MXR - M-108 Ten Band EQ new version  [schematic]

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Henz
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Post by Henz »

Man, I want to post the links from mouser but it doesn't paste the entire link to show you the exact parts. But I think I found the .68 there as well. When i paste, it just pastes mouser.com.

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Post by Henz »

In the descriptions they say: 3,300pf 0805 2012 mm cog dielectric . The .68uf says 1206 3216mm. Should I be concerned about the 2012mm on the 3.3 ? Thanks, James

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Post by floris »

The 0805 series = 2012 in metric, so that is correct.

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Post by Henz »

Hey brother, I really, really hate to keep bothering you but I tend to over think. Should I be concerned of the material? .68uf . Can I just get a multi layer ceramic instead of the poly film? Mouser has many hits with the ceramic. None with the poly film. Thanks brother! Thank you for your time!!

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Post by Henz »

Greetings. This thread may be worn out but I really need some answers before I trash my mxr. When activated, mine is very microphonic. Its in front of my amp. And mine makes a very loud pop when turning off and on. Mine hisses but that has always seemed to have been an issue with these. Hope to hear from someone. Btw, this is prior to the work being done to it as mentioned above. Thanks for your time. Blessings! Jim

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Post by rmroza »

I have an old (1978-1980) MXR M-108 without the overall and gain sliders. It is powered by 120VAC with a 15-0-15 stepdown transformer and I'm reading like 36V!
It is regulated by full bridge. I'm looking to pull out the AC power cord and power it by a MXR brick or ISO brock with 18VDC and want to confirm how to install.
Am I correct in the negative would go to the negative side of C27 and positive to positive side of C26??
PLEASE CONFIRM. Thanks in advance. The vintage pedal sound killer! :)
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mxr_eq10_v10-2.png

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Post by FuzzMonkey »

I believe 'Ground' is the line between what you have ID'd as +18VDC and GND as it is a dual power supply.

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Post by Jarno »

Unless you rip out the power supply and replace it with a DC/DC converter which has two sets of outputs, powering it from a single DC voltage will mean significant changes to the circuit.
But, if there is room inside enclosure, you could fit a 9-36V input dual output DC/DC converter, like one from Traco Power or Meanwell (or CUI, or a number of others). Possible option would be Traco Power TDR 2-2422WI or TDR 2-2423WI
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Post by rmroza »

FuzzMonkey wrote: 18 Nov 2022, 04:28 I believe 'Ground' is the line between what you have ID'd as +18VDC and GND as it is a dual power supply.
Yeah, that's the way I'm taking it also. The other old MXR pedals just need positive B+, so still use a transformer, but just rectify it with 2 diodes and center tap to ground, then regulator to whatever voltage....15V or 12V.

In regard at the EQ, they use a positive and negative voltage! I'm assuming to allow for more headroom is the overall difference of +18V and -18V = 36V!

Now, what happens if you hook up 18VDC like the way I did?? Someone communicated it will effect the circuit overall. I actually did it and it is working and the sliders are doing something....but does that mean the frequencies changed or since you only have the positive, your headroom has decreased as only 18VDC in total available??!?

Can anyone supply a circuit to supply the +18 and -18V to ground off of an already regulated +18VDC input, like from an ISO brick??
Also, if you just apply voltage like I outlined to one of these old pedals, do you need to remove the transformer or lift the legs, or ok to still be in there?!??

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Post by Jarno »

I did, please read
"It crackles....., but that's ok"

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Post by rmroza »

You did generally, but I meant a schematic and/PCB layout!

The vintage MXR pedals all have different voltages also. The Chorus is 12V, The Flange is 15V, and as above, the EQ seems to be +18V and -18VDC!!

Attached are the marked up schematics and where using an already isolated and regulated +18VDC feed from the ISO Brick to the Chorus and Flange to the input and capacitor of the regulator to regulate it further to the voltage it needs.

Have to work out the EQ...wish it was just the same as the other pedals and be easy. Now, need a work around.
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Flange.jpg
Chorus.jpg

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Post by rmroza »

I did some searching around, but not seeing any off-the-shelf dual output +18/-18VDC converters, only up to 15VDC, which should work, but since lower B+, compromise headroom, so not getting the full effect if the pedal

For the EQ, I actually pulled out the 120VAC cord and put in this normally DC jack. It's only rated at 50VDC @ .5A, so why I'm weary of continuing to use them in this manner, but do have an inline fuse, so if any problems SHOULD pop first.

Another jack thru Tayda only shows Max. voltage rating of 12V at .5A and a third, that doesn't even show these spec, fyi.

