early philips colour tv


Posted on Feb 5, 2014

A pair of thyristors in a controlled full-wave bridge configuration provide a 100Hz rectified supply. This is smoothed by an active smoothing circuit to give a DC supply of 1. 56V. The thyristors only conduct for a short period when they are triggered in the latter half of each mains half-cycle. Stabilisation of the DC output voltage against both m


early philips colour tv
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ains voltage changes and load variations is achieved by controlling the conduction angle of the thyristors. The trigger pulses are produced by a silicon controlled switch with a phase control circuit which senses the mains input and the DC output voltages. At switch-on, the phase control circuit provides slow start-up. In addition to the normal CRT beam current limiter in the signal circuits, further protection is provided by reducing the power supply output voltage if the EHT current becomes excessive. A monostable gating circuit inhibits spurious thyristor triggering by spikes which may be present on the incoming mains supply. Over-voltage protection is incorporated. Two diodes A/B of the four-diode encapsulation D4005 along with the two thyristors SCR4018/20 form a controlled full-wave (100Hz) bridge rectifier circuit. Thyristor conduction, and therefore the power into the load is controlled by changing the phase of the thyristor trigger pulses, and by this means a stabilised DC output is obtained. All four diodes of D4005 form a second full-wave bridge rectifier to provide a separate 100Hz supply for the trigger pulse generator SCS4061. The input bridge circuit is such that the receiver chassis is floating at about half-mains potential irrespective of which way round the mains connection is made. Two fuses FS1301/02 are fitted, so that, regardless of which mains input lead is live , there is protection for a short...




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