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

7. A resistor (R171) was connected in series with the EOC line of the
ADC (1C73) in order to prevent momentary output contention which may
occur during power-up.
8. A 4k7 resistor (R173) was connected between pin 7 of IC89 and +5v,
as the output is open collector.
9. Resistors R104, R125, R142, R149, and R153 which were in series with
the ROM chip select lines were replaced by copper links formed on the
component side of the PCB. (S34 to 38)
10. A 10k resistor (R172) was introduced between the analogue input on
the 1MHz bus and 0 volts in order to reduce the input impedance and
hence improve the signal to noise ratio. (See section 6.4)
11. Link S39 was added in order to connect the 470pF capacitor, C58
from the base of Q7 to the emitter of Q9.
12. A 220nF capacitor (C59) was added in series with R90 in order to
AC couple the log amplifier on the cassette interface.
13. A number of changes were made to the Econet control lines in order
to speed up software control. For details of how to bring earlier issue
boards up to the current issue, see the section on upgrading the Econet
system (section 4, modification E).
14. Provision was made for mounting a right-angled phono socket as an
alternative to the free-wired BNC socket normally used for video
output.
15. A 200k potentiometer (VR2) was added in parallel with R32 in order
to adjust the operating frequency of IC99 for the appropriate pitch of
the speech output.
16. At some stage between issues 3 and 4, C34, the cassette output
coupling capacitor was increased from 47nF to 220nF.
5.3.3 Changes from issue 4 to issue 7.
(Issues 5 and 6 never went into production).
1. R114 changed to its present value of 18 ohms 1W, and C42 changed to
33pF.
2. R75 went to its final value of 82k. (The reason for the change in
value of R75 was to control the data carrier detect delay time to avoid
loosing the first bit of the first byte of the first block when
recording data.)
3. The diodes and resistors on the ROM select circuitry which can be
used to produce 1MHz operation were omitted.
4. Links S18 and S19 are made with tinned copper wire.
5. When the video processor ULA was replaced by the first set of
custom-designed ICs, a modification was necessary. S26 was left
unconnected and a wire link was made from the TTX-VDU line (pin 17 of
IC2) to the invert input of the videoprocessor (pin 27 of IC6). Later
versions of the custom IC made this modification unnecessary.
A11 other changes from issue 4 to issue 7 were cosmetic changes
including some thickening up of the tracks to improve the power supply
distribution.
37