MXR - 143 Limiter  [schematic]

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MXR - 143 Limiter

Postby ironman28 » 28 Mar 2012, 23:57

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2 different units here. the first one had a blown CA3280 and a substitute ECG VR. The second unit is very distorted. looks like the VR was replaced with an unidentifible device. Schematic and layout to follow.

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Re: MXR 143 (guts)

Postby pedalgrinder » 29 Mar 2012, 08:11

absolutely sensational i was so happy when i seen this. Didn't think i'd ever get the chance to see this one. Would it be possible get one good clear shot of the track side so a clone board can be made from the pic thanks heaps. :thumbsup
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Re: MXR 143 (guts)

Postby roseblood11 » 29 Mar 2012, 08:27

How does this pedal sound? Can't find a demo of the old version.
Would be interesting to compare it to the reisssue. I think that's not based on the old CA3280 anymore, but it sounds really good.
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Re: MXR 143 (guts)

Postby roseblood11 » 29 Mar 2012, 08:30

Why can't I edit that nonsense? I confused 143 and 134...
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Re: MXR 143 (guts)

Postby ironman28 » 29 Mar 2012, 23:51

I have the factory schematic and layout that I can link here as long as the mods are ok with it. My intent was to redraw the schem and post to avoid IP issues.
I have not used either one yet so I cant give you a review. Here is one: http://www.ovnilab.com/reviews/mxrlimiter.shtml
Heres the track side. I did a direct link instead of an image tag. They are getting cropped on the right side when I post them :hmmm:
http://i1158.photobucket.com/albums/p61 ... CN0189.jpg

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Re: MXR 143 (guts)

Postby pedalgrinder » 01 Apr 2012, 13:00

i've been trying with that pic to bring out black and white but the yellow background doesn no favours any other programs people have had success with here
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Re: MXR 143 (guts)

Postby uncleboko » 01 Apr 2012, 13:17

pedalgrinder wrote:i've been trying with that pic to bring out black and white but the yellow background doesn no favours any other programs people have had success with here

Tried with Photoshop CS5 and not much luck there.
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Re: MXR 143 (guts)

Postby pedalgrinder » 04 Apr 2012, 10:59

these pics are good but i would be guessing a lot of the cap values. the resistors and diodes are ok. I don't really get what he means about two different versions using different ic's. :scratch: hmmmm the 4016 i assume is doing switching for the bypass i don't think it's used as a form of attenuator. Any other thoughts or views from anyone else?
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Re: MXR 143 (guts)

Postby ironman28 » 04 Apr 2012, 20:13

Not two versions, two different units with different construction. Both are CA3280 chips. I have not compared the two part by part to see if there were any differences.
I dont have time to redraw right now so...
http://i1158.photobucket.com/albums/p61 ... ge-001.jpg

I wont leave this up for long so I hope someone has the time to redraw it properly to share with the community. 8)
The schematic calls out a 3080 so ignore that and use a 3280 as built. Also I would omit the AC transformer and the switching around the 4016. I convert these MXR pedals to external 18vDC power and leave the 15v regulator in place

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Re: MXR 143 (guts)

Postby pedalgrinder » 07 Apr 2012, 15:13

iam using a ba6110 in my build. I think it will make a good circuit even better. I can't wait to finish it. I've also got rid of those horrid jrc4558 op amps.
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Re: MXR 143 (guts)

Postby pedalgrinder » 09 Apr 2012, 09:35

something is seriously not right with that schematic. i wonder if mxr try and put people off cloning by putting errors in there schematics. i've had nothing but trouble with it. It was close to ending up embedded into the wall after i threw it as hard as i could. Didn't matter what i did it wouldn't play ball. not happy
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Re: MXR 143 (guts)  [schematic]

Postby pedalgrinder » 15 Feb 2013, 10:03

here are service manual schematic and layout enjoy :applause: :applause: :applause: :applause: :applause: :thumbsup :thumbsup :thumbsup :thumbsup :thumbsup

MXR 143 Service Schematic.pdf
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Re: MXR 143 (guts)

Postby mictester » 20 Feb 2016, 12:11

pedalgrinder wrote:iam using a ba6110 in my build. I think it will make a good circuit even better. I can't wait to finish it. I've also got rid of those horrid jrc4558 op amps.


