Softii problem, only works with a specific 4393 in Q3

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gastaxxx

New member
the transistor in question would seem to be Q3, if I change the positions of the others it varies the sound slightly, but it works correctly, moving Q3 I get a heavily cut sound (gated, I think it's like a bias problem).
Could it be a more sensitive position or could it be an out of range or damaged component?
Thanks
Dan
 
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Q3 is the bottom right one. If I change Q3 with others (and using the q3 transistor in other positions) I have a gated sound, with this the overdrive works well, I can clean the sound by my guitar pot.
 
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I resoldered a second time all resistors on the back and another time on the front side, I don't understand why the position of (theoretically) equal transistors gives such different results. I try to check the values with a multimeter
 
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I expect that the problem you're having is caused by the normal part-to-part variation in JFETs. Catalinbread most likely tested and selected the JFETs in their production pedals, so don't be surprised if you have to do the same.
Q1, Q3 & Q5 are fairly critical, especially Q3.

I visually verified that R4, R10, R12 & R13 are correct.

Measure and report these voltages:

Q3-S (center pin)
Q3-D (pin closest to Q4)
Q4-S (center pin)

One more thing you can try, and only do this if all of your capacitors are rated for 25V or more, run the pedal on 18V.

Where did you get your MPF4393s? They've been out of production and hard to find for some time now.
 
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Chuck D. Bones said:
I expect that the problem you're having is caused by the normal part-to-part variation in JFETs. Catalinbread most likely tested and selected the JFETs in their production pedals, so don't be surprised if you have to do the same.
Q1, Q3 & Q5 are fairly critical, especially Q3.

I visually verified that R4, R10, R12 & R13 are correct.

Measure and report these voltages:

Q3-S (center pin)
Q3-D (pin closest to Q4)
Q4-S (center pin)

One more thing you can try, and only do this if all of your capacitors are rated for 25V or more, run the pedal on 18V.

Where did you get your MPF4393s? They've been out of production and hard to find for some time now.
Click to expand...
It’s a musikding kit, they are pn4393. Thank you, I’ll check voltages tomorrow
 
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Based on anecdotal evidence, Musikding does not test their parts and depending on where they obtain their parts, they may not know if the parts they kit are in-spec.

Even so, just because a JFET is in-spec does not guarantee it will work the SoFTii or any other guitar pedal. If Musikding is buying JFETs and kitting them without verifying they have the right specs for that particular pedal, then you are taking on the risk that the kit won't work. It the builder's responsibility to ensure that the parts they use will work in the pedals they build. I buy lots of JFETs, I test 'em and select the right ones for each pedal I build. It's a pain in the ass, but I knew and accepted that going in.

The good news is PN4393 are still in production so good ones should be inexpensive and plentiful.
 
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Voltages are:
8.91 from transformer. Q3 central pin 2.23, pin near Q4 2.70 and Q4 central pin 7.46.

Yes, for others pedals like fuzz or germanium booster I have different transistor to try, I didn’t know that also jfet are “problematic”, I’ll buy them at next order!
 
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Q3 is saturated. The source & drain voltages are nearly equal. They should be a few volts different. Vgs,off is too high on Q3 & Q4. I suspect the other PN4393s on your board are similar. Even if these JFETs are in spec, and I doubt that they are, they are much too hot for this circuit. I recommend that you buy some from another source. I assume you're located somewhere in Europe. Looks like Mouser has a presence in Europe, so you might want to buy a full set from them.

https://eu.mouser.com/ProductDetail/Central-Semiconductor/PN4393-PBFREE?qs=OlC7AqGiEDnlEKOM7U/Saw==
 
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other volatges are (from left to righ, with another transformer 9.13v, guitar and amp plugged):
q2: 8.91 - 7.70 - 4.24
q1: 4.42 - 3.29 - 0
q5: 4.84 - 2.63 - 0
q6: 8.91 - 6.30 - 4.24
q4: 8.91 - 7.56 - 4.23
q3: 2.79 - 2.24 - 0

this is in the position where the pedal sounds with no obvious problems, but I guess it could sound better with correct jfets. Mouser has very high shipping costs for Europe, I was looking at musikding now and they have these expensive mpf, I try to look in other stores if there is anything else

www.musikding.de

MPF4393, 1,65

MPF4393, N-channel JFET Switching Transistor
www.musikding.de www.musikding.de

I just found a 2n5457, only one, but I can try it in Q3 to check if the problem is jfet in Q3 in other position or others jfet in Q3 (maybe I'm a little too empirical)
 
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Those are good voltages for Q3.

What is Q4-S voltage under those conditions?

If you have JFETs other than the ones that came in the kit, then try those. They do not all have to be the same part number.

The most critical ones are Q1, Q3 & Q5. If you get source voltages around 1V to 1.5V, that's good.
 
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