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Old 4th August 2013, 09:50 PM   #1
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Default How to check a 2SK170?

My Denon PMA900V broke down destroying several obsolete transistors and some resistors. Right now i'm desoldering every transistor to check it so i dont miss any broken parts.

I'm not a huge expert on complex amplifier stages so i'm planning to check every transistor with the hFE function of my multimeter. (except the ones that are already in several pieces).

The first transistor the circuit is a FET. Tho i know some basics about fets, i never really worked with them, so i dont know how to test these. I googled a bit and found some ways tot test with either the diode or the resistance function of a ordinary cheap multimeter.

Either all the 2SK170 are dead, or the tests i found dont work with my multimeter. If have a total of 4 of these (2 per channel) and all show the same measurements including approx 50 ohm between the source and drain. As far as i know only one of the channels was destroyed, so i'm a bit confuesed by these measurements.

Do you guys have any tips on how to check these small FET's. Most of the test i found seem to be focused at more powerfull TO220 MOSFET's. This is the multimeter i'm using.
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Old 4th August 2013, 09:56 PM   #2
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Your meter looks OK to me.
Using the diode test mode, there should be no continuity between the drain and source if the gate has been emptied. It is a high impedance device requiring less than 1nA to hold the drain/source on.
Difficult to say whether faulty or not unless you download the data sheet and build the test circuit.
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Old 4th August 2013, 10:12 PM   #3
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Thanks for your reply! Unfortunately i was not able to find a test circuit. There is none in the datasheet.
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Old 4th August 2013, 10:21 PM   #4
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Look at Pass (forum) on building f4 and f5 or at firstwatt web site Nelson covers the 170 and j74 Isss testing that should tell you if it good requires a 9 volt battery and a current meter to read in the 10 ma range.
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Old 5th August 2013, 04:20 AM   #5
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When the gate is tied to the source the jfet acts like a current source equal to the Idss of the jfet. So, with a milliameter in series, or a voltmeter across an appropriate value drain resistor (100 ohms?) E = I x R, so 0.01 x 100 = 1 volt assuming the idss is exactly 10ma (which it won't likely be). 9 to 12vdc is sufficient... like a 9v battery.
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Old 8th August 2013, 07:05 PM   #6
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Thanks for the tip! They are all in the 7,45-7,55 range so i guess they are fine. Must have been my multimeter.
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Old 8th August 2013, 11:42 PM   #7
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Quote:
Originally Posted by SuperKris View Post
Thanks for the tip! They are all in the 7,45-7,55 range so i guess they are fine. Must have been my multimeter.
That sound well matched in the bl range (should be marked as such on each jfet)
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Old 9th August 2013, 09:38 AM   #8
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They are indeed BL range. Per channel the difference is actually within the 0,04 range, so i guess that is pretty well matched.

While i understand a bit of the purpose of matching i dont know a lot about the maximum difference. All other transistors are non FET's and quite a couple of them are broken. As these are mostly obsolete types but still available if you feel like paying enough.

Some of the ones that are obviously broken are 2SA13061, 2SA1106 and 2SC2581.

Finding exact matching hFE's will be difficult as most suppliers will not know the exact hFE before they are delivered. These components will not be cheap, so i dont feel like ordering multiple pcs and pick the best ones.

What difference in hFE is still acceptable, and will a multimeter in hFE mode be enough to check the hFE?
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Old 9th August 2013, 02:59 PM   #9
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The 2SK170 is a depletion device, with no gate voltage applied you should measure some bulk resistance between drain and source.

Quote:
Originally Posted by JonSnell Electronic View Post
Your meter looks OK to me.
Using the diode test mode, there should be no continuity between the drain and source if the gate has been emptied. It is a high impedance device requiring less than 1nA to hold the drain/source on.
Difficult to say whether faulty or not unless you download the data sheet and build the test circuit.
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