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Reason for and modifying this circuitry

 
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JamesQB



Joined: 23 Aug 2005
Posts: 26

PostPosted: Thu Mar 09, 2006 8:28 pm    Post subject: Reason for and modifying this circuitry Reply with quote

Hi

I have a TV in that has an area of circuitry on the secondary side of the chopper for low mains voltage detection. According to the service manual, it should trigger when mains is below 195V.

I haven't seen this in any TV before and don't quite understand why it's needed. I also think that this may be the cause of very intermittent start-up problems with the set as the mains does dip around here on occasion, and although I have changed all the parts and checked the entire supply, I reckon that maybe it's triggering at voltages above 195V so it's being too fussy. The way the set behaves ties in with the circuit manual fault-finding tree, putting this protection circuit as the main culprit.

I must admit that I can't understand how the mains being low could trigger this circuit. The -PDandFAIL line is active low, so the transistor must conduct in order to activate the fail line. It's connected to B6, a chopper supply secondary voltage rail. Not only would I like to understand how it works, but also what I would need to modify in order to decrease at what mains voltage it fails, perhaps to 180V or so.



If anyone could help please, I'd very much appreciate it.

Thanks,
James

*Edit* Actually, looking at it I think perhaps that when the primary is receiving voltage and the EM field is building, it should induce a reverse-flow in the secondary windings, shouldn't it? This reverse flow will be proportional to the strength of the EM field which in turn will depend upon the mains voltage being rectified and smoothed and applied to it via the chopper MOSFET. As the current reverse-flows through the winding, it should make the base of Q855 go more negative via reverse-connected diode D870, leaving it with no bias and hence not conducting, so not dragging the fail line low and activating the protection. Is that right?

Now, if that's the case then it makes sense how low mains voltage could activate the protection. Interestingly, I found that the mains smoothing capacitor rated at 220uF 385V was low at 198uF and I have now replaced it with a 400V type reading 219uF. Could the 10% lower capacitance be enough to cause the mains-low protection to activate even though the mains hasn't in fact dropped to the 195V threshold? This might explain why the set hasn't always had the problem even at the same property, and why the fault hasn't got better or worse in over a year perhaps?

When the set is switched on from standby, that should be when maximum current is drawn - more than during normal running after start-up - and the service manual does say that the -PDandFAIL line is read by the microcontroller before allowing start-up of the set and during running, so maybe the low capacitance would have more effect, as the set never went off while running, it only refused to start-up intermittently after being put into standby?
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R_ay
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PostPosted: Tue Mar 14, 2006 12:43 pm    Post subject: Reply with quote

James,
Looking at the protection ckt in question, you've figured out it's operation. It is based on a summation potential. I do see it quite frequently on a lot of newer equipment especially Audio. In other words, the summation of plus 3.3V & negative potential generated at the anode of D870 add up to keep the base of Q855 at below the .6~.7 V trigger point. Within the design parameters, the potential dip will trigger Q855. Technically speaking, you can change the trigger points by changing the bias circuitry around Q855. However are you sure it is the culprit? Need to monitor that line. There are variety of other defects that could cause similiar problem in any TV. As a rule of thumb, electrolytics tend to be the suspect but, 10~15 percent tolerance is usually accepted unless tight regulation is called for.

Ray
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