# XB-MB grounding issue



## Mike McLane (Dec 7, 2021)

I just finished an XB-MB.  I plugged in power and got the "red light" before I fastened the jacks and tightened the other nuts, but when I plugged it in to try it out it was dead. . . no LED and the positive jack terminals are NOT contacting the enclosure.  I get an LED when the jacks are ungrounded (removed from the enclosure), but loose it when they contact the enclosure.  Also, if the jacks are ungrounded (active LED) and the guitar cords are plugged in the thing goes dead as well.  Any suggestions?


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## fig (Dec 7, 2021)

Did you remember condoms for the bottom 3 pots?


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## Mike McLane (Dec 7, 2021)

Yes, all insulated.  No STD's here!


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## JamieJ (Dec 7, 2021)

We’ll need to see the underside of the board to get a better idea. Do a really good visual inspection. You might have a solder bridge somewhere.


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## fig (Dec 7, 2021)

_Check pin 1 of the bottom IC....it may be folded under._




Nah...that's the OUT pin..

Jamie's got the right idea.


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## Mike McLane (Dec 7, 2021)

Found it.  The foot switch did not come with an insulator washer.  I thought it may have been internally insulated, but when I put an insulating washer in everthing came together. . . . KINDA.  Now when testing the volume is absolutely off the wall!!!!!!  Its coming from the GAIN knob, plenty of distortion and more volume than I've ever heard out of a pedal.  The VOLUME is "working", but only minimally effective.  EQ seems to be working OK.  I'm going to double check the values of the components.


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## Mike McLane (Dec 7, 2021)

OK.  All the component values seem to check out.  There's no schematic in the build docs (not that I could make much sense out of them), but I'd be looking for some components that if poorly soldered could accidentally create a "jumper" around the volume pot.  But the bottom side of the PCB looks clean and tight.


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## PJS (Dec 7, 2021)

I am a bit suspicious of the insulating washer curing a ground issue.  The button of the footswitch is not electrically connected to anything, so insulating it or not should not make a massive difference.  On some pedals I have had a bit of noise with it insulated that went away when it was grounded.  I am not sure that you fixed much by doing that though.  Hopefully sorting the volume problem will also sort the other symptoms properly.


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## Stickman393 (Dec 7, 2021)

Sounds less like a ground issue and more like a short.

Some switching power supplies that I've used will cut out when exposed to a short...could be that your switch chassis is hot...or one of your potentiometer bodies (though that seems less likely, given that the insulating washer helped the situation)  That wouldn't be noticeable until you offer up a path to ground: the input and output jacks would complete the circuit and cause the power supply to cut out.

Simple test with the multimeter...pull the jacks, put one probe on the switch, and another on the metal of the switch.  See if you get a DC voltage.


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## Mike McLane (Dec 7, 2021)

You're right.  The foot switch insulator was just coincidental.   Pedal is working without it albeit with the same anomalies. . . pedal sounds like its at full distortion. . . GAIN simply functions as a volume control. . . . everything else modifies the tone somewhat.


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## Mike McLane (Dec 8, 2021)

I've spent some time fiddling with this thing.  First, I had my understanding of GAIN vs VOLUME reversed.  The VOLUME seems to work as advertised (EQ as well), but the signal that's hitting the VOLUME control seems off the chart.  If GAIN is designed to go from 0-60, this thing seems to go from 50-150.  No "clean boost" on the low end of the diaI, starts out gritty and gets real raggedy on the top end, like a FET driven pedal.


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## giovanni (Dec 8, 2021)

I built two of these and what you describe is definitely not normal. I can easily get low drive/volume tones. Not sure what the problem may be unfortunately.


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## Mike McLane (Dec 8, 2021)

Dear Mr. PedalPCB, when do you think you will have the schematic up on the build docs.


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## PJS (Dec 8, 2021)

Even without the schematic you can start to work out a few things.  What are the voltages on your IC pins?


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## Stickman393 (Dec 9, 2021)

Mike McLane said:


> Dear Mr. PedalPCB, when do you think you will have the schematic up on the build docs.


Oohhhhh, careful.  You don't wanna Bugg him.

Yup.  Been holding onto that one in my back pocket for the last six months or so.


