MXR M234 true bypass mod

[Fred13]

Hi all,
I'd like to mod my analog chorus to true bypass, but I haven't found neither any schematic nor someone else that did this mod before.. is there anyone that can help / guide me in the right direction? 

[Fred13]

Meanwhile I tried to study the circuit of the pedal and I desoldered the SPDT switch. What I found out is that without the switch the pedal stays always on. The two central pins of the switch are grounded, and the pedal is off when the switch grounds some part of the circuit in one position (the remaining two pins don't seem to be connected to anything).
My idea is to cut input and output pins on the female audio jacks, insert a 3PDT as follows:
1 row to substitute old switch grounding operation
Other two rows to connect directly input to output in one position (pedal off, true bypass, also with pcb in and pcb out grounded), or input jack to pcb in & pcb out to output jack in the other position (pedal on).
What do you guys think? Should this be correct? Could there be any pops doing this?

[ElectricDruid]

It sounds like what you're looking at is something like this:

http://www.geofex.com/Article_Folders/bosstech.pdf

The switch you've desoldered doesn't actually switch the pedal on or off, it just switches a flip-flop that provides voltage to the FETs that switch the pedal on or off. So changing too true bypass isn't as simple as just taking the switch out, although it's not too much harder either.

If you're going to continue down this path (assuming that this pedal does actually follow this scheme, which I don't know for certain) then you need to remove the FETs and solder wire links drain-to-source for the ones that connected the pedal in its "on" position. That wires the pedal permanently on. Then you can wire up a true bypass switch around the PCB. If all this seems overwhelming, then maybe you need to step back and find someone who can give you a helping hand.

And yes, making the pedal true bypass does make it more likely to pop. the whole reason these pedals adopted FET switching in the first place was because of the silent "soft switching" aspect of it. When I was younger, *that* was the holy grail of pedal switching, and this modern fascination with true bypass seems like throwback to an earlier era of pedals that went "CLUNK" when you trod on them!

Tom

[Fred13]

Thanks for the info Tom! I knew that removing the switch didn't turn it into true bypass on its own  I ended up trying the mod I came up with in my previous post. Given the fact that there were absolutely no info about this mod anywhere, I report back what I did:

- desoldered DPDT footswitch
- desoldered input and output female jacks (in order to facilitate next steps)
- on both input and output connectors I cut the lug that makes contact with plugs' TIP (the one circled in red)



- soldered a wire on those lugs and resoldered female jacks to pcb (I named wires INPUT JACK and OUTPUT JACK):



- soldered a wire into the left open holes on pcb:



Wired a 3PDT switch as follows:



Note: the effect is turned off by grounding what I marked as FX switch pins. Since those two are connected together inside the pcb, I only used one to wire the 3PDT switch. Same for the ground pins.
Also, in order to fit 3PDT switch I drilled another 12mm hole about 1inch below the old one (which i plugged with a large enough bolt).

I tested the pedal and it works flawlessly, absolutely no pops, and no more tone suck when the effect is off. By the way, this was the reason I wanted to mod it in first place, I have a few other buffer pedals but with this one I noticed my tone became somewhat dull.. I know that with M234 the tone suck is rig dependant.

Let me know if something is unclear (I did my best ), hope it will be helpful to anyone who wants/wanted to mod one like me! 

[Mark Hammer]

Can I ask why you want to convert t to true bypass?  I'm assuming here that MXR/Dunlop is not in the habit of designing things by accident.

[Fred13]

Can I ask why you want to convert t to true bypass?  I'm assuming here that MXR/Dunlop is not in the habit of designing things by accident.

Because it altered my tone. A video is worth one thousand words 

https://www.youtube.com/watch?v=x4eEzqSRReA

I searched around and what I found out is that it depends on the rig, some say it sucks tone, some say that it was ok until they changed pedals in their pedalboard, some others say it sounds great and don't notice any tone alteration. I can speak for my pedalboard, and it caused some high end loss when bypassed. After modding it my tone is back!

