Geofex Jfet Matcher Question

niggez · February 05, 2008, 01:35:03 PM

Tried search, no luck.
Im not experienced with the pinout of opamps, so i have a problem with the schematic.

Im using a ua741 opamp for the build. My question: In the schematic, there are 2 leads going to + and 2 leads going to - of the opamp. Is this the same pin, or different pin depending on what side of the + / - it goes to?

www.ortodoxism.ro/datasheets/SGSThomsonMicroelectronics/mXssuwx.pdf

This is the opamp im using. So do i connect 2 leads to pin 2 (-) and 2 leads to pin 3 (+) ? HELP!

slacker · February 05, 2008, 01:44:08 PM

It should be like this.

Connect pin 7 to 9volts. Connect pin 4 to ground. They're the wires that are shown going into the sloping sides of the opamp on the schematic.

Pin 2 is the - input and that gets connected to the top of Rset and the bottom of the Jfet.
Pin 3 is the + input and that gets connected to Vmid and to one of the DMM leads.

Hope that makes sense

niggez · February 05, 2008, 02:10:53 PM

tnx. I would have swapped around 4 and 7 but...ill try your version first!

niggez · February 05, 2008, 02:15:26 PM

is it normal to measure 6v without a device plugged in to test??

I tested my 8 fets, and they all have a voltage of 5.8 V. This seems very strange to me...

There is no difference in any of the 8 tested devices, but i tried several combinations in my phase 45 with no luck...

niggez · February 05, 2008, 04:11:34 PM

I figure the voltage without a device being tested should be around 4.5 v?
Could a bad capacitor be causing my probleM? The cap I'm using has been lying around for a long time.

Mark F · February 06, 2008, 02:47:50 AM

When I built the JFET Matcher a long time ago at first I had a bad (leaky) cap & if I recall I was having the same or similar problem. I replaced the cap with a new one & it worked fine. So, yes, the cap may very well be your problem. Hope that is of some help.

niggez · February 06, 2008, 04:02:32 AM

seems like the cap was part of th eproblme, i replaced it and now read 4.10V without a device plugged in.
All my 8 Fets now give me readings from 3.56-3.58 V which still seems odd to me...

niggez · February 06, 2008, 04:56:47 PM

this is weird...i cant find the problem. Ill try it with a brandnew cap in a while, only have old ones lying around at the moment...

niggez · February 08, 2008, 10:21:56 AM

still no luck. And i tried a lot of the jfets in my phase 45, without any result....any ideas what could be wrong?

R.G. · February 08, 2008, 10:29:42 AM

I hate to put it this way, as you're at the predictable frustrated first timer stage, but it's true.

It is very likely that you have made one of the following errors:
(1) pinout connections to one or more of the parts is incorrect
(2) solder connections are either not making contact or are shorting from one to the other where they shouldn't
(3) the wires in and out are misconnected.

It is possible that with only old parts to work with, you have a bad part, but that's not all that likely.

The best suggestion I have is for you to read "Debugging Thread: What to do when it doesn't work." and follow its suggestions.

niggez · February 09, 2008, 08:46:49 AM

I finally got it working, and dont know what the problem was...i looked over the circuit a million times without finding anything, plugged it in, and it works. Maybe theres a loose solder connection somewhere, if it stops to work ill go over all of them...but for now im happy

Thanks for the help all of you.

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Geofex Jfet Matcher Question