Hello everyone!
First I just want to say thank you all to the great knowledge shared on this website, it has really made my first diy project fun.
I bought and assembled the f5 v3 boards with universal psu and on first startup everything seemed fine. P1 and P2 set to zero resistance measured over r5/r6 but when i started biasing one channel I read the DCV on the wrong resistors and went to quick, resulting in the r13 starting to smoke. I figuered out the right resistor to read and with the broken channel disconnected from psu i sucessfully biased the other channel to about 0.55dc voltage and let it stabilize with lid on for several hours, no issue.
I replaced the burned resistor on the first channel and turned pots back to zero and reconnected to psu, but on startup both r13 and r14 start to smoke rather quickly even when reading 0v over r5/r6. No other component are visibly affected.
I am kind of stuck on where I should go now, i guess i will start by carefully examine that i put all the components in correctly. Any advice?
Thanks in advance.
Edit: "Mosfet gate resistor" not "source resistor"
First I just want to say thank you all to the great knowledge shared on this website, it has really made my first diy project fun.
I bought and assembled the f5 v3 boards with universal psu and on first startup everything seemed fine. P1 and P2 set to zero resistance measured over r5/r6 but when i started biasing one channel I read the DCV on the wrong resistors and went to quick, resulting in the r13 starting to smoke. I figuered out the right resistor to read and with the broken channel disconnected from psu i sucessfully biased the other channel to about 0.55dc voltage and let it stabilize with lid on for several hours, no issue.
I replaced the burned resistor on the first channel and turned pots back to zero and reconnected to psu, but on startup both r13 and r14 start to smoke rather quickly even when reading 0v over r5/r6. No other component are visibly affected.
I am kind of stuck on where I should go now, i guess i will start by carefully examine that i put all the components in correctly. Any advice?
Thanks in advance.
Edit: "Mosfet gate resistor" not "source resistor"
Last edited:
I checked the mosfets and they were indeed done so i bought some new ones and installed. Bias set back to zero i can start the amp without any smoke. The issue i have now is that when setting the bias the mosfets don't get warm at all, compared to the other channel when you can feel heat being generated fairly quickly.
Dont know much about electricity but i tried measuring the voltage across the gate/source which read very low numbers compared to the working channel. Should i increase the bias and instead look at the gate/source voltage or perhaps something else is fried?
Thanks for helping a newbie
Dont know much about electricity but i tried measuring the voltage across the gate/source which read very low numbers compared to the working channel. Should i increase the bias and instead look at the gate/source voltage or perhaps something else is fried?
Thanks for helping a newbie
make distinction between bias ( which is biasing voltage at some control pin) vs. Iq - which is standing current, and that's exactly what we are trying to set
put one DVM set to Vdc acorss one output source resistor - there you observe Iq; goal is prescribed value by Pa
put second DVM set to Vdc across output terminals - there you observe output offset , goal is as close you can get to 0mV
then fiddle iteratively with two opposite trimpots to get what is needed
besides other members, I described setting procedure few times in some F5 threads, dunno where exactly
edit: hope you did change all resistors which smoke, including mosfet gate resistors
put one DVM set to Vdc acorss one output source resistor - there you observe Iq; goal is prescribed value by Pa
put second DVM set to Vdc across output terminals - there you observe output offset , goal is as close you can get to 0mV
then fiddle iteratively with two opposite trimpots to get what is needed
besides other members, I described setting procedure few times in some F5 threads, dunno where exactly
edit: hope you did change all resistors which smoke, including mosfet gate resistors
Yes i changed all the resistors that were damaged with the same resistance.
When setting up the working channel i can get 0.59 vdc over the output source resistor and close to 0mV on that channels output offset and it was stable with lid on for hours.
But on the other channel i can adjust trimpots to get 0.59 vdc on resistors but there is absolutely no heat coming of the mosfets.
Out of curiousity i measured vdc over source/gate on healthy channel and i can read about 4 volts. On the "broken" one it barely surpasses one volt.
Should I adjust trimpots on broken channel to achieve the desired voltage across mosfet gate/source (which i think is about 3.6 volts according to f5 article) and disregard the volt reading on the source resistors? or does this indicate another broken component? Hope you get what i mean
which were the correct resistors to measure the voltage across for biasing? I'm building an F5 as we speak, and whilst there are a lot of guides to biasing online, none of them say which version (2 or 3) they are referring to, and in each version the resistors are referred to differently on the PCB. Some say R8 and R7, some say R11 and R12. I am building a V3 F5, so do I need to measure across the source resistors (R7,R8)?
TIA
TIA
The V3 board matches the schematics on p.2 here:
http://www.firstwatt.com/pdf/art_f5_turbo.pdf
So you want to measure across the 3W 0.47ohm source resistors R7 and R8.
http://www.firstwatt.com/pdf/art_f5_turbo.pdf
So you want to measure across the 3W 0.47ohm source resistors R7 and R8.
Just as I though, thanks. In that case, I think I have similar problems with regards to my Jfets, in that the drain resistors seem to get very hot whilst the MOSFETs don't heat up at all. How should I go about testing the Jfets?
R9, R10, R11 and R12 all get extremely hot very quickly.
What's more, P1 and P2 seem to have little to no effect on the voltage across R7 and R8 respectively.
I have de-soldered R7 and 8 now to check their resistance (and to see if they have blown as they did on my left channel). Once I've put these back in I will test the voltage across R3 and 4.
For a bit of background, this is the story so far.
Before first power up, I verified the PSU rail voltages etc, showing 26V+/- without any load. A little high but should be okay.
I then connected up the amp boards to the PSU, tested for continuity in the +/- and ground wires.
