Author Topic: JFET matcher pcb... what's wrong?  (Read 726 times)

Justus

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JFET matcher pcb... what's wrong?
« on: January 30, 2015, 11:23:07 PM »
I have populated the JFET matcher pcb from the NomNom and want to start taking some readings.  However, my DMM auto ranges to mV and the reading seems to jump around alot (-68.0 down to -63.0 and up, never stopping).  When I turn off auto range and set it to V with 2 decimal places I always get the reading of -.03V on every one.  Granted this is the first time I have done anything like this but it doesn't seem like it should be too hard.

Pics of my pcb attached.  Did I screw something up?

Justus

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Re: JFET matcher pcb... what's wrong?
« Reply #1 on: January 30, 2015, 11:24:22 PM »
Back side...

rullywowr

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Re: JFET matcher pcb... what's wrong?
« Reply #2 on: January 31, 2015, 11:12:51 AM »
Did I screw something up?

It looks decently good.  From the pictures, I would say it wouldn't hurt to add a bit more solder on the backside and "reflow" those joints.  I can't see the markings on the chip...which IC did you use?  It also wouldn't be a bad idea to socket the chip so you are less likely to damage it by soldering directly to the pins.  If you "cooked" the IC too long there is the potential for the IC to be toasted.



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Justus

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Re: JFET matcher pcb... what's wrong?
« Reply #3 on: January 31, 2015, 11:43:42 PM »
It's a Texas Instruments TL072 if I recall correctly.  The markings are pretty faint and just didn't show up at all on my cell phone camera.

Would a cooked IC give results like I'm getting?  Because it's such a simple circuit, that's the only thing I could think of too.

rullywowr

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Re: JFET matcher pcb... what's wrong?
« Reply #4 on: February 01, 2015, 11:02:27 AM »
It wouldn't hurt to remove the IC and replace it. Perhaps the best way would be to snip off each leg and then carefully desolder each leg. Use a socket before installing the new one.



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