Acorn BBC Master 128, Service Manual

Page: 34 / 87

Master Series Service Manual
7 Fault finding
This section goes step by step through fault finding in each section
of hardware. It should be studied in conjunction with the circuit
diagram and component location tables in the Appendix.
If any part of the machine is suspected of being faulty, the following
points should always be checked first:
1 no loose connectors and broken cables
2 no broken or shorting tracks
3 ICs plugged into their sockets correctly
4 power supply working and reaching the components concerned
5 all digital signals are either at clean TTL logic levels (greater
than 2.4V for 1, less than .5V for 0), or clean CMOS logic levels (
greater than 3V for 1, less than 1V for 0). On timed signals this must
be true for the period 150ns before phi2 on read cycles and 300ns
before phi2 on write cycles.
Hints for repair:
1 Never solder to a computer which is switched on.
2 Remove all user ROMs before starting (Remember to put them back
afterwards !)
3 Use sharp pointed meter probes to push through solder resist. This
will make finding short/open circuits more reliable.
4 Always suspect connectors.
5 If you find a recurrent fault (e.g. more than 10 machines), let us
know at ACORN so that we can include it in this manual if
appropriate.
29 Issue 1