The 4558 that's in the audio path is a unity gain buffer - not going to affect the audio passing through it at all. The other half of that IC is just used to derive a ½Vcc bias supply - again, not affecting the audio. The other 4558 is part of a "precision rectifier", and is ideally suited for that purpose since it handles overload so gracefully. Changing out the 4558s will have no effect whatsoever on the sound and might actually degrade the action of the rectifier section of the circuit!

I've used this kind of topology for limiter (and compressor) VCAs in the past. They're a neat way of overcoming the deficiencies of the OTAs (by putting them in the negative feedback path around an op-amp), but they have a limited range of control. The VCA range is seldom better than about 15dB. This may be enough for some purposes, but I use two of this kind of VCA cascaded in my guitar compressor to get the range that I need.
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Re: MXR - 143 Limiter

Postby ilcaccillo » 10 Dec 2017, 14:28

Hi,
I was working yesterday on a 143 Limiter from a friend.

I find quite a few problems with the Built-In Power supply.

First there's Electro Magnetic interference (EMI) between the transformer power transformer and the circuit. MXR tried to minimize that by putting the IN and OUT jacks on opposite side to the transformer. But there will always be some EMI hum induction affecting the audio signal with he transformer inside the case.

Second for 220/240V countries, MXR decided to cut costs on a new transformer, and use the 110V one, so they are dropping 110V to 220V bu using a resistor. It's R40 in the schematic, in my pedal this was a 8K2 resistor, not sure about the wattage I thing from 3W to 5W possibly.
Well this resistor gets really really hot, and also nowadays most European countries are not even 220V , the supply is more close to 240V.
I have a 236V AC supply, the resistor was so hot, the case was really hot, and I was afraid it would melt the solder at the resistor legs.
Also the main Electrolytic Filter capacitor C3 is next to this resistor, and these type of capacitors can fail prematurely when exposed to higher temperatures.
I replace the 8K2 resistor for a 10Watts one, it was a really tight and difficult fit. Still gets pretty Hot

Third and Last, Main Earth connect to pedal case, Audio Ground connects to pedal case, audio ground will connect to Main Earth also trhough the amplifier in the end.
So there's 2 paths to Earth, and it's a potential ground loop situation.
Maybe at home you don't have that problem, but it can happen in a gig, in practice.etc
The solution here could be to Isolate Audio Ground from the case, by insulating the Pots and the Jacks (or using plastic jacks), and then connecting audio ground to the case via a 10R resistor in pararel with a 100nf cap.
like this:
Image


To be honest the Best solution it' to ditch the internal PSU completely and like Ironnman28 advised feed 18V from an external wallwart, that will solve all the problems with the internal PSU.
You feed the 18V right after the Diodes CR1 and CR2, into the regulator and filtering.
You have to remove R40, the transformer T1, and the 2 rectifier diodes CR1 and CR2, and install a DC jack or another connector type for the 18V supply.
Problems solved.

Even if you ditch the intrernal PSU or not I would advise improving the Filtering on the pedal by increasing the value of the axial C3 capacitor from 400uf to 1000uf. and adding 2x 100nf COG Ceramic caps one before the 7815 and one after.

Image

I installed them like this:

Image

Image


I also removed all the rotten and disintegrating foam that was glued to the case back cover, and glued just a plastic sheet insulator :

Image
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Re: MXR - 143 Limiter

Postby Noth » 03 May 2018, 15:18

Hi,

I've got an M143 with innefective Attack/Release knobs.
I read on ironman28 website that this may come from a dead CA3080 chip. All caps and transistors seems to be fine.

On my version, the pedal has a CA3080 labeled (with 16 pins) just as indicated on the schematics, and not the CA3280 that everyone else seems to have.
Does anyone know if there was 2 versions of this model ?

I wasn't able to find specifications for this "16 pins CA3080", so I'm not sure I can just replace this CA3080 with a CA3280...
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Re: MXR - 143 Limiter

Postby Noth » 10 Jun 2018, 15:42

Hi again,

I did spend again a few hours on this pedal.
I did check power lines, 15V is fine and Vs=7,5V (I guess it's correct too).

On my original CA3280 I have the following:
pin 6 0,850V
pin 8 1,15V
pin 9 6,9V
pin 10 6,9V
pin 12 7,5V

When switching this CA3280 which might be dead (attack and release have no effect) I have no sound at all and:
pin 6 0,960V
pin 8 1,0V
pin 9 1,12V
pin 10 2,84V
pin 12 2,6V
and worse, Vs goes down to 4,5V

I suspect my "new" CA3280 to be dead too :(

I someone could just check its tensions on the CA3280, that would be just great !

Thanks !
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