F6 Illustrated Build Guide

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#1,622
Acoustic Emotion said:
Can someone please guide me how to check the conditioning of the mosfets? I will desolder them tonight, but how can I measure to see if they are burned inside? Which legs should I measure with the multimeter to check for proper functioning? I checked the datasheet and diagram of the IRFP240 but I am not sure where to measure.

Thanks in advance,

Koos
Click to expand...

if you can measure short (buzzer test on your DMM) anywhere , it's Dodo

if there is no short , read Mosfet testing/matching article at FW site

don't forget that usually you need to heatsink mosfet while testing
 
#1,623
Zen Mod said:
if you can measure short (buzzer test on your DMM) anywhere , it's Dodo

if there is no short , read Mosfet testing/matching article at FW site

don't forget that usually you need to heatsink mosfet while testing
Click to expand...

Thank you Sir Zen Mod! 😀

The First Watt website is a paradise for me, so much new information in the articles about the functions of the components. After reading all of this I will probably have less questions and I can continue without asking too many questions to you guys 🙂

Thanks again,

Koos
 
#1,631
JulesOz said:
Hello lads,

I almost finished my F6 build and I'm now trying to set the bias.
I'm having my meter across the 0.47ohm source resistor but there is no voltage at all (on both side) =(
I did test my PSU and it works fine.

Am I missing something? Thanks for your help!
Click to expand...

I'm just finishing up my build too and made the same observation.

Just need to crank up the bias until the mosfet fires... Look at the gate voltage to see which way to turn the bias pot (it might be around -23V and you want it less negative) and then go for it - slowly - may take quite a few turns but you'll eventually see volts climbing across the source resistor if everything is fine 🙂
 
#1,632
BruceInOz said:
I'm just finishing up my build too and made the same observation.

Just need to crank up the bias until the mosfet fires... Look at the gate voltage to see which way to turn the bias pot (it might be around -23V and you want it less negative) and then go for it - slowly - may take quite a few turns but you'll eventually see volts climbing across the source resistor if everything is fine 🙂
Click to expand...

Thank you very much 🙂
 
#1,633
R4 problem too

Like others i'm (trying to) building a F6 and i have smoke from the R4 resistor after 10-15sec the first time i tried to power it up. The voltage arriving to the board is 24.7v and the board led does not emit light (i guess it is mounted the wrong way)


I have no voltage when mesuring across the 0.47 ohm resistor.


Do you have an idea where could the problem? Should i power it up with a bulb tester or a load?


Thanks!


ps: screws on mosfet have been changed since picture


 
#1,636
eddyvb said:
Check/measure that both output fets are isolated from the heatsink. There is something wrong with these fets. (And is R4 still ok?)
Click to expand...



There is a mica insulator between the fets and the heatsink, i am going to replace them just to be sure.

The burned resistor was measured @18ohms before soldering and after the R4 smoke, still read an impressive 18 ohm, i will replace it too



The transformers deliver 18v AC and after the power supply boards 24.7 DC, i will post pictures of my "amplifier" i surely made a mistake somewhere. Thanks!
 
#1,638
eddyvb said:
Check/measure that both output fets are isolated from the heatsink. There is something wrong with these fets. (And is R4 still ok?)
Click to expand...

Hello eddyvb,

Can you recommend a material that I can place in between the mosfets and het heatsink?

I have no isolation material in between the mosfets and the heatsinks until now, can this be the reason for the burned R4 resistor?

Thanks in advance!

Koos
 
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