Need Help with Vintage MXR Stereo Chorus

[Pigyboy]

A guy brought this by last night and you can see he tried to adapt this old relic to true bypass and add an LED.

I don't imagine it is as simple as plunking the new switch in like with a crybaby, yes? There must be buffers or whatever involved. I haven't plugged it in yet but he says it just goes all swirly.

I wish he had not drilled the hole in the face 

I need a picture of an original or some input just so I can hook it back up with the original switch or if it is possible to finish the mod some info on how to do it would be great.  The only schematic I can find is very poor quality and not much help.

Thanks all!

[Fender3D]

I got this:



you'll need a DPDT though...
hope it helps

[Pigyboy]

Whaa, Thanks man!

So this is for the true bypass mod w/o the LED yes?  I'll look at everything now - Really Cool Fender3D, thanks

[Fender3D]

It's not true bypass 'cause input is still hooked to first buffer...
but that was the way it went at the time...
Since it has a stereo out, you'll need a 3PDT + millennium or a 4PDT to get a "real" true bypass + LED. You might look at this solution:

[nexekho]

Are those 0R resistors on the board?

[Pigyboy]

OK - I see now. 
The one I have here must be real old as the original switch is just a SPDT and the connector has 14 wires instead of 12 like in you diagram. All the wires from this picture line up right to left with your diagram bottom to top up to "A" which is the ground. On my connector then goes
green - 2nd output
orange - 1st output
yellow - input
blue - wired to top peg of switch
white - wired to middle peg of switch

[Pigyboy]

Are those 0R resistors on the board?

Yes they are.

[Fender3D]

Oh I see..
It must be the older pedal with R5101...
It was always engaged, just killed the wet signal when in "bypass" mode...

[Pigyboy]

Yes, this is the Reticon chip before the MN3008.

The schematic I found is for the MN3008.


So, in one direction the switch shorts the wet signal to ground and in the other passes it?  I think I can figure it out but it looks like there was either a part or a wire connecting the two output jacks on the hot tabs. if you look at my last picture you can see a blob of solder with a wire sticking out that had been snipped off by the red cap and there is a similar piece on the other jack that is not so evident in the picture.

I need to find someone with the same model to get a gut shot..

[Scruffie]

R5101 schematic - http://www.6v6power.ru/inf/Chorus/MXR%20Stereo%20Chorus%20(model%20134).jpg

[Pigyboy]

Thanks Scruffie. I see what to do now. I will give it a try and report back ...

[Pigyboy]

Thanks for the link Scruffie - Got it sorted with that schematic. Replaced the original switch and took out the led bezel. It is a bummer he hacked it. 

So I fired it up and it was making a funny whistling sound which is what the guy must have been referring to and upon closer inspection found this




replaced it


And it works great now. I don't notice so much of a tone change with the effect switched out but I don't have it hooked up with any other effects either.  It works just as Fender3D said by shorting the effect to ground.
I am just happy it works and it sounds very nice - quiet, very usable.

Thanks for all the help guys