and check that it is
reaching the VIDPROC IC42 pin 8. The signal should be reasonably
clean, 4V peak to peak. If not then check the crystal controlled
oscillator circuit formed by half of IC43 and X2.
Warning: if the system 16MHz clock does not work, the DRAM RAS and CAS
lines could be held low. This can make the ICs heat up causing
permanent damage. If the oscillator is not working, or the clock is
not reaching the VIDPROC, try to find the fault and repair it as
quickly as possible. Do not leave the computer switched on for more
than 30 seconds at a time.
4 Check the 1 2 4 and 8 MHz signals from the VIDPROC, IC42 pins 4 5 6
and 7 respectively. If any is missing, or is not a clean square wave
with MI logic levels then replace the VIDPROC 1C42.
5 Check that there is a clean 2 MHz signal on the CPU clock input,
IC14 pin 37. If it is missing or incorrect then check for broken or
shorting track.
6 Check that all data and address bus lines are clear of shorts
between themselves, +5V and ground.
32 Issue 1