Acorn Computers Limited BBC A, Hardware Manual, Page: 46 / 47

46
iv) Conduct a detailed visual inspection of the PCB's paying particular attention to those areas which
have a high population of discrete components such as resistors and capacitors. These areas are mainly
around the UM1233 video modulator in the NORTH EAST corner of the main PCB, the DIN sockets at
the rear of the machine and the "Econet" circuitry, if fitted.
It is very common when working on the PCB for some of these resistors and capacitors to become bent
over and consequently the wires on adjacent components may short together. Take great care to ensure
that this does not happen.
It is likely that on later issue PCB's the number of discrete components will be considerably reduced.
v)
It is a good idea to remove the flux residues that will be left behind after soldering. This is best
accomplished by using a proprietary flux removing agent such as RS Components stock number 555-134
together with a fairly stiff bristle brush. A cut down artists paint brush is quite suitable.
Chapter 1 gives information on de-fluxing procedures.
vi)
Check that the heat sink on IC6 (video ULA) is still correctly positioned.
It is likely to have been disturbed whilst the work was being carried out on the board.
vii)
Visually check the entire PCB for traces of solder or fine pieces of wire etc. which may have
dropped onto it whilst it was being worked on. Use a bright light and take your time.
viii) Conduct the above visual check again.
4.4 Link Functions.
Link No. Position. Function.
1 Printer strobe select;
Only present on issue 4 boards onwards.
Selects the type of printer strobe output to pin 1 of PL9 (printer port);
SOUTH Printer strobe output taken directly from CA2 output of IC69 (VIA)
i.e. Centronics standard.
NORTH Printer strobe output with "current sink" capability.
2 Econet Non Maskable Interrupt (NMI)
This link must not be fitted with IC91 (74LS132) in place.
Note this link must fitted in order for the floppy disk interface to be
able to produce a NMI;
FITTED Disables the Econet NMI function.
REMOVED Enables the Econet NMI function.
3 Econet clock base frequency selection;
Only used on issues prior to 4, consult circuit diagram and "Econet" manual.
If the microcomputer is to be used as the network clock source then the clock frequency is set using this
group of 9 links together with link S6. The frequency can be set in the range 75 to 625 kHz. In practice
the frequency setting will depend upon the length of the network. See chapter 3 for a more detailed
explanation.
Link S3 actually sets the base clock frequency which can then be further subdivided by either 4 or 2 using
link 47.
ii)
All peripheral devices should have their own power supply source and should not draw power from the
BBC Microcomputer (via the 1MHz bus)
iii) The timing relationship of the logic signals on the 1MHz bus may alter when fully loaded. This will
happen as a result of the increased rise and fall times which occur as the lines become more capacitively
and inductively loaded. The set up times for the address bus and for FRED and JIM will have a minimum