Tried for the who-knows-how-many time to build this circuit and it is still not working. Please help.
New batch of SMD J201s from Mouser soldered onto adapter boards from Pedalpcb. So components should be good. But when the pedal is switched on, nothing but a whining sound. Reflowed all connections and ran a scalpel between each row.
Q1
D 4.42
S 0.10
V 0
Q2
D 4.8
S 0.12
V 0
Q3
D 4.05
S 0.08
V 0
Q4
D 4.10
S 0.06
V 0
Circuit source:
http://www.runoffgroove.com/uno.html
Circuit layout
https://www.sabrotone.com/wp-content/uploads/2013/05/UnoMarkI.gif
Let me know what else you need from me. Thanks
(https://i.postimg.cc/JtKrxHRQ/IMG-7497.jpg) (https://postimg.cc/JtKrxHRQ)
(https://i.postimg.cc/bdqY19fJ/IMG-7498.jpg) (https://postimg.cc/bdqY19fJ)
(https://i.postimg.cc/wyQ6S7Lg/IMG-7499.jpg) (https://postimg.cc/wyQ6S7Lg)
(https://i.postimg.cc/KRzZHczh/IMG-7501.jpg) (https://postimg.cc/KRzZHczh)
(https://i.postimg.cc/qz5kVfhh/IMG-7502.jpg) (https://postimg.cc/qz5kVfhh)
(https://i.postimg.cc/gLcYdmHR/IMG-7503.jpg) (https://postimg.cc/gLcYdmHR)
(https://i.postimg.cc/cgSx9QKF/IMG-7504.jpg) (https://postimg.cc/cgSx9QKF)
(https://i.postimg.cc/KkxbrxVV/IMG-7496.jpg) (https://postimg.cc/KkxbrxVV)
(https://i.postimg.cc/S2dhgHbK/IMG-7505.jpg) (https://postimg.cc/S2dhgHbK)
Which J201s did you buy? The InterFET ones Mouser has in stock now have a different pinout than the PedalPCB adapter boards.
https://www.mouser.com/datasheet/2/676/jfet_j201_j202_interfet-2887411.pdf
https://www.pedalpcb.com/product/sot23adapter/
I've read that drain and source are interchangeable on JFETs but I don't know how universally that applies.
I got these:
https://www.mouser.com/ProductDetail/onsemi-Fairchild/MMBFJ201?qs=sGAEpiMZZMsVgcksf1EMUjCB5v9MvAEa&countrycode=US¤cycode=USD
https://www.mouser.com/datasheet/2/308/1/MMBFJ202_D-2316065.pdf
So maybe you are right. I'll try rigging something up with wires tonight to switch the drain and source pins and see if it works
Also - it doesn't look like you've adjusted any of the trim pots - they are all set to minimum (or maximum). You need to adjust those until you have the correct voltages or thereabouts according to what's posted on the ROG page. Your drains look right, but sources are too low - does adjusting the trimpots help that?
I have adjusted those trimpots to get the drain voltages as close to 4.5V as I can. It is barely a nudge to get it to drop from 9V down to ~4.5V. The source voltages do vary if I adjust the trimpots but none higher than .13 and that is if I adjust the trimpot completely down (bringing the drain voltages to max) so I don't know what is going on.
I used some wires to switch the drain and gate inputs and nothing has changed.
The new drain still reads as 4.42 and the new source is still 0.10 (same as what it read before). And if I turn the trimpot the entire way down (which brings the drain up to 8.73) the most the source will read is 0.70.
Voltage on an JFET should be (with grounded input):
G 0V
D 4.5V
S 1.2V
Or similar. What is suspicious is that you have close to 0V at source, which indicates the possibility that your adapters are WRONG. Make sure they have correct pin order. If you have a transistor tester, it should be easy, if not use your multimetar set to diode range and measure GD and GS. If they differ more than few %, then pinout is not good.
If adapters are not good, then solder excess resistor or capacitor leads to SMD JFET and convert it to TTH that way. ;)
The adapters have the drain and the source reversed from these specific J201s as GGBB pointed out. I have already reversed the pins and nothing has changed. As the readings are the same, it seems that drain and source are interchangeable for these specific J201s
Try without adapters, solder leads to JFET.
To me, it seems that gate is reversed with source or drain, thus you get 0V on source.
Layout is made expecting JFET use DSG pinout. So check your datasheet for SMD parts, and if their pinout is like this, solder leads like shown and simply put into your circuit. Or just check with MM if your adapters are like this.
(https://i.postimg.cc/LqDngpkW/dsg.jpg) (https://postimg.cc/LqDngpkW)
I do not understand why you are thinking the gate is in the wrong location. These are the ones i bought
https://www.mouser.com/datasheet/2/308/1/MMBFJ202_D-2316065.pdf
The gate is clearly at the top which is where these adapter boards have the gate. It's the source and drain that are in the reverse location, not the gate
Readings with MM for J201
Top pin to lower left 1.837k
Top pin to lower right 1.823k
Lower left to lower right .585k
The gate is effectively the anode of a diode to the channel, either end of which is the cathode. Test lead polarity, therefore, matters when testing from the gate. Polarity won't matter when testing between source and drain as they are just the ends of the channel. A JFET channel is normally conductive, only when the gate voltage is made negative (or less positive) compared to the channel voltage does the channel resistance rise.
