G8 Output Transistor Balancing
A quick, reliable check on the output transistors is to measure the voltages at the two heat-sinks. Under normal conditions one voltage should be about half the other, 190V and 90V. If both collectors are at about 190V one transistor is probably short-circuit. Once you have replaced the failed transistor or if you indeed just need to re-balance the pair you will need to heed the following advice.
To balance the output pair you will need to adjust L5003 but wait, there’s a trap here waiting to cause you trouble. L5003 is at the very top of the panel and you will almost certainly find that the core is stuck fast. Any attempt to force it will result in the little metal-cap breaking off and unlike days of old, you cannot order up a new one. Therefore the following guide is a way to deal with it. I take no credit for the procedure as it was published in the Television service magazine in 1985. The procedure is as follows:-
- Unscrew the four mounting screws then unsolder the whole coil from the upper board (take care not to leave little solder bridges across the print).
- Very carefully cut around the centre of the coil former with a junior hacksaw until you just hear the blade contact the stuck ferrite core (see Fig. 1 below).
- Unscrew one half of the assembly from the other, gently – they are not connected electrically.
- Gently unscrew the protruding ferrite core from one half of the assembly, using hand pressure only.
- The threads of the former can now be cleaned and the core reinserted with a trace of silicone grease on its threads. The whole assembly can then be resoldered to the board. Use a small amount of Araldite to hold the cut halves of the coil together. After refitting the coil, turn the core while observing a stationary test pattem. At the correct core position the picture will be of minimum width and the e.h.t. will be at its correct, maximum value.
The voltages at the two transistor heat-sinks should then be at approximately 190V and 90V. Note that zero voltage at the heat-sink of the “lower” transistor T5532 can also be caused by misadjustment of L5003. The whole surgical operation on the coil takes about five minutes and is easily carried out using the simplest of tools. Always make the output transistor heat-sink voltage test when dealing with this chassis it saves so much heartache.