Vintage MXR 10 band EQ Mod

[Davve1992]

Hi guys! I'm new to the forum (& probably posting this at the wrong place  ) but as anyone ever modded a vintage (blue) MXR 10 band eq to have a DC in jack?? I just got one and the power transformer seems to be buzzing/humming like crazy... I know it can be done, just wondering if any of you guys ever did it and might have some tips!

I know the transformer steps down the 117V ac to 15V DC... There's a whole power section on the board (diode rectifier, etc...)

I can post pictures of the board if it can help!

Thanks

[robthequiet]

Does the pedal work ok otherwise?

[Davve1992]

Yes it does, you can hear it boost/cut frequency behind all the buzzing/humming
Can I link an url for a schematic here? I know some forums don't like that or something

[robthequiet]

Schematic OK as far as I know -- it's nice to work from the same map. Pictures also great, the closer the better.

Always need to be careful working with live power supply , has to be said.

Could be just a bad diode or dried out capacitor, so, before we do major surgery, maybe check for simple fixes, if any.

[mimmotronics]

A friend of mine was asking the same question. I didn't work on it but my assumption would be to board up a regulator circuit at the DC power supply voltage and install the DC jack for it. Interested to see what this thread comes up with.

[Davve1992]

I didn't draw the schematic, so the credit goes to who ever did. It's the closest one I've found online.

The bruned mark on the board and the lazy solder job on the 3 prong wire was done before I bought it... It did resolder and heatshrinked it but the problem is still there & was doing it before I touched anything. There is 2 blue electrolityc capacitor on the otherside of the board.

I do have a DMM and I can check some readings if it's needed btw

[robthequiet]

There has been some heat around the transformer, which also looks like something hit it (or tried to take it apart?). I think I see some heat damage on the pedal case as well, possibly someone leaning an iron on it or there was a smokey/flamey thing going on in the transformer region which is concerning. Nonetheless, checking voltages (carefully) to make sure your +/-15V and ground are healthy wouldn't be too much -- maybe try measuring the ends of the two gray/black diodes and see what they show.

In this case, replacing filter caps would be a simple task that you might want to do anyway on a unit of this vintage, along with the rectifier. +/-15V is popular in the synthesizer world. I've never looked for a 30VDC wall wart but hacking the power supply from that might be doable if you can find one. Much more fun than dealing with mains voltage in a stompbox.

[Davve1992]

Ok guys... There is a steady +16V/-16V coming from the diode bridge. Both zener diode (5245B) read around 12.5V.
There is also a steady +12.34V on every Pin 8 of the opamp and a -12.34V on every Pin 4... I didn't fool around the PT just to be safe...

I just noticed that on the last picture of the board ( with the 2 cap) that the one on top had somekind of leakage coming out of it. Can you guys confirm this or not??

Thanks for all the help so far!   

[robthequiet]

Makes sense generally, your +/-15V AC (correcting myself here  ) after rectification and regulating goes to +/-12.5 VDC. If the hum corresponds to mains frequency, replacing the filter caps C26 -C29 would be my first choice. I can see some brown stuff on that one cap but I can't tell if it's sealant or spilled capacitor innards. There is some fugly looking soldering in there but it may still be fine. If all else is working and you just have some power supply ripple, the caps might take care of it. I lean in the direction of getting an external supply if the transformer might be damaged leading to a potentially catastrophic event.

[PRR]

> have a DC in jack??  I know the transformer steps down the 117V ac to 15V DC...

It eats plus and minus 15V (30V total).

If you have a 24V to 32V DC supply, it can be hacked with a rail-splitter to give proper power.

> the power transformer seems to be buzzing/humming like crazy...

So replace the power transformer?

