Acorn BBC Master 128, Service Manual

Page: 43 / 87

Master Series Service Manual
7.3 Video
To debug the video, all three different types of monitor should be
connected: TV, ROB and composite.
7.3.1 The display scrolls vertically round the
screen. Does it occur on RGB, UHF and Composite
video?
1 If so, check VSYNC is correct on the CRTC IC22 pin 40.
2 Then follow the signal through the exclusive-OR gates IC25 where it
is combined with HSYNC. If the signal does not make good logic levels
at any node, then there is probably a short in that area.
Note: when VSYNC (period 20ms) is EXORed with HSYNC (period 64us), the
former will be difficult to spot within the latter. A way of getting
round this is to synchronise the oscilloscope from the VYSNC output of
the CRTC IC22 pin 40 and then examine the EXOR gate output.
Does it only occur on one of the video outputs?
1 Is the CSYNC polarity link LK5 correct for your monitor?
2 If it only occurs on composite video, check that all the resistors (
R147 to R150 inclusive) to the output transistors are correct in
accordance with the circuit diagram. Do they all have good signals
going into them?
3 If it only occurs on UHF, check the signal as it passes through the
resistor network, IC40 pins 12 to 19 inclusive through resistors R69
75 77 78 84 85 87 and 89.
4 If it only occurs on RGB, check all the signals going to the back of
the connector.
7.3.2 The display is correct vertically, but is broken into moving
diagonal lines.
Does it occur on RGB, UHF and Composite video?
1 If so, check HSYNC is correct on the CRTC IC22 pin 39.
2 Then follow the signal through the exclusive-OR gates IC25 where it
is combined with VSYNC. If the signal does not make good logic levels
at any node, then there is probably a short in that area.
Note: when HSYNC (period 64us) is EXORed with VSYNC (period 20ms), the
former will have the latter superimposed upon it. This will make the
former appear to have glitches. This should be ignored in the
measurement of HSYNC period.
38 Issue 1