F5 Toshiba Output bias Issue - diyAudio
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Old 5th March 2014, 01:04 AM   #1
DGreen is offline DGreen  United States
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Default F5 Toshiba Output bias Issue

I am just finishing up my F5 rebuild after blowing my IRF outputs after 3+ years of trouble free service. Please don't ask how I blew the mosfets, it was an error on my part
I decided to use Toshiba 2SK1530/2SJ201 outputs this time around. When I set the bias to to 0.6 volts the outputs are barely warm to the touch. I was hoping someone could advise where I went wrong on this rebuild. Of course when I built the my first F5 I had no issues on power up (beginners luck I guess).
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Old 5th March 2014, 01:39 AM   #2
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I set my bias with a scope and sig gen.
Apply a sine wave.
Turn bias right down.
Turn on amp with speaker connected.
Look for crossover distortion on output.
If there is turn bias up and check again for crossover distortion.
If not sit back and enjoy.
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Old 5th March 2014, 02:08 AM   #3
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These MOSFETs turn "on" at about half the voltage of the IRF (now Vishay) or Fairchild MOSFETs (same with the Renesas lateral mosfets). Before you solder them in make sure that the potentiometers will have a really low Vgs voltage -- and remember that the P-channel device Source is connected to the positive rail and vice versa.
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Old 5th March 2014, 02:52 AM   #4
DGreen is offline DGreen  United States
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Thank you, I rechecked my boards aginst the BOM and didn't find any mistakes. The pots were at 0 ohms before I started, then gradually I increased resistance to obtain .3, .4, .6 volts across R11 and R12 then I backed off. The mosfets did not produce significant heat ~ 90 degrees only. I did not test the outputs before installing them. Still baffled.
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Old 5th March 2014, 07:01 AM   #5
The Dastardly Dad of Three
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that is seriously odd. What is the voltage that you measure across the Drain and Source pins of the fets when the bias is say 0.3V or so as measured across R11 or R12?
While you're at it may be worthwhile rechecking (with the meter) the insulation between the fets and the heatsink.
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Old 5th March 2014, 03:23 PM   #6
DGreen is offline DGreen  United States
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OK. Will check those voltages on outputs at 0.3 volts when I can get to it. I am using alum. Oxide insulators and the outputs are electrically isolated. I will probably try to replace the Toshiba's with the irfs cause I have a spare pair that I know are good. Then will try again to bias and see how it goes from there.
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Old 5th March 2014, 07:37 PM   #7
AndrewT is offline AndrewT  Scotland
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check the Source Resistor values.
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regards Andrew T.
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Old 5th March 2014, 10:29 PM   #8
The Dastardly Dad of Three
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Andrews suggestion is a very good one - use a multimeter to measure not just eyeball the value. It's easy to for the eyes to mistake .47 for 47
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Old 6th March 2014, 01:17 AM   #9
DGreen is offline DGreen  United States
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Source resistors measure 0.6 ohms.
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Old 6th March 2014, 03:57 AM   #10
The Dastardly Dad of Three
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ok great - time to measure the voltages as suggested in post #5. If you have 0.6 volts across the source resistors and those are 0.6ohms then you have 1 amp flowing through them and presumably thru the fet.

Since your fets are not heating up with this much juice flowing through, we have to measure their Vds to try and understand why.....
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