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## Mike McLane (Dec 9, 2021)

Both IC's read identically:

Source voltage: 9.38vdc

1-3 & 5-7:  4.56vdc
4:  0
8:  9.14


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## PJS (Dec 9, 2021)

They look good.  OK.  We know the IC pinouts.  An audio probe will give you an idea about which op-amps are sounding OK, and which are not.  That will help you narrow down where the problem occurs.  You can follow the traces on the board either visually or with a multimeter and have a good visual inspection of all solder joints and part values in the suspicious area.


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## Mike McLane (Dec 9, 2021)

You'll have to enlighten me on the audio probe.


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## Mike McLane (Dec 9, 2021)

I've found how to construct an audio probe and understand the general principal of tracing through the signal path of the pedal looking for the "bad spot".  I assume that since my issue is with an abnormally hot signal I want to work through the signal path and search for a point where the signal abruptly increases in volume??  Hmmm, guess that brings us back to the schematic doesn't it?

PS Stickman:  Like my new avatar?


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## Robert (Dec 9, 2021)

Schematic is in the PDF.

https://docs.pedalpcb.com/project/XB-MB.pdf


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## Mike McLane (Dec 7, 2021)

I just finished an XB-MB.  I plugged in power and got the "red light" before I fastened the jacks and tightened the other nuts, but when I plugged it in to try it out it was dead. . . no LED and the positive jack terminals are NOT contacting the enclosure.  I get an LED when the jacks are ungrounded (removed from the enclosure), but loose it when they contact the enclosure.  Also, if the jacks are ungrounded (active LED) and the guitar cords are plugged in the thing goes dead as well.  Any suggestions?


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## PJS (Dec 9, 2021)

You can trace through without a schematic, but now the schematic is here it definitely helps a lot
As you go through too bear in mind that in a pedal with gain, you are expecting some volume increases through the circuit.  I am by no means an expert, but would expect the primary gain around IC 1.1, and a bit around IC 2.1, but the gain control should in a working system be able to wind it back a lot judging by the reports from other users


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## Mike McLane (Dec 10, 2021)

WoooHooo for Mr. PedalPCB!  Thx for the schematic. . . . .

I went through pins 1,2,3,5,6,7 on both IC's.  I assigned guesstimated volume values (0-5) to the outputs with VOLUME and GAIN full CW:
1.1 - 5            2.1 - 5(+)
1.2 - 3            2.2 - .5 (barely audible)
1.3 - 3            2.3 - .5
1.5 - 3            2.5 - .5
1.6 - 3            2.6 - .5
1.7 - 3            2.7 - 5(+)

Pins 2.2-2.6 were barley audible.  I tried alternate IC's with no change in results.  Unless these numbers indicate something amiss I think the thing has to be working as intended, but it is hugely loud and GAIN goes very aggressive early in the sweep.  If so, might there be a way to soften the GAIN?


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## Mike McLane (Dec 11, 2021)

I was looking at the schematic for the Tommy III because it has a toggle that selects btw higher vs lower gain.  I see that it is effected by adjusting the capacitance and resistance feeding off of VREF into the "top of the diode clipping network".  Would making mods to R7 & C4 in the XB-MB circuit adjust the gain characteristics of that circuit in the same manner?  Also, I note that in many cases the poly film caps we get from typical sources (Tayda, Small Bear, etc) can easily be of by 20% or more.  If C4 is off by that amount could it account for a significant variance in the gain characteristics.  Inquiring minds want to know!?!?!?


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## Mike McLane (Dec 11, 2021)

OK. . . I pulled R7 (4K7) and put in a 10K trimmer.  Gain doesn't ramp up as fast at 10K, but the tone goes dark.  I'm guessing that's the change in the resistor-capacitor relationship.  Anyway, it doesn't appear that this is the area of concern.  Wherever the GAIN is set the thing is L O U D.  Once you pass 10:00 on the VOLUME knob your beyond useful.  What would be the effect of looking at R14 which appears to "meter" the input signal into IC2?  Any thoughts would be appreciated.


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## giovanni (Dec 11, 2021)

I still have the impression that there is something wrong with the build. I built two of these and the pedal doesn’t sound particularly loud to me, and I have built a fuzz once that was untamable (never figured that one out), so I think I know what you mean. Are you sure you have all the correct components?


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## Mike McLane (Dec 11, 2021)

I checked all of the component values and they check out.  I'm clueless at this point.  I'm going to try remelting all of the solder joints and see what happens.  Beyond that, c'est la guerre.


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