Acorn BBC Master 128, Service Manual

Page: 50 / 87

Master Series Service Manual
7.8.3 The catalogue can be read, but the entries are corrupted.
1 Some computers have 220 pF capacitor C27 fitted from the DRQ line to
ground. Is this fitted? Is its value correct? There will be no place
on the PCB for computers that do not need this capacitor.
2 Are one or two disc drives present? If two, make sure that only one
of them has line termination resistors in.
7.8.4 The catalogue can be read, but not the data (or the directory
cannot be changed in ADFS).
1 (Issue 1 boards only). There should be a small capacitor from the
DRQ line to ground. Is this fitted? Is its value correct?
2 Are one or two disc drives present? If two, make sure that only one
of them has line termination resistors in.
7.8.5 Data can be read but it is wrong (or you get Bad FS map in ADFS).
1 Try the disc drive in another computer. If the fault persists, then
the disc drive is at fault.
2 (Issue 1 boards only). There should be a small capacitor from the
DRQ line to ground. Is this fitted? Is its value correct?
3 There may be a fault in the memory system. Check that the address
inputs to the Memory Controller (IC20 pins 21 to 28 inclusive) are
switching with good logic levels. Check that the outputs: the AA and
AT lines are switching. A good test here is to press BREAK repeatedly
and look at each line in turn.
4 Check that the AA and AT lines are correctly connected to the
address multiplexers, IC29 and IC33.
5 Use a memory test program to validate memory. This is rarely
necessary as the memory chips are highly reliable.
7.8.6 Data can be read correcty but cannot be written reliably.
1 Is the disc formatted for DFS but being used with ADFS or vice
versa? The built in formatter is only for use with DFS.
2 Check the disc drive on another computer if possible.
3 Check that data transitions are appearing on the 1770 WD and WG
pins, IC3 pins 22 and 21. If not then check all connections to the
1770 (IC3) and the 74LS174 (1C4).
4 Follow the signal path of WD to the connector.
Note: a disc drive must be connected at this stage as pull-up
resistors for the line driver outputs are within the disc drive. The
output should be normally high with low going transitions.
45 Issue 1