It's better to use the Diode test of the meter than resistance although that's assuming your DMM has it. It will show the forward voltage drop of the gate diode in millivolts. It will read as a normal silicon diode when the red lead is on the gate and the black lead is on either source or drain (those two really are interchangeable and you should check both).
It's always better to test out of the circuit and if you have to do it in the circuit, definitely with the power off.
Diode Test
Top (red) Bottom Left (black) 0.740V
Top (black) Bottom Left (red) no reading
Top (red) Bottom Right (black) 0.736V
Top (black) Bottom Right (red) no reading
Bottom Left (black) Bottom Right (red) 0.293V
Bottom Left (red) Bottom Right (black)0.315V
Can you use an audio probe to figure out where in the circuit the signal dies?
Quote from: freshmex18 on March 06, 2022, 02:50:38 PM
I do not understand why you are thinking the gate is in the wrong location. These are the ones i bought
https://www.mouser.com/datasheet/2/308/1/MMBFJ202_D-2316065.pdf
The gate is clearly at the top which is where these adapter boards have the gate. It's the source and drain that are in the reverse location, not the gate
We don't think the gate is in the wrong location! I (and maybe others) think that your adapters are not good! I wrote you that. And your MM readings (diode test) just confirm that transistor itself is good (as shown on my image from datasheet). Now do the same test on adapter pins where you soldered transistor, put black probe on third pin and check if red on pins 1 and 2 measure aroun 0.74V. If not, adapter sucks.
with an audio probe, the last place I have signal is the leg of C4, the 250p cap going into the treble pot.
Ah, ok. I misunderstood what you were saying before. I thought the issue was the j201 itself, not the pins coming out of the adapter board.
Diode test on the pins shows 0.753V when the red is on the gate and black is on the source but no reading when the red is on the gate and black is on the drain
Quote from: freshmex18 on March 06, 2022, 06:41:09 PM
with an audio probe, the last place I have signal is the leg of C4, the 250p cap going into the treble pot.
You have the signal before the cap, or after it? Is it stronger after Q2 or before it?
When measuring JFET, put red probe on gate, black probes on drain or source. Try to measure pins on JFET directly, gate should be the lonely one, drain and source are a pair on the opposite side. If you confirm measurement, then measure pins on the adapter. The gate should be the rightmost pin, left and middle are drain and source. If it doesn't measure as expected there, then adapter has the wrong pinout. If it measure good, then we can say adapter is good as well and discard that as the source of trouble.
If I understood well those readings:
Top (red) Bottom Left (black) 0.740V
Top (red) Bottom Right (black) 0.736V
it seems JFETs are OK. So measure adapter pins now and confirm they measure as expected.
I have signal when I put the probe onto the leg of the 250p cap that then goes into the treble pot. (Hole 21,E on the vero layout)
The signal is stronger out of Q1 than Q2
When I wrote "Top (red) Bottom Left (black) 0.740V, Top (red) Bottom Right (black) 0.736V" I was indicating the terminals/pins/leads whatever you call the metal things sticking out of the j201 itself when looking at it with the side with two pins (rain and source) facing down and the side with one pin (gate) facing up.
I did measure the pins exiting the adapter board themselves, which I indicated with "Diode test on the pins shows 0.753V when the red is on the gate and black is on the source but no reading when the red is on the gate and black is on the drain" Gate is the rightmost pin. Source is the middle pin. Drain is the leftmost pin. As i stated, I get a reading of "0.753V when the red is on the gate and black is on the source but no reading when the red is on the gate and black is on the drain." I also get no reading when I reverse the read and black probes from the MM
I'm happy with the JFET test. You now know how to identify the gate pin. That's a win 8)
I saw mention of a whining noise. It could be oscillating. There are x4 inverting stages so the circuit is non-inverting overall. If there is any coupling between input and output then you have a positive feedback loop. The coupling doesn't have to be a short circuit but radiated from output wires to input wires. Even the switch groups of the stompswitch can have enough coupling to cause feedback.
The bias setting problem could be due to the whine waveform which reading on DC will cause the DMM to show a false value.
An easy check for feedback is to temporarily ground the PCB input. The whine should stop and you have only circuit hiss that will change if you play with the controls and you can recheck the bias trimmers for drain and source voltage.
I should probably be more clear about what I wrote earlier. When i said I hear a whining noise, I meant I ONLY hear a whining noise. There is no guitar signal output. So it isn't like a high gain pedal that needs a shielded input wire to quiet the background whining noise like my Diezel VH4 pedal did.
