[HELP] True Bypassing and modding an MXR Blue Box [w/pics]

[HeaD]

Hi all, Can someone help me with my reissue MXR Blue Box? I've some questions:

1 - What is the most effective way to true bypass it? (I've a small pedalboard and I can't use an external true bypass box)

a) There is not enough space to use a 3pdt switch, should I cut the pcb to fit it?

b) I've found a schematics here http://www.tonepad.com/getFileInfo.asp?id=73 but I've noticed some differences... I don't understand if in my pedal there is a buffer circuit... if yes, where it is? Must I exclude it?

c) I can't seem to figure out where the circuit input and output are. 

2 - It's possible to increase the volume output without to introduce background noises and original sound modifications? Maybe increasing the blend pot?

Please help me. Feel free to edit these pics with a paint editor if it is necessary. Thanks so much.

[HeaD]

anyone? 

[zachomega]

That is going to be really tricky with that board mounted switch.  I wouldn't cut the board though.  That is asking for trouble.  Sorry I can't be of more help. 

-Zach Omega

[HeaD]

Thanks Zach... btw i see that many others did it with the phase 90 reissue...

any other ideas? Thanks

[rockgardenlove]

If it's sucking tone, I'd personally try and mod the switching circuit to fix it that way.

[Ben N]

I understand you don't have enough room for an external bypass box, but how about a larger Hammond-say a 1590BB?  You would have to desolder the switch and one (or both, for best mechanical stability) of the jacks, but you would not have to alter the board at all.   Plus, you might be able

Ben

[HeaD]

Mmmh thank you Ben, it's a solution, but I think that there is an electronical buffer circuit in it... I should take away it and understand where the input and output are... 

If it's sucking tone, I'd personally try and mod the switching circuit to fix it that way.

Yes... but, where it is? 

Pheraps need the original schematics.... does someone have it?

[Ben N]

I see I lost my train of thought...  What I was starting to say was that by up-boxing  you might have room to add something useful, such as a switchable 2nd-order low-pass filter in front to improve tracking.

I don't recall whether the Blue Box (original or reissue) has any buffering.  It is certainly not like a Boss pedal, if that is what you mean.  Typically, the classic MXR configuration was old-fashioned mechanical non-true bypass, which left the inpput of the effect always connected to the signal chain, and the output switched between the input (bypass) and the effect.  When they incorporated DPDT switches, they just used the extra pole for switching the LED indicator, but didn't change the actual effect switching.  At least that is my understanding.  If that is the case here, there is no buffer in your signal chain in bypass mode.  However, when the effect is bypassed, the effect input impedance is in parallel with whatever impedances precede and follow it, and that can result in tone-sucking.

That being said, there would be no need to bypass any buffer if there is no buffer.  OTOH, if there is a buffer, it may be needed for operation of the effect.  I would just try to confirm that one pole of the existing switch is connected to the output jack.  If so, you can wire 2/3 of the 3PDT to the board exactly the way the DPDT is now.  Unsolder the input jack and use a chassis mount stereo jack instead.  Now all you have to do is figure out how to wire the third pole of the switch and its throws between the new input jack and the effect input--it is too mmuch for me to explain here, but if you diagram it you can figure it out.  Also, make sure to find the old jack pad on the board that connected to the battery, and wire that to the ring lug on the new jack, to switch the power.

BTW, a buffer would be another solution, and a whole lot easier.

Ben

[HeaD]

Hey again

When they incorporated DPDT switches, they just used the extra pole for switching the LED indicator, but didn't change the actual effect switching.

This is what also I thought. But if you look at the pictures you will notice that the LED is not connected to the extra pole of the switch

[Ben N]

Well, it is a little hard to tell, but it looks to me like the LED is connected to the middle lug of the outside row of the switch--althouigh I cannot see where the other lead of the LED connects to, or where the outside lugs of the switch go.  Wait--is that a double-sided board?

Ben

[HeaD]

Yes, it is a double sided pcb

[HeaD]

no one has answered the output volume question... anyone?

[HeaD]

please... help me

[tokyoburns]

I'm not sure how old this topic is but I am also modding my MXR Blue Box (new post coming soon) I'm pretty sure it already has true bypass. I have measured the voltage in and out using a constant sine wave voltage source and they match when the pedal is turned off. Now that doesn't exactly mean it is true bypass but every schematic I have found only shows true bypass circuits so it seemed to me that it was already true bypass.