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

* Certain other faults on the VDU display associated' with the UHF
output can be cured by adding extra decoupling to the supply to the
modulator to improve its stability. A 10 ohm series resistor with a 4.7
uF capacitor to earth is usually sufficient.
* It has been noticed that problems can occur with some of the 74LS74
ICs, especially from certain manufacturers. If there is any problem
therefore with the cleaness of the clock pulses applied to the 6502A,
or problems with the PAL encoder circuit, or more likely with the RAS
signal, it would be worth checking the output of these ICs to see if
they are driving to the full TTL levels.
* Unfortunately, a number of problems also arise when people have
tried to do their own upgrades and have made mistakes or used bad
soldering techniques. In particular, a number of people seem to get the
printer upgrade on the issue 2 board wrong and therefore this should be
checked very carefully (see section 4). Also it is worth checking the
soldering very carefully, particularly around the area of the IDC
connectors. This is because it is easy to get solder bridges over the
tracks which are fed in between the pins on this connector. The worst
place seems to be in the area of the tube connector.
* If there are problems associated with the PAL or cassette circuitry,
it is worth checking very carefully whether the correct resistor values
have been used. Since there are so many resistors so closely packed
together, it is very easy to get resistors in the wrong places. To
check this, it is best to remove the circuit board from the case
entirely and use a strong light source in order to view the resistor's
colour codes carefully. This is well worth doing, as it can save a lot
of time looking for faults which are basically simple but which would
be difficult to diagnose.
* Our service centres tell us that there is a series of rather obscure
faults which they have detected which is associated with timing
problems with the RAM. One symptom is twinkling characters in mode 7
but not in the other modes of graphics, and another is that when
playing Acornsoft's "Defender" (not the later version of "Planetoids")
some very strange effects occur as the game continues. Also there is a
program of 3D Noughts and Crosses from Beebug which produces a strange
fault, stopping inexplicably at one particular line and giving a No
Room error. These faults, being related to relative timing, can
sometimes be cured by changing the 6502 processor, or the 74LS245 (
IC14) and are more often noticed where the RAM in CAS 1 is a different
type from the RAM in CAS 0, when an A to B upgrade has been done. In
particular it seems to be that the Fujitsu RAMs do not mix well with
the Mostek or Hitachi RAMs.
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