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1. Introduction
(Concept and cautions of troubleshooting)
1) When repairing a trouble in a computer system, it is essential, in the first place, to distinguish
whether the trouble is due to software or hardware. When it is found to be a hardware trouble,
it is then required to judge whether it is due to inside of the interface device – such as, in this
system, PCI (IC28), PIA (IC32), CRTC (IC18), output latch (IC27), signal input ports (IC45,
IC46, IC40) – or due to outside of the interface device.
2) Actions of PCI, PIA, CRTC require setting by the software, and they do not operate unless set-
ting is made normally. In a trouble of computer block, display is not made since CRTC does not
operate, data reading from keyboard and data output to printer are not done because PIA does
not operate, and even control of transmission and reception by SEND-AUTO-RECEIVE switch is
impossible because PCI, output latch, and signal input ports do not operate normally.
3) One of possible troubles in the computer block is a defect of the ROMs (IC29, IC30) in which
software is written. Therefore, when the Computer does not seem to operate normally, replace
the IC29 and IC30 in the first place. (The both ICs are mounted by way of socket. Always re-
place the two ICs at the same time. If specified tool cannot be used when dismounting, remo-
ve with particular care not to bend the lead pins of ICs.)
4) In the description, "H level at TTL level" or merely "H level" refers to DC + 2.4 V to +5 V; "L le-
vel at TTL level" or merely "L level" means DC +0.8 V to 0 V.
5) It must be noted, that normal output signal may not be delivered from the IC also due to trou-
ble at the load side.
6) Since the waveform at check point of frequency is often other than 50 % in duty ratio (such as
TP-3 on main PCB), it is better to use an oscilloscope of triggered sweep system, rather than
frequency counter (However, oscilloscope of synchronized sweep or free-running sweep system
is not proper.)
7) In this system, many connectors are used in the connections of the PCBs, switches and other
modules in order to facilitate assembly, maintenance, checking and replacement. Accordingly,
troubles in these connectors are also probable, such as faulty contact and defective connection
between contact pin and vinyl coated wire within connector plug housing (in the case of solder-
less connection).
Besides, probable causes of computer troubles may also include faulty contacts of the socket in
software ROM (IC29, IC30), and take this point also into consideration when repairing.
8) The IC numbers, part numbers, and connector numbers refer to the numbers on the main PCB
(PCB No. 810529-1) unless otherwise noted.
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