You are thinking of resistance instead of current. Three 500mA fuses in parallel will make the combined maximum current 1.5A. That's obviously the opposite of what you want. There is no way to combine fuses to make the maximum current smaller.
The output of the TL331 should be a square wave bouncing between 0V and 19V. You would see the same shape as this (different voltages, but the same shape). The red line is pin 1, the green line is pin 3, and the blue line is pin 5.
Did you test the circuit for a short? (One probe on ground and one probe at PQ61 pin 1)
Testing without the transistors isn't very useful. Nothing interesting will happen if there isn't a short. You could test the new transistors with a 100-150mA fuse. But how expensive is a fuse? Is it really much less than the transistors? It might be best to just try the new transistors. I assume you are getting more than 1 of them.
That little circuit will run on 5V or 19V or anything in between. You can try it first without a new PD16 diode so at least you won't burn up a diode.
It can't be turned on until you get this circuit fixed.
The easiest way to turn it on without putting the motherboard back in the case is to just remove the power switch board from the case and use that. It's only held on by 1 screw.
By the way, that electron fuse circuit isn't a very good one. You lose a couple volts on the output. For some circuits being tested that might be okay, but I'm sure there are better designs to use.
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