[Mark Hammer]

Assuming the video doesn't lie, and the little speakers on my computer monitor don't lie either, the small drop in top end is audible. At the same time, it seems to be something that would appear and disappear depending on various impedances, cable lengths, pickup loading, etc.  Personally, it wouldn't bug me to leave it as is, and I'm the sort who usually aims for Nile Rogers or Johnny Guitar Watson ice-pick-in-the-forehead tone.  But if it worked for you and solved an issue, a tip of the hat, fred. 

[mcgubbins]

I was thinking of using a Griffin Effects Silent Step mod to convert the MXR M234 to true bypass. Does anyone have experience with this or know if it might work/be possible? It looks promising, anyway.

http://www.griffineffects.com/home/69-silent-step-true-bypass.html

I guess soldering to the PCB and cutting input jack lugs via Fred13's method makes me a little nervous... I don't have a lot of soldering or electronics building experience, though I work with electronics and wiring diagrams all the time at my job.

[ShadSunsCrash]

Thanks for the info Tom! I knew that removing the switch didn't turn it into true bypass on its own  I ended up trying the mod I came up with in my previous post. Given the fact that there were absolutely no info about this mod anywhere, I report back what I did:

- desoldered DPDT footswitch
- desoldered input and output female jacks (in order to facilitate next steps)
- on both input and output connectors I cut the lug that makes contact with plugs' TIP (the one circled in red)



- soldered a wire on those lugs and resoldered female jacks to pcb (I named wires INPUT JACK and OUTPUT JACK):



- soldered a wire into the left open holes on pcb:



Wired a 3PDT switch as follows:



Note: the effect is turned off by grounding what I marked as FX switch pins. Since those two are connected together inside the pcb, I only used one to wire the 3PDT switch. Same for the ground pins.
Also, in order to fit 3PDT switch I drilled another 12mm hole about 1inch below the old one (which i plugged with a large enough bolt).

I tested the pedal and it works flawlessly, absolutely no pops, and no more tone suck when the effect is off. By the way, this was the reason I wanted to mod it in first place, I have a few other buffer pedals but with this one I noticed my tone became somewhat dull.. I know that with M234 the tone suck is rig dependant.

Let me know if something is unclear (I did my best ), hope it will be helpful to anyone who wants/wanted to mod one like me! 

EDIT (Final): You beautiful @#$%ing bastard you! It works!!! ONE POINT TWENTY ONE GIGAWATTS!!! GREAT SCOTT IT WORKKKKSSS!!! Lol great job brother. Getting those damned output jacks desoldered without disrupting any of the SMD components was a massive pain in the ass. In the end i easily desoldered the original switch but the female jacks were giving me a really hard time even with a desolder pump and solder wick so i just clipped the leg at the base on one and elegantly smashed the other one into oblivion through the top. But as i said it works lol. All i need to do now is decide where i want to drill the new hole. I thought about getting some smaller mini 3pdt's but even those might not have worked out and they arent easy to find (and i live 30mins from mouser). So ill probably just do as you did and drill a new hole over the lettering and battery compartment and then use a flat screw/bolt to fill the original hole. I actually already found a couple that would do the job just fine. Again i really appreciate you taking the time to post your findings. Please take a few moments to imagine a gym full of people clapping and whistling for you.... starting now ............................................enough people enough settle down!! All joking aside your post really helped me brother.

EDIT AFTER FINAL....FINAL FINAL: It works PERFECTLY, the high end is better than ever DAY AND NIGHT, NO POP!!!! I can finally remove the loop box, and the loop box wasnt half the fix this TB mod is. Bypassed and engaged it sounds so much better. Damn what a relief.

EDIT: I just realized you used the top of the jacks to get input and output tips to 3pdt, i acutally did the same thing already to add a switch to this pedal for a kill dry effect without having to use a patch cable. Damn im so @#$%ing relieved to have found your post.

