Ross Phaser Clone Univibed.1 problem:vibrato mod isnt workin

[Freddy205]

I built a Ross Phaser and univibed it as per Mark Hammer's advice (search for info on this forum). It sounds great when in univibe mode but when I flick the vibrato switch all I get is a clen unchanged signal, I've tried twiddling all the nobs e.t.c but I can't get it to vibrato!!! Any ideas why.

[Mark Hammer]

Not sure how you did the vibrato mod, but this is what OUGHT to work, in both theory and practice.

Looking at the Tonepad "Ropez" layout from Francisco Pena, there is a dual opamp in the upper right hand corner.  Just "south" of the op-amp are a 470k resistor and a 27k resistor below that (and bumped over a little).  Lifting one end or the other of that 27k resistor will interrupt the path for the clean signal to reach the mixer stage, producing vibrato if there is only a swept phase-shifted signal remaining.

How to accomplish this?  One way is to take the 27k resistor out completely, and install two wire leads to the pads for that resistor.  Solder a 27k resistor to one of the lugs of a SPST switch (toggle, slide or stomp), put some heat shrink around the resistor and lug to secure it and prevent it from snapping during installation, and you're done.  Naturally, the unshrunk heat shrink needs to be slipped over the wire BEFORE you solder it to the free end of the resistor.

Another way of doing it is to use a utility knife or dremel to break the connection between that 27k resistor and the 15k resistor around the corner.  Drill two holes close to the remaining traces and run your leads from the SPST (vibrato/phase) switch to those holes.

Finally, if you haven't already MADE the board, when you go to transfer the resist pattern to the copper board, scrape away the connection between the 27k and 15k resistors, and draw in (with waterproof marker) a pad just to the right of the 27k resistor, and another just below the spot where the 15k, and the 27k just to the right of it, meet.

Francisco, if you're reading this, that might make a simple and feasible change to the Ropez board, applicable to both phase shifter and univibe contexts.  If a person doesn't care for vibrato, just bridge it with a wire link.