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Old 30th April 2011, 07:10 AM   #1
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Default new f5 bias problem

So i've been watching everyone do their F5 builds, and it looks like everyone is doing awesome! not a whole lot of issues, just small things, hat you can turn the amp off, swap something out, and turn it back on, and go to town!

But i've run into an issue that i haven't seen yet.

I'm in the process of trying to get my amp to bias properly, doing one channel at a time. but on one of the channels, as soon as i turn the amp on, R14 instantly bursts into flames! i've replaced it twice now, and i cant seem to find the issue, and was hoping someone would have an answer for me. i dont know if i have my trimmers the wrong direction? everywhere i've read, you set it to 0 ohms, and work your way up, monitering voltages across R11 and R12.

maybe i have any of the transistors backwards?

any information will be greatly appreciated,

Thank you early

Cody
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Old 30th April 2011, 07:34 AM   #2
Zen Mod is offline Zen Mod  Serbia
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my crystal ball is muddy these dayz ( I confess - I'm lazy )........ so - will you enclose some pictures and further info ?

pretty please ........

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Old 30th April 2011, 09:21 AM   #3
Itsmee is offline Itsmee  England
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R14 is the gate resistor, if P2 is at min resistance, that connects one end of R14 to -V, for current to flow, there must be a fault/short around the IRFP240
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Old 30th April 2011, 09:56 AM   #4
Itsmee is offline Itsmee  England
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When you have found the fault, check that P2 has not been damaged from the current flow before powering up again.
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Old 30th April 2011, 03:24 PM   #5
qusp is offline qusp  Australia
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yes, pictures would do nicely, sometimes i have trouble reading just words without them
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Old 30th April 2011, 03:31 PM   #6
Itsmee is offline Itsmee  England
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F5
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Old 30th April 2011, 03:46 PM   #7
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i know i have knock off boards, and would like to apologize to everyone appropriatly. i have great respect for all of you, and at the time of purchasing, didnt even know the amplifier was an F5, and would have absolutely bought the proper one, if i would have realized.

i guess i should have added some more information, although i thought R14 would be important because it arced to the end of its life, twice in a row. I also had R18 burn shortly after it, when i tried firing it up the first time, as well as R1, and R2 turned to a nice golden brown color.

so i replaced all of these resistors, then ran into R14 the second time, and figured i would ask the experts.
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Old 30th April 2011, 04:05 PM   #8
Itsmee is offline Itsmee  England
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Silly question, I cannot tell from the photo, are there insulators between the FETs and heatsink, and how much clearance is there between the PCB and heatsink, I notice that two of the stand offs are mounted on the FETs and two are on top of the PCB.
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Old 30th April 2011, 06:12 PM   #9
alazira is offline alazira  United States
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I don't recall capacitors in the F5 circuit. What value are they?
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Old 30th April 2011, 06:29 PM   #10
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Yes, there are insulators. And I believe its close to 1/4 inch standoffs. The reason for the standoffs being on top is because I haven't had time to go get the proper length bolts, being I'm about a half hour from a place that carries that size.

And the capacitors are just on the voltage rails, not anywhere in the actual circuit.
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