it can be either serious mismatch or damaged part
only you can know how you handle them and what was their history , once when unpacked
- I'm not taking any special care in handling them - no gloves , wrist strips etc.
but I know when and how to bring voltages ....
pull them out and test them per Papa's pdf
only you can know how you handle them and what was their history , once when unpacked
- I'm not taking any special care in handling them - no gloves , wrist strips etc.
but I know when and how to bring voltages ....
pull them out and test them per Papa's pdf
So I finally managed to replace the MOSFETs and now the current across them are well matched.
I got one more problem now. On one of the channel, I can manage to zero the DC at output down to 0.0mV stable (the measurement limit). On the other channel, with the same multimeter, the DC fluctuates, up to about 30mV. What is going on here? I suppose it is not something that can be ignored?
I got one more problem now. On one of the channel, I can manage to zero the DC at output down to 0.0mV stable (the measurement limit). On the other channel, with the same multimeter, the DC fluctuates, up to about 30mV. What is going on here? I suppose it is not something that can be ignored?
I must agree with ZM and 6l6 . Since one channel will zero and measure well then compare and contrast the good channel to the wonkey one . Sound like a weak solder joint or bad resistor to me . Check every thing there is not that much in the F4 that will take a real long time . Please report back please this is education to all of us that are on the side line so to speak.
Thanks all. 
I realised that my "good" channel isn't that good after all, I don't know how I managed to make it stand still the last time. But the jump is under 10mV, while the other side is in tens of mV.
It could be bad connection as you guys say though. It is built without the PCB, all point to point soldering, mostly on perf boards. So there are a lot of places that can go wrong.
The MOSFETs are mounted on heatsink, with the gate and source resistor soldered on it. The connection between the heatsink and the perf boards are made with crocodile clips at the moment, this could contribute to it.
I swapped the boards on both channels. The problem appear to follow the board, as oppose to the heat sink. So that narrows it down. I also thought that may be the trim pot is bad, so I shorted the trimpot but the fluctuation remains.
I also suspect if bad C3 and C4 can cause this. Those capacitors are soldered and unsoldered a few times.
More poking to do.
I realised that my "good" channel isn't that good after all, I don't know how I managed to make it stand still the last time. But the jump is under 10mV, while the other side is in tens of mV.
It could be bad connection as you guys say though. It is built without the PCB, all point to point soldering, mostly on perf boards. So there are a lot of places that can go wrong.
The MOSFETs are mounted on heatsink, with the gate and source resistor soldered on it. The connection between the heatsink and the perf boards are made with crocodile clips at the moment, this could contribute to it.
I swapped the boards on both channels. The problem appear to follow the board, as oppose to the heat sink. So that narrows it down. I also thought that may be the trim pot is bad, so I shorted the trimpot but the fluctuation remains.
I also suspect if bad C3 and C4 can cause this. Those capacitors are soldered and unsoldered a few times.
More poking to do.