I did find an isolated unit with +/-18VDC outputs! However, looking at the spec sheet, it looks like it only handles 6VDC input!! Soo, even using the 9V tap instead of 18V tap of the ISO Brock....could have a problem!! Output current may be a problem also. :O :(

https://www.taydaelectronics.com/datash ... 2237-1.pdf
https://www.taydaelectronics.com/nebj21 ... ocket.html
https://www.taydaelectronics.com/dc-pow ... unt-1.html
https://www.electronics-lab.com/project ... wer-input/

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Post by FuzzMonkey »

You're probably looking at something like the circuit shown in Figure 22 in the LT1054 datasheet:

https://www.ti.com/lit/ds/symlink/lt1054.pdf

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Post by Jarno »

The difference between 15 en 18v will not be audible for guitar/bass signals, and the datasheet for the converter will have the mandatory components to make sure it is stable (but in practice, you do not need any components on there for it to work fine). Simply splice the rails onto the mxr unit. Given that they are two independent outputs in a lot of the cases, you will need to tie the "-" of output1 to the "+" of output2 to make a 0v, and tie that to ground.
"It crackles....., but that's ok"

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Post by rmroza »

Jarno wrote: 20 Nov 2022, 09:46 The difference between 15 en 18v will not be audible for guitar/bass signals, and the datasheet for the converter will have the mandatory components to make sure it is stable (but in practice, you do not need any components on there for it to work fine). Simply splice the rails onto the mxr unit. Given that they are two independent outputs in a lot of the cases, you will need to tie the "-" of output1 to the "+" of output2 to make a 0v, and tie that to ground.
Can you draw a mock up in MS Paint or something??

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Post by Jarno »

Why? Everything is here, and if you are not able to deduce the rest from what is on this very page, I think this is not the project for you. It might be better to try out different projects first to learn.
In my view, but that might just be me, starting from scratch (with circuits or kits which are not too difficult, and work your way up), is a lot easier than hacking/repairing an existing project. I myself am a lot better skilled at designing and building, than I am at troubleshooting.

It might be that this is a bit too difficult for you at this moment, better to realise this and build something else, than getting frustrated. Frustration is not a good motivator.
"It crackles....., but that's ok"

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Post by slammer88 »

Why is it 18V? ...

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Post by floris »

> Why is it 18V?
Larger headroom.
I run mine on 9V without problems.

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Post by ianridd »

Hi all -

I reactivated my dormant account just to add a post to this thread. I bought one of these pedals in a pawn shop and got it home to find it was not working. Shame on me... I know. However, I don't give up on old pedals. You know that doctor in the movies who pounds on the patient's chest and says "Don't die on me!"... thats me with old musical equipment.

I reviewed a lot of online message boards to learn about this pedal, but this thread was the most helpful, by far. So I felt obliged to add the details of my situation to this post, hoping it helps some other foolish soul, like myself, to bring their M108 back to life.

Mine is the black version. And I purchased a Jim Dunlop power supply specifically recommended for this unit. So the power is spec.

Symptoms:
1) Five of the ten bands would not light up (treble side).
2) When pedal was engaged, guitar signal was reduced to near zero and a loud screeching hiss was created. The hiss was constant. It was more screeching than just white noise. Very grating sound. (when off, guitar sounded clear as day... nice bypass, MXR!)
3) one of the five bands in the treble side (non-lighted), was visibly cracked. LED was missing. Leaving two open holes for an LED to plug into.
4) I was able to confirm that the EQ circuit was functioning when the pedal was engaged. Moving the sliders had the intended effect on the sound, just buried under a ton of noise.

I studied the schematic that was provided on this thread (thank you sooo much!) and then cracked my pedal open. Someone in this thread described that their pedal guru told them that their noisy M108 could be fixed by bypassing the LED's. Given that I had a damaged LED, I sensed a connection to my issues. This seemed like something I could experiment on, with a piece of wire, touching circuit board solder joints and bypassing different LED's.

Also, the schematic showed me that the LED's were wired in a series, meaning damage to one would result in all failing. If the whole series stopped passing current, this could result in the type of voltage (or current, sorry I'm not knowledgeable enough to know which is correct in this situation) shift that might cause my issues.

A little experimentation and I was able to figure out which leads on the broken potentiometer did what. I found the two leads that were LED+ and LED-. When I bypassed the broken LED (simple jumper, handheld in position), the noise vanished and the other four LED's turned on. (cue Hallelujah chorus). After this, I realized that the broken LED left its housing exposed and I could see the plugs for the two ends of the LED. As though the old LED was simply unplugged. So for my next trick, I took my wire and plugged each end into this LED plug. Jumper. Cured!

The pedal still has a noise floor that I consider unacceptable for recording. And when I zero all sliders and engage the pedal, the signal drops noticeably. It requires a signal boost just to match it's bypassed level. Naturally, this comes with noise. So I have to say, this pedal is not a superstar. But for live purposes, it seems to be solidly built (like all MXR's). And, when playing live, is there anything in the world more useful than a 10-band graphic EQ? So I'll keep it.

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Post by mauman »

Welcome back!

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