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Hi voltage from PSU
there are two main types of psu
1,one has an opto isolator that tells the psu how much voltage its giving out
ie the output voltage is compared to a reference voltage and the bulb(led) glows brighter ,
which feeds back to the smpsu control chip or circuit
Things to look for here are the opto itself (look along the isolation barrier)
or the highvalue resistors from the ht line down to the comparitor circuit
Beware some have an opto-isolator that is only used for standby switching, but are really of type 2
some have two opt-isolators one for standby switching and the other for voltage control
2,the other type of psu uses proportional feedback
a second winding (lo voltage) on the mains side of the chopper transformer
is rectified(negative) to feed back into the smpsu control chip or circuit.
The things to look for here are the small capacitor (electrolyic 1 to 15uf 25v)
and any other small electrolytic capacitors (say47uf25v) in the primary side
that is positive to earth (ie on the negative feedback winding)
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