MXR Micro Chorus Volume Boost Issue

[sohosteve]

Just got myself one of the re-issue MXR Micro Chorus pedals.
Love the overall sound - however there's a definite volume jump when the effect is engaged.
I believe that there may be a fairly simple mod to remedy this.
Though I can't find one that graphically leads me through the process.

I can't negotiate a schematic but I'm a whizz with a solder sucker and soldering iron

I have a selection of resistors and a few variable resistors that, I guess could serve the situation.

However, on looking at the circuit, this is what I discovered - not only a micro chorus but micro components...!

[The Dude]

Step 1 = Find schematic and upload.

[Mick Bailey]

I see you also posted on DIYStompboxes. Did you try tacking a resistor across R41 to lower the feedback resistor to 240k? If this doesn't work, no harm done. If it is the resistor in the output opamp feedback loop you'll hear a volume drop and it could be worth experimenting to get the volume levels matched.

One caution with SMD boards is that it's really easy to lift pads or damage components if you're not used to working on them. If this happens you're in a mess.

[sohosteve]

Step 1 = Find schematic and upload.

I searched and searched and searched until my eyes were almost bleeding before I even posted my request for help.
The best I could find was a schematic that is probably way out of date and I'm guessing, actually relates to the original issue which makes my new version look like an alien beast by comparison.
Not sure if it will help any but here is the best I could find....

[sohosteve]

I see you also posted on DIYStompboxes. Did you try tacking a resistor across R41 to lower the feedback resistor to 240k? If this doesn't work, no harm done. If it is the resistor in the output opamp feedback loop you'll hear a volume drop and it could be worth experimenting to get the volume levels matched.

One caution with SMD boards is that it's really easy to lift pads or damage components if you're not used to working on them. If this happens you're in a mess.

Hi Mick
Yes, I did, as I was desperate to find a quick fix - if possible.
And thanks for your suggestion.
I have a trim pot - 100k if I remember correctly - would that be the correct kind of value to experiment with?
If not, could you suggest something in the right kind of area?
As the SMD boards are a little tricky, I'd rather add one thing once, that I can tweak, rather than keep adding and removing components until I find the best value...
If that makes sense...?
Thanks for you kind suggestion.

[Mick Bailey]

Did the resistor change make any difference? The problem with the old schematic is that the component references are different and most likely there are quite a few differences between the circuits. If we can establish that R41 is the 270k feedback resistor for the output opamp then this goes a long way to adding a trimmer and series resistor to set the output level.

[sohosteve]

Did the resistor change make any difference? The problem with the old schematic is that the component references are different and most likely there are quite a few differences between the circuits. If we can establish that R41 is the 270k feedback resistor for the output opamp then this goes a long way to adding a trimmer and series resistor to set the output level.

Hi Mick
Haven't had a chance yet as the bit in my iron is a little too large to mess around with an SMC!
I've ordered something more suitable and will report back when I've tried that.

[Mick Bailey]

With SMD experiments I often tack a very thin flying lead to each end of a component and then I can than determine what needs to be done to achieve the desired outcome. I use a solder especially designed for SMD rework that flows really nicely at a slightly lower temperature (186C) than regular solder. Sometimes I leave these leads in place as part of the permanent change. That way it can easily be reversed if necessary at a later stage.

I had a look around for a schematic for the SMD pedal but I can't see anything. Your 100k trimpot is probably too low. I would tack on a pair of leads and use a 1M trimmer in series with a 1.5M resistor. This should give you enough range to get the gain down. When you get the level you want you could disconnect one connection and measure the trimmer+resistor value and sub a fixed resistor if you like. Depends on the space inside the pedal.

[sohosteve]

With SMD experiments I often tack a very thin flying lead to each end of a component and then I can than determine what needs to be done to achieve the desired outcome. I use a solder especially designed for SMD rework that flows really nicely at a slightly lower temperature (186C) than regular solder. Sometimes I leave these leads in place as part of the permanent change. That way it can easily be reversed if necessary at a later stage.

