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thank sarma. before i read your reply i already decided to remove the transformer(dont know what's the transformer of smps called?) to do some continuity testing only not check the resistance cause im not sure what is the usual resistance of each coil(what is the typical resistance of a coil? ) then i put it back in the board after checking for shorted pins and everything,i plug it and turn on the power botton. somethings happened, the fuse keeps bursting. i remove the transformer and turn it on again. the did not burst.

i measure the resistance of the transformer. i found out that all the resistance of the coil is equal/greater than 0 but not greater than 1 ohms in all coils.
I assumed that the transformer is shorted. is it? and if it is shorted why it creates a high pitch without bursting the fuse but after i remove it from the board and put it back without doing anything wrong the fuse start bursting?  ???

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Hi
should we now feel that the fuse is not blowing after second time soldering?

coming to smps tranformers, the parameter to be checked is inuctance of the primary and secondary coils, turns ratio (Np and Ns, and cross insulation of secondary wrt primary.
as you switch the rectified DC using a high frequency of the order of Few KHz, DC coil resistance has no relevance.  some times when operating frequency goes to few hundreds of KHZ, turns come down to 20etc, and windings will be paralleled in order to increase cureent carrying capacity.

i was asking you to test smps exclusively with an artificial load, say two 12Volt bulbs, in series to make 24 volts .or a resistor as i suggested,

mean time you can check for vertical section driver IC or final stage transistors, anf the final transistorof line output  for short.




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Hi,
i didn'tmean disconnect all sec connectios of smps tfmr. i only said that the load , i mean, the DC load on 20V output-- if there is a short in the load , this type of sound comes.

I was asking you to test smps exclusively with an artificial load, say two 12Volt bulbs, in series to make 24 volts .or a resistor as i suggested,

Sarma

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