Acorn BBC Master 128, Service Manual

Page: 39 / 87

Master Series Service Manual
7.1.3 There is a sort of display with rolling lines and/or diagonal
stripes.
1 Check the MA and RA lines from the 6845 CRTC IC22. There are 14 MA
lines from MA0 (pin 4) to MA13 (pin 17) inclusive. There are 4 RA
lines from RA0 (pin 38) to RA3 (pin 35) inclusive (RA4 is not
connected).
These lines form the video RAM addresses in the various screen modes.
Check that they are not stuck, and that they oscillate between good
logic 0 and logic 1 voltage levels. If not then first replace the
6845, then look for shorted tracks.
Check that all are getting through to the CRTC/MUX IC31.
2 Check VSYNC pin 40 IC22. This line should pulse low every 20ms.
3 Check HSYNC pin 39 IC22. This line should pulse low every 64us.
4 Check that CSYNC (both HSYNC and VSYNC superimposed) is available at
IC25 pin 8 and also at pin 4 of SK9, the RGB connector.
5 If the RGB display alone is not working then the CSYNC polarity for
that particular monitor is probably incorrect. Alter LK5.
6 Check all video connections.
7.1.4 There is a cursor stuck at the top left corner of the screen,
but nothing else.
1 Disconnect any second or co-processor which may be fitted.
2 Check all connections to and from the keyboard and keyboard encoder
IC16. Replace IC16.
7.1.5 The screen says
AC0RN MOS
ACORN ADFS
but nothing else.
1 Disconnect any second or co-processor which may be fitted.
2 Try typing CTRL F BREAK. If this works then either connect up a disc
drive and put an ADFS disc in it or type
*configure nodir RETURN
Turn the power on and off. The machine should now work.
3 The configuration memory may have been set incorrectly or is faulty.
Turn the power off and on again whilst holding down the R key. Then
press CTRL F BREAK. All previous configuration commands will be reset
to zero.
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