Yikes!! FET Substitution? Could use some FET Guru's here... - diyAudio
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Old 23rd December 2003, 09:36 AM   #1
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Default Yikes!! FET Substitution? Could use some FET Guru's here...

Got a beautiful old vintage SS amp to restore, but looks like someone has removed a few components. In particular, there is a SIP packaged dual JFET at the input (Q701). Manual calls for a M47F (C), whatever the hell that is...I certainly can't find any info on it besides the fact that the schematic shows it a dual N-Channel FET.

What I know about FET's could fit in a thimble. Any suggestions on what I could do to revive this thing? Wayyy to pretty to just have collect dust.
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Old 23rd December 2003, 09:44 AM   #2
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If it matters (at this point...) the pinout of the 7-pin SIP is D-G-S-(NC)-S-G-D

Thanks for any input guys..
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Old 23rd December 2003, 02:20 PM   #3
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You could try replacing it with some other dual JFET with an offset well below 14mV and an IDSS well above 1mA. (An offset of about 14mV or more will overdrive the DC bias loop and the gates will go in conduction when IDSS is less than about 1mA.)
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Old 23rd December 2003, 08:19 PM   #4
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try a 2sk389
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Old 23rd December 2003, 09:03 PM   #5
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I have been digging and digging. Apparently, Toshiba no longer has the datasheet for the 2K389. Hell, I can't even find out what package or pinout it uses, much less if it meets the parameters listed by MarcelvdG.

Anyone got a datasheet stored on a server somewhere? Or perhaps tell me if its parameters (and pinout) are suitable..??
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Old 23rd December 2003, 10:36 PM   #6
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try here for the data sheet:
http://perso.wanadoo.fr/jm.plantefeve/comp.html



Cheers and good luck
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Old 24th December 2003, 08:09 AM   #7
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Apparently its offset is a bit too high, but you can increase the DC loop control range, for example by lowering R753 and increasing C711 by the same factor. With R753=47kohm and C711=1uF, the DC loop control range becomes about 30mV, which should be enough.
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Old 24th December 2003, 09:45 PM   #8
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rickpt...thank you for the link!!


MarcelvdG, I see...you were referring to the differential G-S voltage...Duh! It wasn't obvious to me at first.

No reason why I can't tweak the servo loop...got all the components I need to do so. However, there are three Idss class ranges for the 2SK389 device...the 'GR' with a range of 2.6 to 6.5mA, the 'BL' with a range of 6 to 12mA, and the 'V', with a range of 10 to 20mA. Which one to hunt for?
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Old 24th December 2003, 10:05 PM   #9
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It probably doesn't matter much.
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Old 25th December 2003, 01:44 AM   #10
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Your assistance is most gracious MarcelvdG, and greatly appreciated. I got to find the parts and order them, but I believe this will do the trick.

There's more wrong that this though...although the fuse doesn't blow, the output of one channel is sitting at the Neg rail, so I've got some more troubleshooting to do before I claim victory. However, this input stage with the unknown FET was the big concern.

Thanks again...I'll post once I have parts and figure out where the fault is in the output stage.
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