Mulja tv repair portal
Equipment Repairs => CRT TV forum => Topic started by: Technomaniac on June 11, 2009,22:29:36
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This Telefunken had a black screen with a brightish vertical line illuminated down the centre, meaning the horizontal output stage was not getting coupled to the deflection yoke, or the yoke itself was faulty. I knew that the horizontal output stage was functioning as otherwise there would be no light at all. Note that the problem could have been in the path to the live end of the yoke, OR in the earth end. Either one. One needs to be very careful using an oscilloscope on the high voltage pulse at the live end, as it is usually the same point as the collector of the horizontal output transistor, and the pulses there are usually greater in amplitude than the voltage rating of the oscilloscope or its probe, you usually check the earthed end first. The bottom end had a string of components in series-parallel and one of these was a capacitor 0.3mF 400V. On one end I had a large amplitude waveform, on the end closer to ground, nothing. The cap was open-circuited. When removed there was found a split in its jacket, surrounded by blackened polyester. The fault could not have been so easily found with an ohmmeter as there are other components in parallel with it, which would give misleading readings. And not so easily seen, as the components are quite close together. I found plenty of 200V caps on hand which were the same size physically, but had to fit a .33 630V to have enough room. You must leave a bit of clearance for cooling, and because of the high voltages between the components. There was a 2W resistor partially connected across it and it had discoloured from heat but we couldn't replace it as the value was unknown but it looked like it wasn't far out of tolerance and the picture was OK. The many fractured joints were resoldered before boxing it up.