Hitachi C1422T Wartungshandbuch Seite 4

SAFETY AND ISOLATION
1. Under no circumstances should any form of repair or maintenance be attempted by any person other than a
competent technician or engineer. The following precautions should be observed:
2. For purposes of servicing, the chassis should be supplied from an isolation transformer of at least 150W rating.
3. If disturbed, the original lead dressing should be restored. This is particularly important due to the 'hot coil'
nature of the chassis. Lead dressing will also have a bearing on the EMC performance of the chassis.
Components marked ! on the circuit diagram are safety approved types and have special safety related
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characteristics. Only the manufacturers replacement components should be used. Replacement with alternative
or 'up-rated' parts e.g. higher power resistors definitely does not guarantee the same level of protection and
may create a fire, electric shock or X-radiation hazard.
Components not bearing the ! mark should still be replaced with the originally fitted type and should be
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mounted in the same way.
The Isolation Barrier
The chassis is a 'hot-coil' chassis whereby the line deflection coils are NOT isolated from the mains since the PSU does
not provide B+ rail isolation. Consequently, the mains isolation barrier is more convoluted than in a conventional
chassis and a much larger portion of the chassis is non-isolated. Figure 1 shows the isolation barrier position in the
chassis.
It can be seen that both the DST (T401) and the scan coils now straddle the isolation barrier and are, as such, safety
critical components. Also, the opto-coupler (I402) is required to pass line drive from I501 on the 'cold' side to the line
drive stage on the 'hot' side. The isolation barrier therefore consists of T802, T401, I402, C804, R804, R805, the scan
coils and 6mm air gaps. To maintain barrier integrity care should be taken not to reduce any air gaps, e.g. by protruding
wires, following component replacement. Deflection yoke lead dressing is important to maintain double insulation
across the isolation barrier. The position of the deflection lead sleeving is maintained by a tie wrap. Should this be
removed for any reason, the sleeving should be slid towards the deflection yoke and tie wrapped into position.
Semiconductor Device Handling Precautions
The chassis contains devices which may be damaged by static electrical charge during handling, particularly I501, I602,
I701 and Q801. To avoid damage, soldering irons should be earthed and service engineers should ideally wear wrist
straps earthed through a 1M resistor or at least discharge themselves to an earthed point.
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