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How to identify fake JFETs?
How to identify fake JFETs?
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Old 5th June 2019, 08:53 AM   #1
andyr is offline andyr  Australia
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Default How to identify fake JFETs?

I'm wondering if anybody can advise me what tests I can do to determine whether a parcel of 500x 2SK170s I bought from a supposedly reputable source ... are genuine ... or fake.

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Andy
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Old 5th June 2019, 09:28 AM   #2
jan.didden is offline jan.didden  Europe
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How much did you pay? That'll be some indication.

Jan
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Old 5th June 2019, 09:43 AM   #3
merlin el mago is offline merlin el mago  Europe
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Measure if Idss is in the grade range, also visual inspection helps.
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Old 5th June 2019, 09:47 AM   #4
Nisbeth is offline Nisbeth  Denmark
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How to identify fake JFETs?
Quote:
Originally Posted by merlin el mago View Post
Measure if Idss is in the grade range, also visual inspection helps.
That's a start, but the problem is that the easiest way to fake a 2SK170 is to remark a similar part with inferior specs. Without getting too paranoid about it, if Idss is out of range it indicates fakes, but unfortunately Idss in range doesn't prove beyond all doubt that they are genuine
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Old 5th June 2019, 09:48 AM   #5
Mooly is offline Mooly  United Kingdom
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How to identify fake JFETs?
Checking a J-FET is a J-FET shouldn't be to difficult, but checking exactly what it is supposed to be is an entirely different matter.

This is the problem with parts of unknown province, you always wonder as that element of doubt is always there.

You could rig a test circuit up and check they behave correctly at a basic level, check that they are symmetrical and operate the same either way around. Check the pinch off voltage.

You could even build a dynamic test circuit (small single J-FET amplifier).

Next procure some known genuine devices and compare performance.
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Old 5th June 2019, 09:49 AM   #6
merlin el mago is offline merlin el mago  Europe
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Visual inspection, if I remember correctly Salas published pics with the real unit and fakes.
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Old 5th June 2019, 10:07 AM   #7
PMA is offline PMA  Europe
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Quote:
Originally Posted by andyr View Post
I'm wondering if anybody can advise me what tests I can do to determine whether a parcel of 500x 2SK170s I bought from a supposedly reputable source ... are genuine ... or fake.

Thanks,
Andy
Quote:
Originally Posted by merlin el mago View Post
Measure if Idss is in the grade range, also visual inspection helps.
Yes, as Merlin has already said, Idss measurement will for almost sure tell you if the part is genuine or not. If the Idss measurement is followed by Gm measurement, then you may be sure what you have.

About 10 years ago I bought a set of "2SK170BL" and "2SJ74BL" parts from an internet seller in Taiwan. They were all fakes. I tested them in a test-setup derived from one of my preamps
PRE3 complementary-differential preamplifier
All the parts were both very noisy an had very high distortion in the test circuit.
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Old 5th June 2019, 10:08 AM   #8
jan.didden is offline jan.didden  Europe
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EUVL Patrick would know how to check.

Jan
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Old 5th June 2019, 10:13 AM   #9
PMA is offline PMA  Europe
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Simple JFET test circuit attached. Measure both Idss and Vp.
Attached Files
File Type: pdf testfet.pdf (129.2 KB, 223 views)
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Last edited by PMA; 5th June 2019 at 10:15 AM.
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Old 5th June 2019, 10:26 AM   #10
EUVL is offline EUVL  Europe
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Quote:
Originally Posted by jan.didden View Post
EUVL Patrick would know how to check.

Jan


PMA already showed you a simple test.
You can check the Idss against Vp from the datasheet.
That is a good start.

If you want to test more parameters then take a look here :
FET Measurements with Just A DMM

Or send a few to someone who has a curve tracer and willing to test for you.
But if you paid less than 3USD a piece then it is very likely to be fake.


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