May be because it was 5am, the "good" channel actually has some DC, so I guess matching resistance doesn't work.
The amp began to smell, the "hot" MOSFET got really hot. And after a turning on the amp a few times the fuse blew.
The amp began to smell, the "hot" MOSFET got really hot. And after a turning on the amp a few times the fuse blew.
voltage across each Rs can't lie ;
if all resistors are of same value , and all mosfets are having same biasing voltage at gates , for wrong ( excessive , in this case ) current - you can blame only mosfet
if all resistors are of same value , and all mosfets are having same biasing voltage at gates , for wrong ( excessive , in this case ) current - you can blame only mosfet
Do you think it is a simple case of device mismatch? Could static or heat to the device cause this?
Thanks. 🙂
Thanks. 🙂
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
It's humid here, statics are rare. They are mounted on heat sink before I solder them. I also ground them when I solder them. I think it is quite unlikely that I destroyed them.
I'll pull them out. Thanks. 🙂
I'll pull them out. Thanks. 🙂
If you had static damage you would have a lot more drama than this.
Have you considered that the offset could be coming from the Jfets?
😎
Have you considered that the offset could be coming from the Jfets?
😎
What is going on here? I assume "Pa" thought it is some other circuit? My electronic awareness is very limited. 😀
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?
in what time frame it "fluctuates" ?
however - check all connections ( reflow solder joints if needed ) , then - if problem is still there - replace TL431 ........ then few surrounding caps
however - check all connections ( reflow solder joints if needed ) , then - if problem is still there - replace TL431 ........ then few surrounding caps
Fluctuate as in the numbers keep jumping and won't stay still. Over time, I think the value also drifts.
The voltage between across the TL431 seems to be stable, the current across the MOSFETs are also stable.
I will check the solder joins.
The voltage between across the TL431 seems to be stable, the current across the MOSFETs are also stable.
I will check the solder joins.
I agree with ZM -- check your connections and soldering.
I had a problem similar to what you describe and it was nothing more than a bad solder joint on a source resistor.
I had a problem similar to what you describe and it was nothing more than a bad solder joint on a source resistor.
repetitio est mater studiorum
especially when soldering is in question - nothing can replace mileage
especially when soldering is in question - nothing can replace mileage
I agree completely! Go slowly, be careful, check your work.
There will still be some mistakes, we're only human, after all...
There will still be some mistakes, we're only human, after all...
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. 🙂
The selling point of pc boards from the first was repeatability . It may be easier to buy the boards and start over re-using the fets . For 22 dollars and shipping it a very good deal. It will save you more than that in blood pressure meds. Regards