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Old 4th June 2004, 09:54 PM   #1
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Default how to check mosfet dead or no?

hi, having problems with zen v4, build it a year ago, sounds cool, have 120 000uf

not long ago dead. I try to fix, fixed, again dead.. again fix. again dead, have no money each time replace all parts now I have 20 mosfets and dont know how to chek it is dead or good. I have only didgital multimeter.
pls help anybody..
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Old 5th June 2004, 12:57 AM   #2
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Several things you can do. Since you must have Gate
resistors in series with the Gate, you can check for
any current through them (in other words, any voltage
across the resistor). Since the Gate should not conduct, any
current through the resistor says that the Gate is broken.

If it's an N channel part, you can attach both the Source and
the Gate to ground and apply some positive voltage at the
Drain. If any current goes through it, it's broken, since an
unbroken part will be shut off.

You can apply the well known matching technique (documented
ad nauseum), and if it fails that test, it's broken.
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Old 5th June 2004, 07:43 AM   #3
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Talking I'm not dead!

No need to check I'm not dead!

Mozfet
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Old 5th June 2004, 08:13 AM   #4
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Default Msft

Thanks Mr. Nelson. for suport.
very sensitive devices they are..


also I did horible mistake- forget so sold in two film capasitors C8/C10 as usualy I before turning zen V4 on for the firs time turn in cd,preamp speakers put some volume and turn Zen V4 in. I played for about 2seconds,and silent.. why every time I biurn zen there are no signs of burn fire, smoke.. silent death.. what parts I could damage?

Mozfet- easily.. I dont want check You. thats for sure!
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Old 7th June 2004, 09:46 AM   #5
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The easiest way to check mosfet is to check it's internal drain-source diode.\
Connect you multymeter (setted to diode or resistor measurement) to D-S in both polarities - on one polarity it shoud conducted, in another - no.

According to my (and not only my ) experience (I working every day with mosfets in the power electronic area) - in 95% mosfet is "OK" if it's internal diode is also "OK".
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Old 10th June 2004, 06:30 AM   #6
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I second Altor. The integral diode between D-S is always the first to go...unless of course static got to the G-S first. With the multimeter set to diode test...a good one will have a V d-s at forward bias (N-channel: +probe at S) at around 0.5 to 0.7 V and reverse bias is OL. If it is shorted out as is almost always the case...it's 0V! At the resistance function...forward bias will be low resistance compared to reverse bias.
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Old 12th June 2004, 06:49 AM   #7
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Hi,

Use your multimeter to charge the gate capacitance by forward biasing the gate /source with the meter leads, in ohmmeter mode, then check the resistance from the drain to source, it should be zero or close to it with the gate charged.

Then short the gate to the source to discharge the gate capacitance and re check the resistance from drain to source, it should max the meter reading.

Here's a link that might explain it more clearly, I find this to be the fastest and simplest method, however he also shows another form of test anyone with little equipment available can do which is said to test the pinch off voltage.

easy mosfet test info

Good luck.
Chris

PS:

I know you have little equipment, but those mosfets keep blowing for some reason, do your best to find out what it is. Maybe some here who know the design can give you some tips on what to check out?
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Old 12th June 2004, 02:14 PM   #8
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Quote:
Originally posted by classd4sure
Hi,


I know you have little equipment, but those mosfets keep blowing for some reason, do your best to find out what it is. Maybe some here who know the design can give you some tips on what to check out?

big thanks classd4sure. very informative data, thanks for support and help. I put out all elements and have clear PCB(thanks to God and Nelson- schematic are simple as posible. so only few(20-35pieces components only)
I will check PCB tracks again. I checked it twice, but no faults found..

components have no fault too. power suply is dual mono and in case of bad PS I testing on othrer chanel PS.

I have troubles with other apms or this zen, but find problems very quick( 20min or later) and fixed it

tomorow I will try to take parts on and again turn on this amp.

why bias isnt stable and after 3-5 second goes down tranzistors...
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