Service Manual

6
• When inserting or removing printed circuit boards (PCBs), cable connectors, or
fuses, always turn off power to the affected portion of the equipment. After power is
removed, allow sufficient time for the power supplies to bleed down before
reinserting PCBs.
• When troubleshooting, remember that FETs and other metal-oxide semiconductor
(MOS) devices may appear defective because of leakage between traces or
component leads on the printed circuit board. Clean the printed circuit board and
recheck the MOS device before assuming it is defective.
• When replacing MOS devices, follow standard practices to avoid damage caused
by static charges and soldering.
• When removing components from PCBs (particularly ICs), use care to avoid
damaging PCB traces.
PROCEDURE TO ESTABLISH THE ABSENCE OF VOLTAGE
Follow these simple steps for establish the absence of voltage:
• Before starting work on the equipment, it shall be isolated from the main power
supply. This disconnection shall always be checked by visual inspection. Further
precautions shall be taken to ensure that the main supply cannot be restored while
work is being carried out. After the main supply has been disconnected, all other
lines such as the control, interlocking and modulation lines shall be disconnected if
they carry dangerous voltage. Moreover, the antenna or the antenna transmission
line shall be disconnected from the antenna terminal device to prevent the
introduction of dangerous voltage due to antenna pick-up. When disconnection of
the antenna or antenna transmission line is not possible, other suitable precautions
shall be taken, for example, earthing, when necessary at several places, to
establish absence of voltage. These earthing connections shall be very short
compared with the wave-length.
• Capacitors, which are connected to a circuit isolated from its supply, shall be
discharged (and have their terminals permanently short-circuited, and the casing
earthed) during the whole period of the work.
• The electrical charge retained by electrical machinery when stopped may, in certain
cases, be sufficient to cause a severe shock. This shall be taken into account when
making connections to an apparently "dead" machine. Therefore, all machinery
shall be discharged and earthed using an adequately insulated lead for this
purpose. The discharge operation shall be repeated several times.