Hitachi C28W440N, Service Manual

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inverted for this IC, is fed from pin63/64 of IC200. The outputs of IC600 are used for
vertical deflection.
B.C.L CIRCUIT (BEAM CURRENT LIMITER)
Beam current limiting is employed to protect the circuitry in the receiver, the CRT
and to prevent excessive X-ray radiation in fault conditions. The current drawn by the
CRT is monitored by the current drawn through the winding of the fly-back
transformer that produces the EHT for the CRT anode. The end of the winding (Pin 10)
is returned to IC200 Pin 34, here according to the beam current info, low or high,
brightness and contrast of the picture is set not to load the CRT with edge and
maximum ratings. The beam current drawn by the CRT passes through Q602 and
develops a voltage on the collector proportional to the current (V=IxR).
The voltage on the collector will vary depending on the beam current being drawn
reducing the brightness and contrast of the picture. If the voltage is sufficiently
negative (indicating very high excess beam current) the output will be reduced,
reducing the picture brightness and contrast.