Chuck D.Bones Mojito Mk3 switching issue

[Kris100]

Hi All, I just build this Mojito Mk3 overdrive based on the attached vero layout from dirtboxlayouts and while it generally works when engaged, the bypass switching is not working well - ie when I hit the footswitch, there is always a bit of delay before the pedal turns on (signal seems to quickly fade in), same happens when I turn it off. I wired it in true bypass following Madbean's wiring scheme which I am also adding as attachment for reference. The layout has not been verified before and the thread on Mojito MK3 in this forum no longer includes the schematic so I can't really troubleshoot apart from checkiing my build against the layout which seems to be good. I checked the following:

- strip cuts and I double checked there's no continuity across any of them
- placement of links and continuiuty across them
- all component placement including polarity of diodes and caps
- I double checked there's no unintended shorts between strips
- all components were tested before being soldered in (I'm using some salvaged caps from old tape players etc but always test before using)

Perhaps there's someone here who built this on vero and can relate and share some ideas for troubleshooting or see something obvious on the layout I used which is not correct

 

[z2amiller]

Doublecheck your footswitch wiring. When there's a bypass related problem it's the footswitch 99.99% of the time. (Because at that point the signal isn't even going thru the effect)

My guess is that it's wired in a way that ground is only connected to your board when the footswitch is activated. When it isn't activated, it has no power - so once you hit the footswitch it effectively has to 'boot up' - from what I can read from this circuit that means charging the 100uF input filtering capacitor through the 47R filtering resistor (just a few milliseconds) and then charging the 10uF VREF capacitor thru the voltage divider made by the two 47K resistors (much slower - just over 2 seconds) So make sure that ground is always connected to the effect and the only thing that the footswitch is switching is the bypass and the LED.

(Also check for possible shorts when it's bypassed)

 

[Kris100]

Thank you for the suggestion. I went around the board and measured voltages which seem fine - both when board is bypassed and when it's engaged there's constant ca9v present. There's also no brief dip in voltage across ICs when I hit the footswitch. All ground connections are solid and board is grounded at all times. I suspected the footswitch at first but when I check for continuity it looks to be working fine - solid switching across all poles with no shorts etc.

 

[comradehoser]

I'm no electronics engineer, and have no experience with vero per se, but it might be useful to offer a look at the solder side as well to see what's up with the connections and cuts. Particularly as it strikes me that it would be easy to make a transposition/mirroring error in vero.

You can also just try rewiring the 3PDT and see if that makes a difference. Robert's way is straightforward and logical for me, and out of habit, probably, I stick with that.

Having said that, usually 3PDT issues are usually binary: it works/it don't work. But, I have had weird ass crackles and volume jumps due to poor soldering, so there's that. In this case, I concur that it sounds like it might be related to something with a capacitor charging, and @z2amiller 's idea makes sense to me, but if you have 9v after d7 with the switch off, then that rules it out.

The thing that solves my problems 9/10 times is to grab a schematic, and go through the troubleshooting order of operations:

1. Visual check of components and connections

2. Check every single component value against the schematic, and check for correct orientation of polar/pinout components (capacitors, diodes, ICs, and transistors). Since it looks like you have a lot of vintage caps, I always check the readings on those before installing in the circuit. Could be a bad cap, maybe?

3. Device-assisted checks:
1) voltage readings to the ICs and transistors (DMM)
2) continuity checks between components, off board elements, and ground (DMM)
3) audio probe (homebrew)

4. Reflow

 

[Kris100]

I'm no electronics engineer, and have no experience with vero per se, but it might be useful to offer a look at the solder side as well to see what's up with the connections and cuts.

You can also just try rewiring the 3PDT and see if that makes a difference. Robert's way is straightforward and logical for me, and out of habit, probably, I stick with that.

Having said that, usually 3PDT issues are usually binary: it works/it don't work. But, I have had weird ass crackles and volume jumps due to poor soldering, so there's that. In this case, I concur that it sounds like it might be related to something with a capacitor.

The thing that solves my problems 9/10 times is to grab a schematic, and go through the troubleshooting order of operations:

1. Visual check of components and connections

2. Check every single component value against the schematic, and check for correct orientation of polar/pinout components (capacitors, diodes, ICs, and transistors). Since it looks like you have some vintage caps, I always check the readings on those before installing in the circuit.

3. Device-assisted checks:
1) voltage readings to the ICs and transistors (DMM)
2) continuity checks between components, off board elements, and ground (DMM)
3) audio probe (homebrew)

4. Reflow

You're right, I ahould have made some picks of the underside of the board when it was all apart, will look into that.