I set the P1 and P2 to minimum (all of them wouldn't go lower than about 6Ohms weirdly). I couldn't find any information on setting P3 without a oscilloscope so I set it to minimum as I had seen suggested.
Powered up and immediately blew r7 and 8 on L channel. R channel was stable.
Unplugged, took L channel off and removed blown resistors, verified P1 and 2 were at minimum, now they showed about 1Ohm, even without changing the pot. This seems to be a theme, where P1 and 2 on BOTH boards don't show consistent resistances.
I check all pot valued on both channels, then removed P3 from both as I had been recommended to do so.
I powered up again and R7 and 8 blew again on L channel. I removed L channel just to focus on R channel whilst I wait for new resistors to arrive.
Attempting to bias R channel, I powered up. All fine. I turned P1 slowly half a turn, voltage across R7 increased a bit. Did the same for P2 and R8. Then turning P1 another half turn, R7 started to smoke. I turned P1 back half a turn and no change. I powered down
I went back to reset, set P1 and 2 to minimum again and they now show 18ohm. I turned it back down to minimum, this time wouldn't go below 5 ohms. Now whenever I try to bias R channel, R9 R10 R11 and r12 all get extremely hot, and the MOSFETs don't seem to heat up at all.
I think the cause of the troubles may lie in my inconsistent potentiometer situation.
What's more, P1 and P2 seem to have little to no effect on the voltage across R7 and R8 respectively.
I have de-soldered R7 and 8 now to check their resistance (and to see if they have blown as they did on my left channel). Once I've put these back in I will test the voltage across R3 and 4.
For a bit of background, this is the story so far.
Before first power up, I verified the PSU rail voltages etc, showing 26V+/- without any load. A little high but should be okay.
I then connected up the amp boards to the PSU, tested for continuity in the +/- and ground wires.
I set the P1 and P2 to minimum (all of them wouldn't go lower than about 6Ohms weirdly). I couldn't find any information on setting P3 without a oscilloscope so I set it to minimum as I had seen suggested.
Powered up and immediately blew r7 and 8 on L channel. R channel was stable.
Unplugged, took L channel off and removed blown resistors, verified P1 and 2 were at minimum, now they showed about 1Ohm, even without changing the pot. This seems to be a theme, where P1 and 2 on BOTH boards don't show consistent resistances.
I check all pot valued on both channels, then removed P3 from both as I had been recommended to do so.
I powered up again and R7 and 8 blew again on L channel. I removed L channel just to focus on R channel whilst I wait for new resistors to arrive.
Attempting to bias R channel, I powered up. All fine. I turned P1 slowly half a turn, voltage across R7 increased a bit. Did the same for P2 and R8. Then turning P1 another half turn, R7 started to smoke. I turned P1 back half a turn and no change. I powered down
I went back to reset, set P1 and 2 to minimum again and they now show 18ohm. I turned it back down to minimum, this time wouldn't go below 5 ohms. Now whenever I try to bias R channel, R9 R10 R11 and r12 all get extremely hot, and the MOSFETs don't seem to heat up at all.
I think the cause of the troubles may lie in my inconsistent potentiometer situation.
How did you verify that P1 and P2 were set at minimum? Did you measure the
resistances across R5 and R6 to check their values are low?
Did you measure dc offset during the process? Having R9-R12 very hot could
mean high dc offset.
Please check you have proper parts placements.
When you get a chance, please take some bright and clear photos and post them.
Others might spot something.
resistances across R5 and R6 to check their values are low?
Did you measure dc offset during the process? Having R9-R12 very hot could
mean high dc offset.
Please check you have proper parts placements.
When you get a chance, please take some bright and clear photos and post them.
Others might spot something.
Yes I measured across r5 and r6. I have checked and checked the part placement and it is all correct, that is one thing I am totally confident of. I will post pictures anyway when I can.
I measured DC offset, never over a volt during my first runs, but on the last one it was showing 17Vdc! That's what makes me think the output devices are dead
I measured DC offset, never over a volt during my first runs, but on the last one it was showing 17Vdc! That's what makes me think the output devices are dead
I realise this now, thank you. What effect would having it set to minimum have?
Please forgive the potentially newbie questions, I'm still learning the about circuits and circuit rlationships.
If P3 is set min value, you basically end up with one jfet without degneration
and it will look like a badly matched pair of jfets.
If your amp is already biased up and you turn P3 all the way, then you'll
probably run into trouble, with the bias going up on one side and down the other
(and likely quite significantly) and you'll have nasty dc offset...
Of the other hand, if P1 and P2 are all the way down (ie, measured R5 and R6 are
low) then the outputs should still not be conducting.
and it will look like a badly matched pair of jfets.
If your amp is already biased up and you turn P3 all the way, then you'll
probably run into trouble, with the bias going up on one side and down the other
(and likely quite significantly) and you'll have nasty dc offset...
Of the other hand, if P1 and P2 are all the way down (ie, measured R5 and R6 are
low) then the outputs should still not be conducting.
Thanks for this. I did some more investigating today and have found that the MOSFETs on R channel are shorting, and the Jfets on the L channel are bust, explaining the different behaviour of each channel. So now it's time to wait for new ones to come from the US.
I measured the resistance between the output and the GND of each board also today (power off and cables disconnected from board) which showed 3ohms on the L channel (dead Jfets) and 23ohm on the R channel (dead MOSFETs). Can you tell if this sounds about right given the listed problems with the channels?
Low resistance on R channel would explain why R7 and R8 burn up immediately, because current can blast through the board fairly unresisted. This may also mean the MOSFETs on this channel have been fried too I suppose.
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