Mojito deluxe gain/crunch

[comradehoser]

This may or may not be a troubleshooting thread.

How much crunchiness should I be getting out of this? I am only really getting very tame breakup with boost engaged.

Unity without boost engaged is almost full CW, so it could maybe an issue with Q2 if Q1 is responsible for boost.

Seems pretty quiet and mild overall, so I'm thinking something went awry, as I don't always get on with SMD, and I thought I could rock it with some mmbf4392. Well. Separated a leg off one, so gives you an idea of my proficiency. How fryable are smd transistors?

On the other hand, I am also prone to "is it supposed to sound that way-itis" so I need some independent confirmation, and the only demo I unearthed was a frickin slide player, damn. I am not a slide player.

Ran an audio probe, and there's nothing that jumps out.




Gain pot maxed:

Q1 (bottom) mmbf4393
1: 3.03
2: 3.04
3: 2.48

Q2 (top right). Mmbf 4393
1: 1.95 (variable)
2: 3.10 (variable)
3: 0.5 (variable)

IC 1 ca3130ez
1: .088
2: 4.51
3: 4.23
4: 0
5: 0.88
6: 4.42
7: 9.02
8: 4.75

Ca3130 from Mouser, Mmbf4393 from Ppcb or mouser.

Pins 1 and 3 of Q2 look a bit low compared to Q1, and the fluctuation/instability with gain up make me think that could be it? Q2 is stable with gain down. But what do I know?

I miiiight have one last little smd buddy to try, but I didn't think so. I have bad luck with these guys in particular.

[Edit: tried reflowing Q2 following with what has worked for me in the past: liberal flux on the legs and a retouch with a solder loaded tip. Now I have no output at all ]

 

[comradehoser]

THIS LITTLE PEDAL!!!

Schematic I forgot to post last time:


I am officially not knowing what to do.

Resistors and capacitors look correct and in the proper orientation.

Found an extra mmbf4393, tried to sub out Q2, lifted the SMD traces.

Soldered resistor legs to the SMD legs and put in the through-hole slots. Checked out on my transistor tester as a jfet.

Continuity checks out through the whole top of the circuit. Only part I didn't check thoroughly was the power section and the diode loops with the bass pot coming off pin 2 of the IC.

I have a signal on leg 3 of Q2. Everything off of pin 1 and 2 is pretty much inaudible (vol, treble pots), as is the bass pot.

Oddly Q1 got quieter as I turned up the gain pot. Could I have installed it upside down?

Voltage checks out at 9.4 v between r102 and R17, but on the other side of R17, is down at 0.1v. All legs of Q2 read under 1v. Q1 and the IC pretty much are the same as before.

Overall pretty quiet through the circuit. Nothing over unity. Not hearing any clipping to speak of.

Stumped.

 

[comradehoser]

I remembered my troubleshooting advice to other people and went ahead and reflowed all of my joints including Q1 legs (but not Q2). Now I have clipping!

But I still don't have volume. It might barely get to unity with volume and boost cranked.

 

[Nostradoomus]

What was the vgs off when you checked the jfets with your tester?

 

[comradehoser]

I don't really remember, honestly. I just soldered everything together for Q2 and wanted to make sure the connections were good and it registered as a jfet before soldering into the through-hole slots.

I will check this evening, though.

 

[Nostradoomus]

Could just be misbiased…it’s usually the dang jfets

 

[comradehoser]

Now it's reading as a diode with the little hook probes and my transistor tester. It's soldered in, though, I don't know if that makes a difference

 

[Nostradoomus]

Yeah you can’t measure them in circuit like that. Have you got any spares?

 

[comradehoser]

Yeah you can’t measure them in circuit like that. Have you got any spares?

Nope, it was my very last one. Can desolder and test if it's crucial, though.

 

[Nostradoomus]

Nope let’s not go that far just yet haha. What are your jfet voltage measurements looking like now?

 

[comradehoser]

Q1
1: 3.022
2: 3.033
3: 2.533

Q2
1: 0.415 variable
2: 0.520 variable
3: 0.288 variable

IC about the same

Bad cap? I am starting to think it's another component or trace around the jfet, since they both exhibited this low variable voltage behavior. But continuity checked out, so ...?

 

[comradehoser]

I also found the old 4393 on my bench. Soldered some legs on it

What was the vgs off when you checked the jfets with your tester?

 

[Nostradoomus]

If you're okay with a bit of refuckulating, I'd put that one in Q2. The vgs off looks good. Maybe @Chuck D. Bones can chime in though before that but it seems something is out of spec.

 

[comradehoser]

Ok, can do.

R15 @ 470K is the bias resistor for the gate/JFET correct? (What is in there currently is in fact a 470K). I did have audio signal on the gate, so is the fault within the JFET, not amplifying correctly to legs 1 & 2? Or R15 doesn't have a stable ground path or something?

