Acorn BBC Master 128, Руководство по обслуживанию

Страница: 41 / 87

Master Series Service Manual
b) A 17 way connector rising to a keyboard with the encoder circuitry
on it.
1 All of 1 to 4 in the previous section.
2 Check for a faulty connection from the main board to the keyboard
which may cause the entire keyboard to be affected.
7.1.9 All the keys are working, but CTRL BREAK behaves as
BREAK. (CTRL BREAK should invoke the configuration settings.
)
1 Check that all power-down components around the CM0S clock/RAM chip
IC11 are present and have the correct value.
On some early computers, C11 needs to be 100uF instead of 10uF, D8
should be changed to IN4001, and Q3 should have a 1k8 resistor between
its base and emitter.
2 Check that the chip enable pin 13 of IC11 is normally high with just
occasional excursions low. If not then check the system VIA IC8 for
the RTC chip select pulses which should be on pin 16.
3 Real time clock chip control lines should be in the following state
at all times when not being accessed by the user.
CE IC11 pin 13 high
AS IC11 pin 14 low
DS IC11 pin 17 low
4 Check that R/W IC11 pin 15 wiggles when alternate reads and writes
are performed.
7.1.10 The computer powers up in terminal mode, and *configure will
not change it.
1 Check 1 and 2 in the section above.
2 Check all connections to the clock/RAM chip IC11 for open circuits
or short circuits.
7.1.11 The computer works, but random dots or characters appear on the
screen.
1 Type a short program or piece of text into the machine - about two
lines in any screen mode are adequate. Leave it for a few minutes and
then see if the screen has been corrupted. If it has then there is a
memory system fault. If it hasn't then enter a longer piece of program
or text - enough to fill a screen and repeat the test.
Any fault showing itself will be due to a memory system failure. These
are rarely due to the RAM chips themselves as these are usually very
reliable. Check the following very carefully.
36 Issue 1