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Firstline TV Model: 1433GR

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Author: Tonyelec

Firstline TV Model: 1433GR
Dead set - 12V zener short-circuit
This is an important correction to someone else's tip and applies to all chassis with a similar electrical arrangement. If the 12V zener goes short-circuit, this is a sure sign that the HT voltage is too high - the diode is meant to do this and forms part of a cheap but effective safety device. Do NOT replace the zener with an uprated component; use the correct type and then replace the two electrolytic capacitors in the power supply. Check the value of the safety resistor connected between the LOPTX and the 22V supply (from which the 12V is derived), then replace the 3W resistor in series with the zener. This is likely to be burnt; since the value depends on the model, you may have to experiment with values between 4R7 and 15R to get a shade over 12V at the zener's cathode. The HT should now be in the region of 110V, 112V, 145V, and 155V for 14", 20-21", 25" and 28" tubes respectively. If you are unlucky, you may find some or all of the following devices need to be replaced as well (in order of liklihood): the standby switching transistors, the sound OP chip, the frame OP chip, associated safety resistors, and the line OP transistor.

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