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Function Generator Amplifier root cause failure mode analyses

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monette999

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Dear All,
this is my first thread here. I am Bob and it is long time ago since I made my elecronic degree.
Today I wished the my brain would remember better.

I have a Max038 in my Function Generator which is bad.
The signal come in @ BHout. It is a sinus, square, triangle with a level of
1Vpp. It runs in the gate of the (Q107, Q108) 2SK117. The source level is 14V DC. The drain level is zero.
How can I test a FET is the attached circuit. I assume it is a differential amp. But it is not clear to me how it works.

By the way the unit is a Victor VC2002. i can post all the data sheets if needed.

Best regards
bob


Is is a J-FET is N type.
 

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audioguru

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The dumbo who drew the schematic shows a bipolar transistor instead of a Jfet.

The Toshiba datasheet for the 2SK117 specs a drain to source current of 1.2mA to 14mA when the drain is +10V and the gate is shorted to the source. The drain current is 0.1uA when the gate is -0.2V to -1.5V.

The FETs should be matched so if one is needed, buy a few and pick one that matches the one that still works or pick two that match.
 

monette999

New Member
Thank you audioguru,
may you give me a lesson how the differential amp with FET 2SK117 works.

I have to correct my statement. I have 14V at the Drain.
1Vpp at the Gate. The source shows no level only noise.


Does it means the SK is bad.

Best regards
bob
 

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audioguru

Well-Known Member
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The sources of the JFETs are joined together to make a differential pair. They work like a teeter-totter. When one FET conducts more then the other FET conducts less.
The signal at the sources is an extremely low level voltage because it is current swinging back and forth between the FETs, not voltage.

Simply measure the DC voltage at the output and report its polarity and voltage.
 
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