I had a look around for a schematic for the SMD pedal but I can't see anything. Your 100k trimpot is probably too low. I would tack on a pair of leads and use a 1M trimmer in series with a 1.5M resistor. This should give you enough range to get the gain down. When you get the level you want you could disconnect one connection and measure the trimmer+resistor value and sub a fixed resistor if you like. Depends on the space inside the pedal.

Mick, that is fantastic advice!
And I do actually have both the components you mention - though no low temperature solder. However, my fine tip solder bit just arrived so I'll experiment with your kind suggestions.
Huge thanks.
PS - SMD circuit schematic search - well it's lovely to know that it wasn't just me being a numpty then!

[sohosteve]

MXR Micro Chorus Re-issue - Volume Boost - Working Modification

The operation was a success!!

A little fiddly - especially as I'm not an electronics engineer, have failing eyesight and don't have the steadiest of hands.
This was also my first attempt at working on an SMD board, which, for me, was a daunting process in itself.
But thanks to the great advice received here, I pressed on regardless.

1. Ordered a very fine tipped soldering bit for my vintage Oryx Iron that was gifted to me, secondhand, almost forty years ago!
2. Soldered the middle and one outside leg of a 1m trimpot together
3. Attached a 1.5k resistor to that (dual) leg and heatwrapped them together
4. Bent both of the legs (one leg actually being one end of the 1.5k resistor into a position where they would easily line up with each each of R41 after neatly folding and heatwrapping the resistor out of harm's way on the top of the trimpot
5. Tinned the end of each leg
6. Very carefully (and with the utmost haste - less than a second per connection) added a tiny blob of solder to each connection of R41
7. Lined up the first leg with one side of R41 and carefully heated the leg until I saw the solder melt.
8. Checked the connection held firmly before repeating the process for the other leg.

Possibly a miracle, but for me it worked first time.

And using the trimpot I can now take the signal from zero, to way too loud, as I see fit!

As the job is such an unobtrusive one and I want to see how the pedal responds in a live situation, I shall be leaving the trimpot in situ.
I may even be tempted to drill a small hole in the side to access it when the pedal is back together....

I've attached some shots of my handiwork in the hopes that this will prove helpful to others.

Huge props to all the very good folk who helped my on my way, especially Mick Bailey, who's stellar advice proved invaluable and led me through a potentially disastrous process!

[Mick Bailey]

Nice it all worked out and your description is here for others to discover. It would be worthwhile securing the pot if you're leaving it in place to alleviate stress on the connections.

[sohosteve]

Nice it all worked out and your description is here for others to discover. It would be worthwhile securing the pot if you're leaving it in place to alleviate stress on the connections.

Yes, I was thinking the same thing - though the legs are bent making it sit flat on top of an IC just underneath it.
I was wondering if it would be worth using some kind of adhesive between the IC and the underside of the pot (as both are flat) but wasn't sure if that may cause the IC to overheat.
I also wasn't sure what kind of adhesive would be strong enough to hold it but semi permanent enough to be pried free without causing damage. Not that I've any intention of removing it...

[Mick Bailey]

Looking over your pictures I was thinking that there may have been enough clearance to epoxy the trimmer to the side of the foot switch. Anyhow, its working well and sometimes an afterthought can lead to problems.

It doesn't need much just to stabilise the trimmer and the opamp beneath won't get very warm. A dab of neutral-cure silicone either side of the trimmer or low-temp hot-melt would do the trick.

[sohosteve]

Looking over your pictures I was thinking that there may have been enough clearance to epoxy the trimmer to the side of the foot switch. Anyhow, its working well and sometimes an afterthought can lead to problems.

It doesn't need much just to stabilise the trimmer and the opamp beneath won't get very warm. A dab of neutral-cure silicone either side of the trimmer or low-temp hot-melt would do the trick.

Thanks again for your consideration and advice.
I think I'll avoid attaching it to the foot switch in case that decides to die at some point in the future!

All good!

Cheers!