Acorn BBC Master 128, Service Manual

Page: 45 / 87

Master Series Service Manual
7.3.6 The display works in all modes except 7 and 135.
These modes work very differently from the others, using the SAA5050.
There are a number of faults which only exhibit themselves in these
modes.
1 There is a cursor which can be moved, but no text. Check the 6MHz
input to the SAA5050. It will have a slightly variable duty cycle,
certainly not 50%, but no part of the waveform should be narrower than
about 40ns. If so, check all components and signals around the 6MHz
generator IC39 and 1C25.
2 The text is at all fragmented. Check as above.
3 Not all colours are present. Check all connections to the VIDPR0C
1C42. Are the pull-up resistors R51 R54 and R56 present and the
correct values?
4 Not all characters can be obtained. Check all data bus connections
the the PBC IC21 pins 21 to 28.
5 Lines of normal size text appear cut in half. Check VSYNC at the
CRTC IC22 pin 40.
6 Rounded characters appear unstable. Check the components and signals
around the 6MHz generator 1C39 and 1C25.
7) Lines are broken up. Check all connections to the PBC
IC21. 7.4 Real time clock
7.4.1 Gives strange characters.
1 Type in the correct time and try again.
7.4.2 Does not increment.
1 Check that the variable capacitor VC1 is in the circuit. Check that
32.768 KHz is at IC11 pin 2. If it is not, check all the values in the
oscillator circuit X1 VC1 C6 C7 R17 and R24.
2 Are all connections to the system VIA correct?
3 Are all the components in the chip select/power-down circuit fitted
correctly ? See 7.1.9.
7.4.3 Loses a lot of time.
1 Is the battery connector PL8 securely fitted with all three pins
engaged? Are the battery leads properly soldered to the battery clips?
2 Is the battery flat? If it is then replace it in its holder next to
the speaker inside the computer, being careful to insert the new
battery with the same polarity as the old one.
3 Is there at least 2.6V at the clock chip with mains power switched
off?
If not the battery may still be flat, but check the FET Q4 and
surrounding components first.
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