Hey. Thanks for replying !

I finally had time to check the pin 8 of the LM324, ans it was between 1.4 V to 8.2 V depending of the position of the range pot.

It does not seems to fit your values.

I double checked the filter matrix switch and there does not seem to be a problem there…

However I found this old thread on Harmony Central http://www.harmonycentral.com/forum/forum/guitar/acapella-29/1776103- where the problem is same as mine. One of the guy suggest to unsolder the range pot cause the soldering might be broken, it would make sense and I will try that tomorrow.