An internal fuse is not required to pass UL or CE. As far as keeping costs down, this is one of the more innovative designs I have seen. I do not like designing this far into the bare bones territory, I do not think it is good idea doing it, but if that is what you are into, this is about as bare bones as it gets.
Adding to Mitchekj's explanation, lets ignore the optoisolator for a moment. R5 turns Q1 on, which allows current to flow through the primary. If the current increases more than 70mA, Q2 turns on and starts turning off Q1 regulating the current. Basic two transistor current regulator.
As the voltage in the control winding increases C2/R6 turn off Q2, which causes the current to decrease, which eventually allows Q1 to turn on and the cycle continues. R7/D2/C4 form the flyback for the transformer.
D3 and C3 form a half wave rectifier from the control winding, creating some DC voltage on C3. If this voltage gets high enough, R4 will turn on Q2, turning off Q1. This control the maximum output power. That is where the "short circuit protected" claims comes from.
As the output voltage increases, the current through U1's input increases and its output transistor turns on which turns on Q2, which keeps Q1 from turning on. That is how the output voltage is regulated.
ZD1 would make sense if it were backwards. That would cause the U1 to turn on quite abruptly at a set voltage, which is what you need.
All that said, I would not recommend fixing this. Far too many parts are dead and you may still not have identified the real culprit. One of the features of bare-bones designs is that when they fail, lots and lots of things fail because by design, everything is on the edge of not working. The risk/reward of fixing simply does not make sense. Buy a used one or even a new one and be happy. The voltages here are dangerous and if you screw up you can cause some serious hurt to yourself or others or to your possessions.
---55p
Adding to Mitchekj's explanation, lets ignore the optoisolator for a moment. R5 turns Q1 on, which allows current to flow through the primary. If the current increases more than 70mA, Q2 turns on and starts turning off Q1 regulating the current. Basic two transistor current regulator.
As the voltage in the control winding increases C2/R6 turn off Q2, which causes the current to decrease, which eventually allows Q1 to turn on and the cycle continues. R7/D2/C4 form the flyback for the transformer.
D3 and C3 form a half wave rectifier from the control winding, creating some DC voltage on C3. If this voltage gets high enough, R4 will turn on Q2, turning off Q1. This control the maximum output power. That is where the "short circuit protected" claims comes from.
As the output voltage increases, the current through U1's input increases and its output transistor turns on which turns on Q2, which keeps Q1 from turning on. That is how the output voltage is regulated.
ZD1 would make sense if it were backwards. That would cause the U1 to turn on quite abruptly at a set voltage, which is what you need.
All that said, I would not recommend fixing this. Far too many parts are dead and you may still not have identified the real culprit. One of the features of bare-bones designs is that when they fail, lots and lots of things fail because by design, everything is on the edge of not working. The risk/reward of fixing simply does not make sense. Buy a used one or even a new one and be happy. The voltages here are dangerous and if you screw up you can cause some serious hurt to yourself or others or to your possessions.
---55p
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