M800 - No Volume

[DrumBuster]

Hello everyone, after neglecting my project for almost a year, I finally built my M800, when trying it out, I noticed I had no volume. Bypass works great, j201s all biased to the right voltage (4.5v ish) save for one of them not doing it. It stays stuck at 2.7, is it possible that is the culprit? I got a 2N5458 just in case is a faulty transistor. I check every solder and ground and everything seems fine, led lights up. What it can be the issue? Thanks

 

[DrumBuster]

Can you confirm what is written on the JFet's, ie J201's. This pedal is Fussy about what is used!
We need to see the Solder side of the Board.

 

[DrumBuster]

Got them off Small Bear back in oct 2019, this side of the board has a rosin mess caused by the rosin core solder. I’m in process of cleaning it with 80% isopropyl alcohol

 

[music6000]

Got them off Small Bear back in oct 2019, this side of the board has a rosin mess caused by the rosin core solder. I’m in process of cleaning it with 80% isopropyl alcohol

You will have to bend those bottom 3 pots up & Clean under that!
Can you write what is on the JFET's?

 

[DrumBuster]

Can you write what is on the JFET's?

Sure, it says J201 07 2.

 

[DrumBuster]

You will have to bend those bottom 3 pots up & Clean under that!
Can you write what is on the JFET's?

Can I brush under it?

 

[Chas Grant]

Does the LED light when you engage the effect?

 

[music6000]

Honestly, I don't use Isopryl or a Brush.
I use CRC Automotive Contact Cleaner, It's Brilliant!
Just spray & watch the Crud fall off. In your case, you may have to spray half a dozen times!
In the USA, you can purchase CRC Contact Cleaner.

 

[Chas Grant]

Honestly, I don't use Isopryl or a Brush.
I use CRC Automotive Contact Cleaner, It's Brilliant!
Just spray & watch the Crud fall off. In your case, you may have to spray half a dozen times!
In the USA, you can purchase CRC Contact Cleaner.

Do you have an issue with it removing certain component markings? I used it on a PCB with white Kemmet caps and it removed the markings on the top on the caps.

 

[DrumBuster]

Honestly, I don't use Isopryl or a Brush.
I use CRC Automotive Contact Cleaner, It's Brilliant!
Just spray & watch the Crud fall off. In your case, you may have to spray half a dozen times!
In the USA, you can purchase CRC Contact Cleaner.

I have CRC electronics cleaner, I should get CRC 5103, right?

 

[DrumBuster]

Does the LED light when you engage the effect?

Yes! The blue led is super bright!

 

[Chas Grant]

OK you have power to the circuit. I was wondering because the ground wire from the DC jack to PCB has a questionable solder joint. You may need to reflow all the solder joints. Also do you have an extra J201, if so try swapping each one out, one at a time.

 

[DrumBuster]

OK you have power to the circuit. I was wondering because the ground wire from the DC jack to PCB has a questionable solder joint. You may need to reflow all the solder joints. Also do you have an extra J201, if so try swapping each one out, one at a time.

I have a 2n5458, some guy recommended it to me to replace the faulty j201 if needed, if the other transistors are biased at 4.5v, they work, right?

 

[DrumBuster]

I have a 2n5458, some guy recommended it to me to replace the faulty j201 if needed, if the other transistors are biased at 4.5v, they work, right?

The tone trim doesn’t affect the transistor, does the transistor need to have 9v by default, right?

 

[Chas Grant]

The tone trim doesn’t affect the transistor, does the transistor need to have 9v by default, right?

Not Necessarily. A JFET is constructed differently than a BJT transistor. The biasing trimmers are setting the voltage on the drain. With no input to the pedal you should have zero volts on the gate. Voltage on the drain should be same as voltage on the source. When you apply the input and it is going negative, the JFET will start to restrict the electron flow through the device and as you reach pinch off voltage, will stop the electron flow. Now drain voltage will go up and source voltage will go down. An easy way to think about it is like a garden hose. Turn it on and water flows freely, a JFET with out gate voltage. Now start to pinch the hose with your hand, water flow is reduced, pressure up stream of pinch goes up. When you pinch hard enough, flow stops. This is like a JFET as gate voltage is applied, when gate voltage reach Vgs off it doesn't conduct.

So by bias voltage, I assume you are talking about drain voltage, which is pin 1 on a J201.

