Yesterday my monitor momentarily flashed on and then only the power LED functioned as normal ie green with signal and amber without. No display. It was quickly identified that the backlighing had failed by shining a bright light on the screen to faintly identify a video image.
Dismantling the unit and testing revealed a short circuit bi polar 2SC5707 in one of the two backlight inverters. Why did it fail? Checking around the circuit no other problem could be found, no shorts and no failed caps.
Tracing the circuit of the inverter revealed a simple push-pull free running switch inverter design. Each inverter switches two transformers. The attached diagram is simplification intended to reveal a strange omission. The diodes D1 & D2 are usually included in this type of design to ensure that the feedback winding connected to the bases of the two transistors has a complete circuit and they also protect the transistor emitter base from excess reverse voltage. However in the monitor inverter these are missing!!!! Meaning that the feedback /drive current can only flow through reversed bias junctions.
I have double checked the circuit and cannot believe that this is the case. Flowing drive current through the reverse voltage zener of th emitter base junction is definitely not a good feature, not recommened by manufacturers/designers. This would reduce the transistor life with higher EB junction temp.
I presume this is done to save two diodes worth very little ie 1N4148 at a few cents.
I would be interested in any comments.
Also it can be difficult to source the 2SC5707 transistor. Currently they are available cheaply from eBay.
I will replace the pair of transistors in the failed inverter and speculate whether to insert diodes D1 & D2, this would increase drive current, perhaps a single resistor should be added to limit base current. I will post further.
Dismantling the unit and testing revealed a short circuit bi polar 2SC5707 in one of the two backlight inverters. Why did it fail? Checking around the circuit no other problem could be found, no shorts and no failed caps.
Tracing the circuit of the inverter revealed a simple push-pull free running switch inverter design. Each inverter switches two transformers. The attached diagram is simplification intended to reveal a strange omission. The diodes D1 & D2 are usually included in this type of design to ensure that the feedback winding connected to the bases of the two transistors has a complete circuit and they also protect the transistor emitter base from excess reverse voltage. However in the monitor inverter these are missing!!!! Meaning that the feedback /drive current can only flow through reversed bias junctions.
I have double checked the circuit and cannot believe that this is the case. Flowing drive current through the reverse voltage zener of th emitter base junction is definitely not a good feature, not recommened by manufacturers/designers. This would reduce the transistor life with higher EB junction temp.
I presume this is done to save two diodes worth very little ie 1N4148 at a few cents.
I would be interested in any comments.
Also it can be difficult to source the 2SC5707 transistor. Currently they are available cheaply from eBay.
I will replace the pair of transistors in the failed inverter and speculate whether to insert diodes D1 & D2, this would increase drive current, perhaps a single resistor should be added to limit base current. I will post further.
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