I checked all components (both placement and values, including measuring all caps before soldering in as they obviously could be an issue). Problem with checking this build is that schematic is not available, just the vero layout where the person who made it included a link to ppcb forum thread where schematic was meant to be available. Vero layout was not verified to be correct but I assumed that even if its wrong I can fix it with the schematic... Never knowing it was taken down from the forum thread

I'll keep checking but appreciate all your advice.

 

[comradehoser]

What Mark __?__ is the PPCB Mojito Deluxe board?

https://docs.pedalpcb.com/project/MojitoDeluxe-PedalPCB.pdf

There's a schematic there, in any case.

 

[comradehoser]

You could also shoot a polite PM to @Chuck D. Bones who is the originator of the circuit.

 

[Kris100]

What Mark __?__ is the PPCB Mojito Deluxe board?

https://docs.pedalpcb.com/project/MojitoDeluxe-PedalPCB.pdf

There's a schematic there, in any case.

This one is some previous iteration of this circuit. I'll do more checking just to be sure this isn't some stupid mistake on my end and if I don't see anything I'll reach out to Chuck to see if he would be willing to help. I didn't want to bother him to be honest

 

[The Grand Wazoo]

The thing that solves my problems 9/10 times is to grab a schematic, and go through the troubleshooting order of operations:

Thank you, @comradehoser, your order of operations list was worth the price of admission. You’ve given me the OSI model for diy pedals. “Start at layer one!”

 

[comradehoser]

I mean that’s MY order of operations. Other folks might have slightly different ones. I used to have “bang with hammer” and then “bang self with hammer”, but this is the productive sequence for me.

 

[z2amiller]

Can you measure the VREF voltage when it fades in/out? (Maybe at pin 6 of IC1?) Probably best to measure against the DC jack sleeve since your GND might be doing weird things.

When you say that it fades out - does it fade out when you bypass it? Or when you turn it off?
Does changing the volume knob affect anything when bypassed (including fade-out?)

My guess is still that something is causing it to dump VREF which has a ~2 second charging time.

 

[Kris100]

Can you measure the VREF voltage when it fades in/out? (Maybe at pin 6 of IC1?) Probably best to measure against the DC jack sleeve since your GND might be doing weird things.

When you say that it fades out - does it fade out when you bypass it? Or when you turn it off?
Does changing the volume knob affect anything when bypassed (including fade-out?)

My guess is still that something is causing it to dump VREF which has a ~2 second charging time.

I did, Vref is 100% stable with no changes in voltage that my DMM can pick up - stays around 4.4v with general voltage about 8.7V going in after the polarity protection diode. When I say that signal fades in and out it's not slow - it literally takes fraction of a second, it's not slow and gradual taking 1-2 seconds. Still it's perceptible enough that you couldn't comfortably use this during a song withour hearing the brief pause. The issue happens when I press the footswitch to bypass the circuit or engage it. I plugged the voltage in well beyond 2 seconds for the pedal to charge up before testing. When bypassed, changing volume does nothing as expected. It does not affect the switching - including the issue with switching.

I spent more time on this today, went through the below steps:

1. replaced the 3pdt footswitch with a new one just in case, didn't help
2. scraped and cleaned the underside, especially and "inbetweens" so no strips get accidentally shorted - still the same. I am including some pics post cleaning for reference.
3. I replaced all electrolytics (measured those I took off which looked fine) - still the same.
4. I decided to replace all caps with regular modern ones (which I also measured before soldering in) - still the same, the caps I took off measured within spec btw. Pics of the board after all rework also attached.

I think I've done what I could, unless I can be sure the vero follows original schematic exactly and there's no errors this could be difficult to troubleshoot further - which is the big question mark, it could easily have some component values missed or need other fixes

 

[comradehoser]

Trust me, systematically going about the troubleshooting order of operations will help.

According to the Mojito Deluxe schematic--and I doubt the architecture could vary immensely much in your version even though I have no idea what work the inserted TL072 in your version is doing--it could maybe be something around the transistors since they are boosting signal into the clipping section and boosting signal after clipping. Maybe if one of them is not being activated correctly, it could cause a signal lag while it turns on? I'd check the pinout and orientation. JFETs have funky orientations. Sometimes they work in an incorrect configuration, but not optimally.



I do note, also, that the CA3130 pins 4 and 7 are implicated in the power section prior to power running to VCC for the transistors and VRef to pins 2 & 3 of the CA3130. Might check that the chip is kosher and the connections/components around it also.

 

[Chuck D. Bones]

I replied in a DM. The problem was due to an error in the Vero layout. One incorrect connection of an anti-pop resistor. I do not know if that is the only error on the board. I generally do not support other peoples' board designs based on my schematics.