[Davve1992]

Makes sense generally, your +/-15V AC (correcting myself here  ) after rectification and regulating goes to +/-12.5 VDC. If the hum corresponds to mains frequency, replacing the filter caps C26 -C29 would be my first choice. I can see some brown stuff on that one cap but I can't tell if it's sealant or spilled capacitor innards. There is some fugly looking soldering in there but it may still be fine. If all else is working and you just have some power supply ripple, the caps might take care of it. I lean in the direction of getting an external supply if the transformer might be damaged leading to a potentially catastrophic event.

The brown stuff is coming from the inside though & the other one seems to be bulging on top picture#4-5 (or is it an impression?)... The Brown-Spew isn't there... C28-29 are the ones next to the diodes & seems to be fine as well but my DMM can't check any capacitor 

[Davve1992]

> have a DC in jack??  I know the transformer steps down the 117V ac to 15V DC...

It eats plus and minus 15V (30V total).

If you have a 24V to 32V DC supply, it can be hacked with a rail-splitter to give proper power.

> the power transformer seems to be buzzing/humming like crazy...

So replace the power transformer?

I thought about the rail-splitter but I didn't start looking for a schematic though and about the transformer: if I can't take it out and mod the power supply properly, I've found a guy on eBay who sells NOS PT.

[PRR]

> guy on eBay who sells NOS PT.

My advice: don't get clever, take the easy way out.

There was nothing wrong with the original supply. Except they didn't build it to last decades. Whatever has gone wrong since can be fixed easier than re-inventing this wheel.

[mimmotronics]

> guy on eBay who sells NOS PT.

My advice: don't get clever, take the easy way out.

There was nothing wrong with the original supply. Except they didn't build it to last decades. Whatever has gone wrong since can be fixed easier than re-inventing this wheel.

I agree with PRR. In a repair situation the first thing I would do here is identify and replace the power filter caps. A lot of times hum is present because the caps' dielectric is drying out so its reactance starts to change, causing it to inaccurately filter the power supply ripple.

Of course, if I owned this pedal I'd be annoyed with the power cable and would probably mod it to accept a DC power supply input...but thats just me!   

[Davve1992]

I'll change the two blue filter cap during the week & I'll keep you guys posted

The value isn't labelled on the cap though and the schematic did't come from MXR/Dunlop since "they don't have this information since Jim Dunlop bought MXR"... Most schematics (flanger/chorus/delay/etc..) I've found online suggest a 400uF Cap but the one linked on this page says 470uF... What do you guys think??

[Plexi]

My humble opinion: I would move to a dc-jack and jump all the PS stage.

I've always be afraid of pedals with PS inside... Idk. 

[mimmotronics]

Just noticed they didn't label the caps....maybe the value's on the underside? If not, my intuition would be to use 470uF..but that's purely guessing on my part.

[thermionix]

the power transformer seems to be buzzing/humming like crazy...

It's not clear to me if you're talking about hum in the audio signal or the transformer physically vibrating and buzzing acoustically, because that happens too.  Assuming the former, yeah you likely need filter caps, but I would change that transformer too.  It is charred.  It may still be working, but all that burnt carbon stuff is conductive.  I would scrape and clean any charred material off the PCB while the transformer was removed too, and check for damaged traces.

[Davve1992]

the power transformer seems to be buzzing/humming like crazy...

It's not clear to me if you're talking about hum in the audio signal or the transformer physically vibrating and buzzing acoustically, because that happens too.  Assuming the former, yeah you likely need filter caps, but I would change that transformer too.  It is charred.  It may still be working, but all that burnt carbon stuff is conductive.  I would scrape and clean any charred material off the PCB while the transformer was removed too, and check for damaged traces.

No physical vibration coming from the PT as far as I can tell! Rubbing alcool should take care of the char? The buzzing/humming sounds like a rippling/bad ground type of sound but 10x louder then the audio signal (guitar)... Every ground connections are good so far!

[robthequiet]

From personal experience, what can happen with transformers is the coating on the windings deteriorating leading to shorts in the coils. Really great magic smoke generator.

Given that you seem to have reasonable voltages this probably hasn't happened. Thermionix has some good advice, imo.