I can not begin to tell you how happy i am to have found your post. I had gone through several chorus pedals and decided i just wanted to stick with the MXR m234 but the high end loss is just unacceptable so i built a simple loop box to temporarily solve the issue until i could trace the board accurately with a schematic (ive tried contacting anyone and everyone who might have one and they either refused to supply it *shocker* or didnt have the correct one). Ive been building pedals for about 5 years but TBing this damned pedal has been nothing but brick walls until now. I hope you wouldnt mind answering a few questions. Ive already learned more about how to go out this specific mod in the last 10 mins than in the last year thanks to your diagrams (some things i had right others i didnt but now they make much more sense). I wanted to know if your wiring the tip pins on the input and output to the 3pdt as the "input jack and output jack" or are you wiring them somewhere else down the line on the PCB? Also did you remove the NPN transistors being used for buffers then jump them or does your mod bypass them altogether (i imagine it would bypass them altogether but i want to be clear). Im really looking forward to hearing back from you with ANY info your willing to spare. Have you still noticed restoration to the highs since doing this mod? Anyways, "patiently" waiting for a response like a kid on Christmas over here. Great work brother!

EDIT 2: Also im wondering how your 3pdt fit into the enclosure with the pcb not cut. Im wondering if i could find a lower profile switch. Ive got several on hand just cause im always building something but they seem like they would be a real tight fit but im about to do a test fit and remove the dpdt to see what kind of space im working with.

EDIT 3: LOL i was so excited when i saw your post that i missed out on a few things. The switch being lowered makes perfect sense. Im already using the battery compartment space for the other switch anyways so i might as well do the same for the modified foot switch.

[ShadSunsCrash]

Anyone considering this mod due to high end loss with you M234 (or any other modern MXR pedal for that matter) i can vouch for its authenticity 100%. So glad i found this thread!

[archimedes]

Just want to say thank you for this post! I really liked this pedal but not the dodgy buffer so I sold it - twice as it happens. Looks like I shall be on the hunt for a used one and a low profile switch - I really don't fancy making another hole in the case. Thanks. As it happens before finding your post I spotted one done in what looked like the same way on Reverb - that's what led me here. It went while I dithered so I shall have to get the soldering iron out again. 

[clarisso11]

I'd like to also say thank you for this, it really works!


I'd like to add that it is possible to do the mod without having to drill a second hole, using a low profile 3PDT switch, and carefully bending the lugs over a bit. A little fiddly, but definitely possible (I might also re-route the blue and brown cable through the holes, to make it look neater):



Just out of curiosity, will this affect the Thru jack functionality in any way?

[archimedes]

I'd like to also say thank you for this, it really works!


I'd like to add that it is possible to do the mod without having to drill a second hole, using a low profile 3PDT switch, and carefully bending the lugs over a bit. A little fiddly, but definitely possible (I might also re-route the blue and brown cable through the holes, to make it look neater):



Just out of curiosity, will this affect the Thru jack functionality in any way?

Do you happen to know the make and / or model of the switch you used? I would like to do this mod without drilling and relocating the switch. Thanks

[clarisso11]

This is the one I used (shop based in the UK): https://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=116_128&products_id=2708

One of those Taiway low profile switches should also work, if you're based in the US: https://lovemyswitches.com/taiway-3pdt-latched-foot-switch-low-profile-solder-lugs/

[archimedes]

This is the one I used (shop based in the UK): https://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=116_128&products_id=2708

One of those Taiway low profile switches should also work, if you're based in the US: https://lovemyswitches.com/taiway-3pdt-latched-foot-switch-low-profile-solder-lugs/

Super, thanks! I am in the UK so that’s great. Just have to find myself a pedal now. I let mine go cos of the bypass

[archimedes]

Just reporting back on my experience with this one. Well we already know it works and mine does too now so that is great! I can’t say I enjoyed the process very much though, it’s been a while since I modified pedals and they were all nice chunky through hole constructions before, working around those tiny SMDs was very scary and as someone else reported above those jacks are not keen to come out. All this is a reflection on my lack of skill rather the mod itself which is exceedingly cool and groovy thank you With the low profile switch mentioned above there was no need to drill the case so from the outside the pedal is unchanged. Once modded the affect of the pedal on tone is much more noticeable, which is to say that with the effect level at zero there is a noticeable change of tone, no worse than any other chorus I would say but you wouldn’t want it there all the time, hence the mod. Thank you