MXR Blue Box Assistance/Question

Dr Tony Balls · Post by **Dr Tony Balls** » 29 Dec 2022, 21:15

Howdy All,

Been messing around with the MXR Blue Box circuit and i'm running into an issue. Basically i'm getting a decrease in volume the higher the note is that i'm playing. Notes played on the low E string are loud and clear and as I move up in frequency the volume drops to the point where an open high E is almost inaudible. This happens on multiple guitars and is slightly improved by using the neck pickup and/or rolling back the tone control, but its still present regardless.

Its been AGES since I built one of these and I dont remember this being the norm. Is it? It doesnt seem to be from various demo videos.

For reference i'm using the sabrotone vero layout (https://www.sabrotone.com/request-mxr-b ... ro-layout/) and building the circuit stock. Ive added a gain stage at the end of the circuit (fixed gain SHO type) for volume makeup, but even bypassing that gets the same results.

Schematic: http://www.tonepad.com/getFile.asp?id=73

Ideas? Thoughts?

CheapPedalCollector · Post by **CheapPedalCollector** » 29 Dec 2022, 21:53

Did you maybe mix up C3 and C4?

Dr Tony Balls · Post by **Dr Tony Balls** » 30 Dec 2022, 14:20

CheapPedalCollector wrote: ↑29 Dec 2022, 21:53 Did you maybe mix up C3 and C4?

Nah, those are correct. I should mention that i've checked my build a few times now. Accuracy to the layout, and that all the parts are of the correct value. I started with 2N3904 transistors and had the problem, then tried replacing them with MPSA18 (the reissues came with those) but no difference. Also reflowed the entire board to no avail.

CheapPedalCollector · Post by **CheapPedalCollector** » 30 Dec 2022, 15:32

Strange, have to get the scope out to check signal strengths, or there isn't enough gain to keep the squelch circuit open perhaps.

grrrunge · Post by **grrrunge** » 30 Dec 2022, 23:07

It sounds like you have a too low corner frequency in a high cut filter somewhere. Try removing c3 and c4, and see if the issue persists.

Dr Tony Balls · Post by **Dr Tony Balls** » 31 Dec 2022, 16:38

grrrunge wrote: ↑30 Dec 2022, 23:07 It sounds like you have a too low corner frequency in a high cut filter somewhere. Try removing c3 and c4, and see if the issue persists.

THANKS for the assist. I removed C4 and high pitched squealing all around, but when I replaced it used a different cap which solved the issue just fine. I measured the original cap and it was 100nF, not 1nF! Turns out a certain dummy (me) had a 100nF cap in his 1nF drawer, and when I went to inspect the build and check part values I didnt see the little dot that made it read ".1K". I read it as "1K" aka 100 pF and thus the user error.

MXR Blue Box Assistance/Question [SOLVED]