MXR Micro Chorus volume booooost

[phx]

My MXR (reissue) Micro Chorus worx great, accept for the volume boost. I wan't to put an extra volume pot to it. Anyone got schematics/tips? Like the values of trimpot and reistor(s) to use??

[Mark Hammer]

There should be a 270k feedback resistor on the output stage.  Lowering that value a bit will decrease the gain and give a little less output level.  240k should be about right, without screwing up all the other things that stage is doing.  YOu can arrive at 240k by tacking a 2M2 resistor in parallel with the 270k.  That way you won't screw up anything trying to unsolder it.

[phx]

Ok so no volume pot, just replace a resistor. I can do that 
but... can't seem to find out wher/what resistor to replace.... can you help me?

[Mark Hammer]

The schematic I have shows one TL062 and one TL061 on the board, with no other op-amps.  The 270k resistor in question connects pins 6 and 7 on the TL062.  There should be a 30pf cap also connected between pins 6 and 7, and a .33uf (330nf) cap connected to pin 7.

Now, this is based on the schematic for the original 1980 pedal.  I would imagine the layout of the board might be different for the reissue, but whether the circuit is the same, I can't say with certainty.  Certainly, availability of delay chips had changed over the intervening years, but I can't see any reason for the output stage to change very much.

Let us know if you don't see anything corresponding to what I described and we'll figure something else out.

[sohosteve]

Did anyone ever get anywhere with modifying the MXR Micro Chorus to remedy the volume jump when the effect is engaged?

If so, could someone please guide me through it, graphically?
Can't read a schematic but I'm pretty handy with a soldering iron.
Though as you'll see, from my gut shots, this is a modern build utilising micro components...   

Any simple way around this...?

[highwater]

I can't do this graphically right now, but I'm pretty sure that the 270k resistor Mark was referring to is the one labeled R41 on your board (in the lower-left corner of your picture, says 274 on it). The two caps he refers to are *probably* the two to the right of R41 in your picture.

As far as the micro components go, I can't help you... I've only worked with SMD bits on a board that was already pining for the fjords.

[sohosteve]

OK, I've had to order a smaller bit for my iron before attempting to tackle the parallel resistor across R41.
But I'll get there in a day or two when the new bit arrives and report back.

Huge thanks for the advice.
And for making me smile ;-)

[slavikator]

There should be a 270k feedback resistor on the output stage.  Lowering that value a bit will decrease the gain and give a little less output level.  240k should be about right, without screwing up all the other things that stage is doing.  YOu can arrive at 240k by tacking a 2M2 resistor in parallel with the 270k.  That way you won't screw up anything trying to unsolder it.

I soldered a 2m2 resistor in parallel to r41. The boost is decreased but is still present. Should I try lowering it to 220k or 200k?