MXR M-120 Schematic sought

[km6xz]

Anyone have a print of this? I could not find one via a Google search....
Thanks

[doctor]

Anyone have a print of this? I could not find one via a Google search....
Thanks

Here it is. Quality is not so good, but better then nothing...

[km6xz]

Thank you very much, that is a big help! The quality is fine.

Edit after looking more carefully at the schematic: Unfortunately this schematic is for a much earlier version that has little similarity to the new surface mount version. I am sure it will useful for some because I've found no other schematics of any versions.

[J M Fahey]

Well, this schematic (thanks Doctor) shows an incredibly advanced (for its age) switched-capacitor filter, entirely built out standard, over the counter parts. Amazing.
Without seeing it, I'm quite certain the "modern" version is simulated with DSP, and sold using the MXR label, legally so by whoever-owns-it-now.
Oh well, the power of Mojo.

EDIT: dear KM6, can you post some picture of the "reissue"?
Just for fun.

[km6xz]

Hi JM
I can take some photos when I get back home tonight, its all buttoned up now. The new version uses an expanded version of the original, with the hex inverter and analog gates for the filter function but adds a lot of features and controls.
I got it working, found a SMD resistor in the bias divider circuit that was open and one section of the 4016b analog switch was not switching. I did not have a SMD 4016 so used a 4066 and all came back to life.
Playing through it and measuring the filter and modulation it is an impressive little box. It sounds good, like a real manual sweep discrete filter. This version has a complex discrete SMD transistor switching and output section, and about twice the parts count as the original. The board looks well laid out and uses good standard parts. It took a lot longer to figure out the mechanism of fault than it should have. Last night, a friend of my GF brought over a no-name tremolo box that had no effect and was presented as a challenge since he had two tech give up on it. The last one wanted $125 and was going to replace all the active devices.
Always up for a challenge, before Roman could get his first cup of tea, it was done. The real problem was the first tech who worked on it, turned a minor pot problem into a more serious problem of swapped and mis-wired parts.

I think when I open my little part time shop I will have two prices, 1 for virgin units never molested by hacks, and 2, for those that have been. Luckily for me, the MXR only had its foot switch rewired and the surface mount pc board was intact. I suspect the cause of the problem was connecting to the wrong power supply.

[52 Bill]

...The real problem was the first tech who worked on it, turned a minor pot problem into a more serious problem of swapped and mis-wired parts.

I think when I open my little part time shop I will have two prices, 1 for virgin units never molested by hacks, and 2, for those that have been....

I absolutely love this!