Also I'm not so sure a 2N5458 will substitute for a J201 in this circuit. A J201 has a Vgs off of -.5 to -1.5, a 5458 has a Vgs off of -1 to -7, what you would have to do is test a few out and find one that is in the -1 range. If it was a JFET buffer, than most likely it would work, but this is being used as an amplifier.

 

[DrumBuster]

Not Necessarily. A JFET is constructed differently than a BJT transistor. The biasing trimmers are setting the voltage on the drain. With no input to the pedal you should have zero volts on the gate. Voltage on the drain should be same as voltage on the source. When you apply the input and it is going negative, the JFET will start to restrict the electron flow through the device and as you reach pinch off voltage, will stop the electron flow. Now drain voltage will go up and source voltage will go down. An easy way to think about it is like a garden hose. Turn it on and water flows freely, a JFET with out gate voltage. Now start to pinch the hose with your hand, water flow is reduced, pressure up stream of pinch goes up. When you pinch hard enough, flow stops. This is like a JFET as gate voltage is applied, when gate voltage reach Vgs off it doesn't conduct.

So by bias voltage, I assume you are talking about drain voltage, which is pin 1 on a J201.

Also I'm not so sure a 2N5458 will substitute for a J201 in this circuit. A J201 has a Vgs off of -.5 to -1.5, a 5458 has a Vgs off of -1 to -7, what you would have to do is test a few out and find one that is in the -1 range. If it was a JFET buffer, than most likely it would work, but this is being used as an amplifier.

Thanks for this info! I will check tomorrow with the meter, can’t find mine so will have to borrow one. Really hope is just a solder issue. Will have to order new JFETs if mine are fried, saw Mouser has them new for $3.49.

 

[Chas Grant]

Thanks for this info! I will check tomorrow with the meter, can’t find mine so will have to borrow one. Really hope is just a solder issue. Will have to order new JFETs if mine are fried, saw Mouser has them new for $3.49.

Read up on JFET's in the forum, Chuck's Boneyard. He knows his stuff STONE COLD. I have a working knowledge of them, but really didn't see them until I started down this rabbit hole of a hobby awhile ago. Most other components, I have worked with for quite a long time, those JFET's though, they have bitten mine, and most peoples, behinds once or twice.

 

[DrumBuster]

Read up on JFET's in the forum, Chuck's Boneyard. He knows his stuff STONE COLD. I have a working knowledge of them, but really didn't see them until I started down this rabbit hole of a hobby awhile ago. Most other components, I have worked with for quite a long time, those JFET's though, they have bitten mine, and most peoples, behinds once or twice.

Will do! Thanks, I saw the PF5102 are super close on VGS(off) but it has a different idss. Will ask Chuck just to be sure.

 

[Chas Grant]

One other thing to try is an audio probe. You can make on easily. I've got one on a jack and one on an alligator clip lead. I've also seen people suggest using an old multimeter probe ( which is a great idea. But its basically a 100nF cap with a short stiff wire on one lead and a wire thats long enough to allow you to work and have it connected to a jack so that you can run it into the amp. Connect the jack ground to pedal ground, now the probe is the input to the amp and you can check for audio at various places in circuit. I find it easiest to start a input and work my way to output. If you look at the schematic you can follow the audio patch through the circuit, its usually pretty straight. If you lose the signal you can find out where and concentrate there.

 

[DrumBuster]

One other thing to try is an audio probe. You can make on easily. I've got one on a jack and one on an alligator clip lead. I've also seen people suggest using an old multimeter probe ( which is a great idea. But its basically a 100nF cap with a short stiff wire on one lead and a wire thats long enough to allow you to work and have it connected to a jack so that you can run it into the amp. Connect the jack ground to pedal ground, now the probe is the input to the amp and you can check for audio at various places in circuit. I find it easiest to start a input and work my way to output. If you look at the schematic you can follow the audio patch through the circuit, its usually pretty straight. If you lose the signal you can find out where and concentrate there.

Interesting, will make one tomorrow, I’m sure I have a 100nf cap somewhere, is there a video / tutorial I can check out?

 

[Chas Grant]

If you Google "audio probe pedal" videos will come up, along with how to make them. Cap size isn't too important, any size cap above 10n to 220n will work. It's primary purpose is to keep DC from being passed to the amp if you touch the wrong lead.