JFet tester question please...J-201 results are in!...

[momo]

Im using the RunoffGroove fetzer tester.


My Vp readings are way off, I get the source voltage plus a few......I test for 9v dead on source voltage and I get 9.07v out of the Vp reading.
In this case the idss is .70v.

Why am I getting the battery voltage on the Vp reading?
I did the circuit to accomodate J201 pinout.
[url=https://postimg.cc/V0nM9VgK]]http://runoffgroove.com/fetzervalve.html#11/url]

My Vp readings are way off, I get the source voltage plus a few......I test for 9v dead on source voltage and I get 9.07v out of the Vp reading.
In this case the idss is .70v.

Why am I getting the battery voltage on the Vp reading?
I did the circuit to accomodate J201 pinout.
[url=https://postimg.cc/V0nM9VgK]

[idy]

Maybe tell us what voltages you see on all three pins.
You would expect to see 9v at the Drain and something low at source.
Idss is measured in milliamps, not volts. It is "current drain to source measured at the source, ....or maybe ss stands for steady state? But it is current not volts.

[momo]

So I think the problem is me....hehe
and I don't know how....lol!

So all the voltages seem good,
I get battery voltage at the Drain
I get nothing on the gate
I get 0.70v on the Source for example, that varies by transistor.

So I don't understand how to calculate then...
As I saw I multiply by 10 the source and thats my Ma
How do I calculate the other value Vp?

[idy]

So I think you are talking about:



I would do this: With the thing configured for Idss
disconnect the D from the Resistor and switch (which goes to gate)
Insert your DMM between D and that R and G. On the setting for Millamps: Amps is series measurement; which you could also get by reading Volts and doing a very little math.
That number should be Idss....

Sounds like you are getting there.

[momo]

Oh boy,
So my circuit is point to point.
Everything is soldered as I see in the diagram.
If I remove the Drain, its like removing the + on the battery and then no current...
don't get it.
I have 3 test points, the +, the - and the Gate...
Sorry I really dont get it..

[momo]

Ok, got it....my bad of course. The gate was not connected to the resistor junction....all is well thanks.

[momo]

The raw values off my volt meter..
One of them in there will get much attention!..a few great ones I think.
I did the test all at 12v as that is what I will be using.

[Rob Strand]

You can see the degree of match after sorting by VP.

Good VP match in the central region, based on VP - VP_av column,


Comparison to ROG's values and summary of JFET measurements and parameters,


The device numbering is from your table starting #1 then moving across the columns then down to the start of the next row (which would be device #4 on row 2).

I've separated out the *measurements* from the estimated JFET parameters.   They aren't quite the same as
the test jig screws the numbers up a bit.

The E% rds0 is the error of the channel resistance compared to the average.  The values should be low
if the JFETs all come from the same batch.  High E% rds0 might indicate a noisy measurement. You can
re-check the measurements on the extremes but over all it looks OK.   Overall the rds0 values are a little
high, due to slightly low IDSS.

FWIW, I've found adding a 1uF poly cap across the meter terminals can help make the measurements more
stable.

You can see your raw ID test values are a little lower than ROG's values.  We don't know what the finer points are of the test jig.  I don't know what your DMM input resistance is.  It affects the estimated parameters a bit but it won't affect the ability to match.

[momo]

Wow
Thanks so much!
So I have to ask, I have a phase 90 on standby for years....could not match j-fets.
Those that match in the green zone could work?
many thanks.

[Rob Strand]

Wow
Thanks so much!
So I have to ask, I have a phase 90 on standby for years....could not match j-fets.
Those that match in the green zone could work?
many thanks.

No problem.    The J201 isn't ideal for a phaser but it's not bad either if know the J201's are good ones (your JFET have VP ~ 0.84V).  What you can do is tweak the circuit a bit to better suit the J201s lower VP.

Here's an old thread showing a simple mod for low VP JFETs,
https://www.diystompboxes.com/smfforum/index.php?topic=122690.0

If you skip the long conversation and go to post #44,  you can see reducing the resistor to the trimpot can help the circuit work better with lower VP JFETs.  The simpler mod is the middle one,
https://www.diystompboxes.com/smfforum/index.php?topic=122690.msg1159079#msg1159079

The thread used this schematic.  You can translate the mods to any build,
https://generalguitargadgets.com/pdf/ggg_p90_sc.pdf

The JFET in this thread had a very low VP.  Yours aren't so low so you need to use a larger resistor (and a smaller cap).  For VP of about 0.84V you might want to try 220k with a 150nF cap.   For some fine tuning, try a resistor value up or down, re-bias the trimpot and see what you think.   The cap affects higher sweep speeds.

The second mod, shown the one on the right, is easier to adjust the trimpot, you will need a resistor larger than 220k for sure, maybe 270k or 330k.   For your JFETs the first mod might be OK.

Oh, there's a chance you might need to tweak the 3.9M as well.  Another angle is to reduce the 4x47n cap to 22n or 33n.   Yes, unfortunately this is where it gets complicated using the J201s *and* trying to match the original.[/s]

[Rob Strand]

Wow
Thanks so much!
So I have to ask, I have a phase 90 on standby for years....could not match j-fets.
Those that match in the green zone could work?
many thanks.

No problem.    The J201 isn't ideal for a phaser but it's not bad either if know the J201's are good ones (your JFET have VP ~ 0.84V).  What you can do is tweak the circuit a bit to better suit the J201s lower VP.

Here's an old thread showing a simple mod for low VP JFETs,
https://www.diystompboxes.com/smfforum/index.php?topic=122690.0

If you skip the long conversation and go to post #44,  you can see reducing the resistor to the trimpot can help the circuit work better with lower VP JFETs.  The simpler mod is the middle one,
https://www.diystompboxes.com/smfforum/index.php?topic=122690.msg1159079#msg1159079

The thread used this schematic.  You can translate the mods to any build,
https://generalguitargadgets.com/pdf/ggg_p90_sc.pdf

The JFET in this thread had a very low VP.  Yours aren't so low so you need to use a larger resistor (and a smaller cap).  For VP of about 0.84V you might want to try 220k with a 150nF cap.   For some fine tuning, try a resistor value up or down, re-bias the trimpot and see what you think.   The cap affects higher sweep speeds.

The second mod, shown the one on the right, is easier to adjust the trimpot, you will need a resistor larger than 220k for sure, maybe 270k or 330k.   For your JFETs the first mod might be OK.

Oh, there's a chance you might need to tweak the 3.9M as well.  Another angle is to reduce the 4x47n cap to 22n or 33n.   Yes, unfortunately this is where it gets complicated using the J201s *and* trying to match the original.[/s]

It just occurred to me, for the J201, there's more to just adjusting for VP.

The J201 has a high resistance and it needs the extra sweep.   As it turns out you might need to *increase* the 1M resistor on the trimpot, to say 1.2M to 1.5M.   After that the trimpot should take-up the slack.   However, the trimpot adjustment is going to be a total pain to set - very small adjustments make a big difference.   There is a way around that - if you go ahead I can continue.

[momo]

Thanks much for your time and effort!
I have other Jfets, some fit the tester pinout in reverse, some have source on one side so If I can't test them, I would build another tester with right pinout.
I have: 2n5458, 2n5952, bf244a, mosfet bs170.

[CheapPedalCollector]

2n5458 and 2n5952 are both used in phase 90 circuit, try those.