Super weird thing on MXR CAE MC-401 (Booster/Line Driver)

[CrazyHorse]

Hello there! This is my first post, although I'm regular visitor. I really like the feeling on this forum, so I'll start to share. 

I've built a DIY MC-401 (CAE Boost/Line Driver) following this schem




Ommitted parts are in red. Just ommitted this to use another type of LED switching because I couldn't find J117 FET's. Instead, use a DPDT with millenium bypass for On/Off LED.

Here is the thing: the effect works perfectly. But when it is turned off, a volume swell appears (going from low to high, like a fade in).

This only happens when the potentiometer is from 8 o'clock to the end. The "fade in" increases as you turn the pot clockwise.

If the pot is set to 7 o'clock (we can call it unity gain or "no boost") there is no fade in. So, if the pot is in fully counterclockwise when the effect is turned off, the signal level remains constant as should be.

With the pot in any other position, a "fade in" shows up.

Any ideas for debugging this?

Greetings
Luqqas.

[rullywowr]

First thought is that there is a wiring issue regarding your bypass.  If the DPDT is wired correctly, the effect should be totally disconnected from the bypass signal.  If there is a "fade in" then it seems that the effect is still hooked up somehow or the Millenium is screwing with the sound.

I'd suggest rechecking your bypass wiring and Millenium bypass circuit to ensure it's correct.  Break it down to the simplest parts, like seeing if the circuit works OK (by itself with no switch) on a testing rig.  Then check your bypass circuit. 

Oh, and welcome to the forum!   

[CrazyHorse]

Thanks for the answer.

Circuit works perfect. Also I've checked millennium and desolder the relative pin on DPDT and still same issue.

I'm an average diyer but this is completely new for me.

[rullywowr]

If you disconnect the in/out from the PCB to the switch do you get clean bypass?  What Dpdt wiring are you using?

[CrazyHorse]

If you disconnect the in/out from the PCB to the switch do you get clean bypass?  What Dpdt wiring are you using?

Good point. Check and report what happen.

How I did not think about that? 

[CrazyHorse]

Ok. Perfect clean bypass.

I'm using this DPDT wiring:

control     fx out     out       [both "out" pins are linked]
    x            x          x

    x            x          x
  out          in       fx in

[rullywowr]

Google Dpdt true bypass.  Stinkfoot.se has a great diagram.

[CrazyHorse]

Google Dpdt true bypass.  Stinkfoot.se has a great diagram.

Ok.

But the method that I'm using it's true bypass.

                                      [both "out" pins are linked]
millen.                                         millen.
control     fx out     out                  control       fx out         out
    x----------x          x                       x               x-----------x

    x----------x          x                       x               x-----------x
  out          in       fx in                    out             in          fx in

            effect off                                      effect on

[rullywowr]

Google Dpdt true bypass.  Stinkfoot.se has a great diagram.

Ok.

But the method that I'm using it's true bypass.

                                      [both "out" pins are linked]
millen.                                         millen.
control     fx out     out                  control       fx out         out
    x----------x          x                       x               x-----------x

    x----------x          x                       x               x-----------x
  out          in       fx in                    out             in          fx in

            effect off                                      effect on

I would perhaps try another DPDT diagram (and leave off the millenium for now).  Once you verified that it works OK, then add the millenium.  Doesn't look like the effect is grounded when in bypass (I could be mistaken but can't see how you did your millenium).

Check out this page at stinkfoot.se for a great writeup of how to do DPDT true bypass for MXR effects (still applies).
http://stinkfoot.se/archives/537