I am still new to this hobby and have a very limited understanding of how circuits actually work as should be evident by my lack of understanding previous comments. I can basically plug and play components into a vero layout but I only partially somewhat understand how a circuit actually works. As such, I will frequently need clarifying information so I can follow along.
So by saying "temporarily ground the PCB input" you mean I take the input wire, add it to the ground clip of the test box along with the ground wires, and then play with the pedal knobs to see if there is any change in the whining and also use the MM to recheck the J201 voltages and make any necessary adjustments, correct?
Yes. Ground anywhere in the input signal feed path.
I'm not saying you have a feedback problem, but it's worth checking. When a circuit oscillates due to positive feedback, it can be a very high frequency and a spikey waveform that can block audio frequency in the transistors. Without an oscilloscope that would show that waveform, we have to look for other clues that point to it.
Then it's back to an audio probe to follow a signal through stage by stage from the input as already suggested.
So the signal comes out of Q1. Is it louder than when it went in? Does it change when you adjust the trimmer for Q1? About Q2, it goes in strong and come out weak. Does adjusting Q2 trimmer change anything? It should be strong enough to give overdrive out of Q2.
These "FETs with trimmers" circuits produce nothing unless the trimmer is in the right range, and you adjust by ear for a sweet spot which is not always mid voltage on the drain.
If you lose sound after the treble cap that suggests (either a bad solder joint or a )solder bridge shorting the signal out right there, in that "tone stack" of the three pots and three caps. Power off, test for a short (continuity to ground) from the leg of the Cap that goes to the treble pot.
Remember bad solder joints don't always look bad.
Also, with some "fender-ish" tone stacks, when all the knobs are on zero, nothing comes through at all. Make sure B, M, T are on at least halfway before deciding its broken!
Have you double-checked items values..??
(e.g. 1k instead of 1M Q3 Gate bias resistor could easily result into practically no signal..)
Grounded the input wire and the biases for all four resistors did not change. Still reading at ~4.5. Playing with the knobs did not do anything to change the sound coming out of the amp.
I have checked and rechecked every component value and everything is correct
Now, get this. I used an audio probe and signal going in and out of Q1 was about the same volume. Signal into Q2 was louder than out. So I adjusted the bias until signal was louder and overdriven. Went to Q3 where there was no signal and adjusted the bias until there was sound. Same for Q4. Voila! Pedal makes sound come out of the amp. Tone stack works properly. Went and checked the drain voltages of the now adjusted Q2, Q3, and Q4 and all of them read 0v. So...now what? apparently, 4.5V is too much adjustment as it chokes the signal from reaching the output of the pedal. So...where do I aim for proper adjustment? I know everyone says by ear but I don't know what I am listening for. It doesn't help that there are only a few videos of this pedal online with sound clips.
Congratulations.
Adjust by ear: you put in a guitar signal (or guitar level signal like a looper).
The best method is to use your probe to listen to each Q1 with a probe: Drain of Q1, then D of Q2, then etc.
As you turn the trimmer, you will hear nothing, then a weak signal, then the signal starts to sound good, then it sounds worse, then nothing. You use your ear and pick a point in the middle that sounds right to you.
Repeat. Sometimes a little on the weak side is OK if the circuit is clipping too much later on. So you may go back and tweak.
Trimmers on the Drain is not an ideal set up, and commercial pedal builders use matched FETs that work with fixed Rs. There is no guarantee that your four FETs will ever sound "just like" somebody else's. It's a neat trick that allows hobbyists to get usually pleasant and useful sounds from a mixed bag of FETs.
Now I am wondering what the issue is since I build a Keeley Katana that had the same issue: output way below unity with the bias set at half of the supply voltage but dropping it way down to ~10% of supply voltage made it sound great. Does that mean I have bad luck getting transistors within spec since those are 2N5484 and not J201s? Or is that just the nature of different production runs and time that specs vary enough and you just have to adjust your builds accordingly? Is there a general rule of thumb that you should adjust every transistor to above unity and then tweak from there instead of to half of the supply voltage?
After adjusting by ear, current biases are Q1 0.42V, Q2 0.08V, Q3 0.07V, and Q4 0.06V. Doesn't that seem unusually low as the schematic says to bias at ~4.5? I can't crank up the amp and hear it at proper volume as it's so late but at below-bedroom levels it sounds good.
Having a drain resistor variable is a simple answer to the wide gate control voltage spec of the JFET. To fully turn off a j201 (with 0v gate) the source volts can be anywhere from 0.3v to 1.5v. You don't want them biased fully off so the numbers suggest a working bias must produce a source voltage >0.3v and that 1v would be a rough average to hope for.
Probably to suit any random j201 that meets its specs the source resistor should also be a trimmer.
To my mind, the drain trimmer might be better at a lower value with a fixed resistor in series. Now when there are designs with fixed drain resistors it's commonly chosen for about 18k, so I'm not sure why 100k is picked for the trimmers. I'd have tried 10k fixed and a 10k or 20k trimmer. The object is to make fine adjustments easier to carry out.