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- Nov 28, 2011
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Good.One lead of the thermistor is disconnected, nothing is getting hot, X4 & X5 checked the same.
Somewhere in that circuit, the voltage is dropping when you try to draw current from the output. We need to know where that voltage drop is occurring, and fix it. I will explain a bit more below.If I may ask what does going across a capacitor, resistor or a transistor tell you?
OK, that's good. That means that the mains transformer, the bridge rectifier (D1~4) and C2 are all OK.1. Across C2: No load = 28.4V; With load = 25.3V
That part of the unit is the unregulated supply circuitry. Voltage from the mains transformer is rectified by the bridge rectifier and smoothed by C2. This forms an unregulated supply which becomes the input for the regulator.
The voltage across C2 needs to be higher than the output voltage, to give the regulator some headroom so it can keep the output voltage steady as the load current varies.
The voltage across C2 will drop when more current is drawn, because it's unregulated. Those numbers look good.
OK, that's good too. X1 is the current limit transistor. If too much current is drawn, it starts to turn OFF and reduces the voltage available to the regulator. This reduces the voltage available to the load (the thing that's connected to the power supply's output) which will generally force it to draw less current. That's how current regulation works.2. Between collector and emitter of X1 (positive probe to the emitter): No load = 25 mV; With load = 0.43V
So there's not much voltage being dropped across X1 even when the output voltage is very low, so the current limiting circuit is not the cause of the problem.
OK. This shows the operation of the regulator. When there is no load, the regulator is able to regulate as normal. There is about 11V across the regulator transistors; this corresponds to the difference between the voltage across C2 (about 28V) and the output voltage (about 15V). Some other voltage is also being dropped somewhere else, but most of this difference is being dropped by the regulator. That's correct behaviour.3. Between collector and emitter of X4 or X5 (positive probe to the emitter): No load = 10.9V; With load = 0.67V
When the load is increased and the output voltage drops, the pass transistors (X4 and X5) in the regulator conduct more strongly, to try to keep the output voltage at 15V. In this case they are conducting about as hard as they can, because there is only 0.7V across them. But they still aren't able to keep the output voltage correct.
So voltage is being dropped somewhere else in the circuit.
You need to set the output to 15V, connect the load resistor, and measure the voltage at each of the highlighted points in the circuit, from left to right, to find out where the voltage is being dropped. Measure with your multimeter's black probe to the 0V rail (the negative output terminal).
You should see about +25~27V at the positive end of C2 (which I've labelled "A"), and from your description, you will see less than +3V at the positive output terminal (which I've labelled "G"). Somewhere in between those points, a lot of voltage is being dropped. We need to find out where!