Acorn BBC Master 128, Руководство по обслуживанию

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Master Series Service Manual
7.1.6 BASIC is printed on the screen, but no > prompt appears.
1 Disconnect any second or co-processor which may be fitted.
2 Check that the IRQ line on the CPU IC14 pin 4 is not stuck in one
logic state. If it is then check the address and chip select lines of
the system VIA IC8.
7.1.7 The BASIC prompt > appears, but the keyboard does not respond.
1 Check the address and ship select lines of the system VIA IC8.
2 Check all connections to the keyboard encoder IC16.
3 If all else fails then try a different keyboard. If this works, look
for broken leads and tracks on the original one.
4 Check that the IRQ line IC14 pin 4 is not stuck low or high. The
memory controller IC20 and CMOS clock/RAM IC11 both have direct
connections to the IRQ line. The latter part has a link LK4 which can
be used to isolate it from IRQ.
Note: the system VIA IC8 generates an interrupt every 100th of a
second. If this interrupt is not cleared by the operating system each
time it occurs the IRQ line will appear to be stuck low. This could
occur if some other installed software is faulty or if an address or
data line to the VIA is faulty.
7.1.8 Not all keys work on the keyboard.
One of two possible keyboard circuits may be fitted:
a) A 40 pin keyboard encoder IC16 on the main circuit board and two 15
way connectors (possibly with an intermediate buffer board) joined to
the keyboard with ribbon cables.
1 If more than one key is affected, check for a faulty connection from
the keyboard encoder IC16 to the keyboard.
2 If more than one key is affected, check for a broken track in the
middle of a row or column of the keyboard matrix.
3 If one key only is affected, check for a broken track at the end of
a row or column in the keyboard matrix.
4 If one key only is affected then check the keyswitch itself.
Note: the beginning of a row or column is the end of a PCB track which
is immediately connected to the encoder circuitry. The end of a row or
column is the other end of the PCB track. Because of the layout, the
beginning and end of a track are not necessarily the parts of the
track which are visually nearer or farther from the encoder circuitry.
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