 

[comradehoser]

If you're okay with a bit of refuckulating, I'd put that one in Q2. The vgs off looks good. Maybe @Chuck D. Bones can chime in though before that but it seems something is out of spec.

Pulled the second, here's how it reads in the tester:


Not too different.

 

[comradehoser]

Soldered in a socket. Neither JFEt really makes a difference. Nor does it really if there is nothing there at all.

Tried to jumper different points around Q2 and the treble pot. Didn't change a thing, but I discovered I could audio probe just by alligator clipping to the output jack tip tab and clipping a resistor lead in the other end to touch points.

Result of the audio probe is that there is volume throughout the circuit until r13 (10k). At which point it assumes it's final diminutive level.

Maybe that's what is supposed to be happening and Q2 is the gain récupération stage, but as noted, nothing is going on. Something isn't right, but I can't figure it out at all.

I am sure there an underlying systemic fact that explains all of this, but it's got me super frustrated.

Next step I think might be to order some through hole PN4393 and see what goes down.

 

[comradehoser]

Took off the 4393 SMDs in case I somehow messed up traces or damaged them in my attentions, put in sockets and 2n5457s.

Swapped in the other CA3130EZ.

Still sounds like quiet ass. Strangled clipping and not even getting to unity with all pots maxed and boost.

On the interesting side, if I hold pins 1 and 3 on Q1, a slight increase in crappy gain happens.

 

[Chuck D. Bones]

This may or may not be a troubleshooting thread.

How much crunchiness should I be getting out of this? I am only really getting very tame breakup with boost engaged.

Unity without boost engaged is almost full CW, so it could maybe an issue with Q2 if Q1 is responsible for boost.

Seems pretty quiet and mild overall, so I'm thinking something went awry, as I don't always get on with SMD, and I thought I could rock it with some mmbf4392. Well. Separated a leg off one, so gives you an idea of my proficiency. How fryable are smd transistors?

On the other hand, I am also prone to "is it supposed to sound that way-itis" so I need some independent confirmation, and the only demo I unearthed was a frickin slide player, damn. I am not a slide player.

Ran an audio probe, and there's nothing that jumps out.

Gain pot maxed:

Q1 (bottom) mmbf4393
1: 3.03
2: 3.04
3: 2.48

Q2 (top right). Mmbf 4393
1: 1.95 (variable)
2: 3.10 (variable)
3: 0.5 (variable)

IC 1 ca3130ez
1: .088
2: 4.51
3: 4.23
4: 0
5: 0.88
6: 4.42
7: 9.02
8: 4.75

Ca3130 from Mouser, Mmbf4393 from Ppcb or mouser.

Pins 1 and 3 of Q2 look a bit low compared to Q1, and the fluctuation/instability with gain up make me think that could be it? Q2 is stable with gain down. But what do I know?

I miiiight have one last little smd buddy to try, but I didn't think so. I have bad luck with these guys in particular.

[Edit: tried reflowing Q2 following with what has worked for me in the past: liberal flux on the legs and a retouch with a solder loaded tip. Now I have no output at all ]

Q1 & Q2 voltages both look screwy. The gate should be at zero volts on both.

Let's refer to the pins as D, S & G, not pin numbers. I'll assume that you're referring to the pin numbers on the schematic. To be clear, this is the pinout:



IC1 voltages all look good. When measuring DC voltages with the power on, make sure the input is shorted to gnd and set GAIN to zero. We don't want picked-up hum to taint the measurements.

 

[comradehoser]

Q1 & Q2 voltages both look screwy. The gate should be at zero volts on both.

Let's refer to the pins as D, S & G, not pin numbers. I'll assume that you're referring to the pin numbers on the schematic. To be clear, this is the pinout:

View attachment 87505

IC1 voltages all look good. When measuring DC voltages with the power on, make sure the input is shorted to gnd and set GAIN to zero. We don't want picked-up hum to taint the measurements.

1,2,3 referred to d,s,g.

Well, like I always say, it's always a bit of an honor to have makers chime in on a problem solving thread, and thank you very much for being generous with your insights and some learnings for me.

Went ahead and reflowed every single thing with solder on it for the third time before I took fresh readings running socketed through-hole 5452s (since I didn't have the pn4393 yet), with input shorted, gain at zero, but I decided to take it for a jog before all that, and, well.

I don't know if it's a case of the creation recognizing its Maker or what, but it sounds like a proper gainy dirt pedal with unity around noon now and hopefully will stay proper.

I'll mess about with it to see what the gain range is like when I get the 4393s, and I also socketed the BA482 so I could give a report on those compared to three other possibilities of diodes.

But for now, I am very happy marking it from mysteriously problematic to mysteriously SOLVED.

 

[Chuck D. Bones]

I used BA482 because they have the same I-V curve as the JFETs BJ used in the pedals that the Mojito is based upon. He wired D & S together and used the gate-channel junction as a diode. Other diodes will work and the differences are subtle. LEDs will not work well because Vf is too high.

What drain voltages do you have now on Q1 & Q2?