XB-MB grounding issue

[Mike McLane]

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?

 

[PJS]

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

 

[Mike McLane]

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?

 

[Mike McLane]

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!?!?!?

 

[Mike McLane]

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.

 

[giovanni]

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?

 

[Mike McLane]

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.