Sunday, 1 January 2012

Philips 190S backlight fault

A faulty Philips 190S, the display would come on for a couple of seconds, then go blanks. Typical of a backlight fault.

The backlight was coming on ok, I have seen bad capacitors cause this fault but following a strip down all looked ok.

All seemed ok using an in circuit ESR meter.

I found the circuit diagram on line and this was useful for the next step of fault finding. The fact the lamps came on indicated most of the inverter circuit was ok. The panel has four lamps powered in pairs. If you unplugged one par the fault did not change at all, screen lit up briefly, if you unplugged the other pair only then the panel did not light up at all, this indicated this bank of lights was faulty.

The data sheet for the lamp driver chip showed that if all lamps do not light it shuts the circuit down. This seemed to be what was happening. There is not much in the backlight circuit after it splits to the two banks of lights. A check of the driver transformer for the half that did not light showed it open circuit, fault found !

The picture shows the faulty transformer removed, the output windings (two pins on their own) were open circuit. (confirmed low ohms on the other working half).

A quick ebay search for the board number showed that transformers were available for ~£5 the pair deliviered (the two are not interchangeable due to physical pinout). However £10 shipped would get you the whole board. It was nice to fault find to the component level but it seemed daft not just to get the whole board.

A month later the board arrived and in it went, all working ok.

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