BBC Microcomputer Service Manual Oct 1985 Section 1 BBC Micro ...

6.4 Most Common Faults
In the following section, we shall try to give some ideas which have
been colllected from various people who have been doing a good deal of
servicing and repair work on BBC Microcomputers. There will be no
particular order to the comments but reading through all of them should
give some useful ideas about faults which are likely to occur.
* A common reason for getting sound-on-vision effects is that the
power leads have become intermittent. To check whether they are giving
a problem, a quick flick with one finger is what the experts recommend.
If this causes the display to flicker then switch off the unit, remove
the power leads, pull back the insulating sleeves, solder along the
area where the wire is crimped by the terminal, and replace them, being
careful not to exchange the 0 and +5 volt connectors.
* It is possible for the ROM sockets to develop bad contacts. This is
sometimes caused by heavy-handed use of the "butterfly" carrier boards
which were used at various stages to put two 8K eproms into one single
16K socket. The only solution for this is to replace the ROM socket
entirely, and you would be well advised to use the best quality socket
available. This is not an easy task unless you are experienced in the
use of desoldering equipment.
* The most common reason for the cassette system becoming inoperative
is a damaged LM324. Another problem is with the clock input to the
serial processor and it is worth checking that this is the correct
frequency (ie 1.23 MHz = 812 ns periodic time). Another problem which
sometimes occurs is that the value of R75 needs to be changed. The
optimum value is different for different issues of the serial processor
ULA because of the variation in impedence of pin 15 to earth. This
affects the timing between receiving a high tone lead-in and asserting
the data-carrier detect on the ACIA. For ICs numbered 2C199E and
2C199E-3, R75 should be 100x or 56k as required for consistent loading
of data. For 2C199E-7, R75 should be 82k.
* One simple problem, but unfortunately fairly common, is that the
pins of ICs tend to get bent as they are pushed into the sockets. If
you have isolated the fault to a particular area, then this is something
to look out for. It is also not unknown for the IC socket itself to
have a pin bent underneath. This may not have been noticed by quality
control if the IC socket was empty at the time of production eg speech
IC socket.
* If you wish to get two BBC Microcomputers to send programs to each
other on the cassette system, then it is possible to do so by a direct
connection, provided a 1.5k resistor is connected between the signal
line and ground. With the later issue boards, that have the 220nF
output capacitor on the cassette system, a smaller value may be
necessary. The resistor is necessary to adjust the relative phase of
the two tones of the cassette signal.
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