XB-MB grounding issue

Mike McLane

Mike McLane

Active member
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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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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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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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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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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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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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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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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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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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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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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