SOLVED Chop Shop Biasing

[Ctrl4Smilerz]

I just finished boxing up the my Chop Shop, and I'm having some trouble biasing the jfets. I have my multimeter on this setting:


Putting the black probe on the threads of a screw hole, and touching the red probe on the biasing pad, with the sag all the way CCW I get 3.3-4.9 volts on Q1, and 4.4-5.0 volts on Q2, but it jumps around a lot. I read elsewhere that you want 6 volts. Also when plugged in, I don't get any sound when engaged (not sure if that is related to the bias, or another problem). Here is the board:




As always your help is very much appreciated

 

[temol]

Trimmer set maximum CCW has max resistance and sets the lowest possible drain voltage (for a given R value). For a 9V supply and 20k trimmer it may be between 2V-5V, depending on the parameters of the transistors (probably below 0.5V with 50k trimmer). Trimmer set max CW should read 0R and set the max voltage - equal or very close to the supply voltage (minus voltage drop on a 1N5817 diode).
There is no resistor in series with the trimmer, which could affect the adjustment range and the inability to adjust "up".
Therefore, the first suspicion is - measurement with the trimmer at zero (max CCW) and "biasing" with Sag potentiometer.
The resulting measurement is in the range from about 0.5V to 5V - which is similar to the description of the problem.

 

[Ctrl4Smilerz]

Can you post a picture of the other side of the board? Something is off somewhere if the audio signal is not there at all when engaged.

1. Can you confirm that the trimmers are 50K, should have 503 printed on the side.
2. Confirm that you had/ have the Input & Output jacks in the correct sockets.

Trimmers say 503 on the side, and I double checked the I/O wiring.

Trimmer set maximum CCW has max resistance and sets the lowest possible drain voltage (for a given R value). For a 9V supply and 20k trimmer it may be between 2V-5V, depending on the parameters of the transistors (probably below 0.5V with 50k trimmer). Trimmer set max CW should read 0R and set the max voltage - equal or very close to the supply voltage (minus voltage drop on a 1N5817 diode).
There is no resistor in series with the trimmer, which could affect the adjustment range and the inability to adjust "up".
Therefore, the first suspicion is - measurement with the trimmer at zero (max CCW) and "biasing" with Sag potentiometer.
The resulting measurement is in the range from about 0.5V to 5V - which is similar to the description of the problem.

I set the sag all the way CCW and checked the voltage and both extremes for the the trimmer. If the Mouser JFETs have different pinouts, would that prevent the signal from passing when engaged?

 

[Laundryroom David]

This has been mentioned before but I don't know if we ever got a definitive answer (and by that, I mean someone actually using them throwing one on a semiconductor tester and verifying)...

The datasheet for the InterFET J201's currently sold by Mouser shows the pinout all wrong.

View attachment 43427

This would be SDG, where the correct pinout should be DSG.

Has anyone used these and can confirm whether the datasheet is incorrect or the pinout is indeed different?

I could not get mine to bias with the InterFET J201s. I switched to surface mount (on adapter boards) and it biased on the first try. FWIW. Etc etc etc.

 

[Laundryroom David]

I set the sag all the way CCW and checked the voltage and both extremes for the the trimmer. If the Mouser JFETs have different pinouts, would that prevent the signal from passing when engaged?

I think the through holes from InterFET are crappy. I’ve used them in a few different circuits and had too much grief for it to be entirely operator error. Before you tear anything up, get some Mmbfj201s on adapters and try them out.

 

[Locrian99]

This is very picky about J201's being within the required IDSS.
Where did you get your J201's ???

From the Builder, Thanks Guillaume - FC :
Let's start by sorting the JFETS. As we saw on the datasheet, J201's can have an Idss value from 200uA to 2mA. That is a huge range. For Q1, you want a J201 that has an Idss from 600-800 uA. For Q2, you want one with Idss about 400-600uA.
So you'll have to see what you have to work with. You might be unlucky and have all J201's with crazy values like 1500uA. In that case, the drain resistors will need to be changed.

With the trim pots is the sorting still necessary? I did on my build but I was also thinking that was so you could use the original resistor values and get in the ballpark of the correct voltage?

 

[music6000]

I could not get mine to bias with the InterFET J201s. I switched to surface mount (on adapter boards) and it biased on the first try. FWIW. Etc etc etc.

I don't trust them either, that's why I purchased NOS J201's with correct pinout.

 

[Ctrl4Smilerz]

I just ordered some pre-soldered SMD J201s. Hopefully that will solve the problem.

 

[geoffrey]

Was about to build mine and then read this thread and got depressed because the only J201's I have are a handful of InterFET's.

I said, F- it , let's just see what what happens. After I boxed it up I just threw in the first 2 J201's that were in the pile (didn't test them at all) and was able to bias them almost perfectly (Q1 6.5 and Q2 6.7).

Did I just get lucky here? Sounds fantastic, btw.

 

[geoffrey]

Was about to build mine and then read this thread and got depressed because the only J201's I have are a handful of InterFET's.

I said, F- it , let's just see what what happens. After I boxed it up I just threw in the first 2 J201's that were in the pile (didn't test them at all) and was able to bias them almost perfectly (Q1 6.5 and Q2 6.7).

Did I just get lucky here? Sounds fantastic, btw.

View attachment 43590
View attachment 43591

Ok, I just realized I didn't get lucky. I purchased my J201's from Barry at GuitarPCB. He states on his site that these are hand tested and in proper range. He has them on sale right now if anyone wants to go that route.

 

[Ctrl4Smilerz]

I got SMD JFETs and I am still struggling to get them to bias. With the new JFETS, I now get sound when engaged, but no clipping. Also there is no sound when the SAG is full CCW. My biasing procedure is:
1) SAG Full CCW
2 Pedal plugged in and engaged
3) Black probe in screw hole
4) Red Probe on biasing pad
5) Test trimmer full CW to full CCW
I'm still getting results between 3-5 volts. I suspect there is something wrong elsewhere but I'm struggling to figure out what.

Here are some fresh pictures:

 

[Ctrl4Smilerz]

I think I got it!. I tried biasing it using the negative terminal on the DC jack against the biasing pad and was able to get both JFETS to about 6.3 volts. Plugged my guitar in and it sounds great!