Vintage MXR 10 band EQ Mod

[Ice-9]

Personally I would take the easiest possibly best solution, change the caps and Tx for new. If you really want to convert to a DC input jack then you can still run it from a 9v DC pedal supply if you build a voltage doubler and inverter with a LT1054 a few caps and diodes. This would get you a bipolar +/-18v Dc fro a single 9v Dc in. You could use a couple of small 15v regs as well to regulate it to +/-15v DC.

LT1054 DATASHEET http://www.ti.com/lit/ds/symlink/lt1054.pdf

Check out page 18 figure 22 for the bipolar voltage doubler.

[Davve1992]

Personally I would take the easiest possibly best solution, change the caps and Tx for new. If you really want to convert to a DC input jack then you can still run it from a 9v DC pedal supply if you build a voltage doubler and inverter with a LT1054 a few caps and diodes. This would get you a bipolar +/-18v Dc fro a single 9v Dc in. You could use a couple of small 15v regs as well to regulate it to +/-15v DC.

LT1054 DATASHEET http://www.ti.com/lit/ds/symlink/lt1054.pdf

Check out page 18 figure 22 for the bipolar voltage doubler.

Could this layout be usefull? They say that changing the diode should bring the pedal down to +15/-15V. http://tagboardeffects.blogspot.ca/2014/01/bipolar-voltage-converter.html

[Ice-9]

Prettymuch exactly as I suggested, so yes it would be good, bring it down to +/-15V and your good to go. Tidier to use 15v regs that diode drops.

[Davve1992]

Hey guys! Just to let you know that it was indeed the filter caps that were dead. I couldn't get any reading from my DMM. The ripple/hum/buzzing is gone, there's only a small hiss from the opamps so far!
Both pictures are the dead Sprague Capacitor.

[robthequiet]

Job well done! 

[Davve1992]

Thank you guys! Your input was helpfull! 

[ElectricDruid]

> 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!   

I agree with PRR too, but he didn't say replace the caps (although it's not a bad idea in this case). He suggested replacing the power transformer. It looks to me like it's taken a bad knock at some point (dent in the top, frame coming loose) and that is likely to knocked the windings loose. As soon as that happens and you put a oscillating magnetic field around it, they move about like crazy and you get an evil buzzing sound. I've got a old tranny like this on my bench that I had wired up for a +12V test supply, but the noise is so painful I can't stand to use it. It's a pity since technically it still works, but it's not doing what it's supposed to any longer (like "be silent").

Replace the transformer, and while you're unsoldering stuff on that bit of the board, put new supply smoothing electrolytics in it too. PRR is right - there's nothing fundamentally wrong with it.

HTH,
Tom

[PRR]

It wasn't clear if the transformer was buzzing, or the pedal was buzzing (which can be crapped-out caps allowing rough DC to the circuit).

Agree that abused parts should be suspected and possibly replaced before they die.

[ElectricDruid]

Ok, fair enough. Could be caps, could be tranny. Do both, and then see what's happening. It's a basic process of elimination and not terribly sophisticated, but it usually works!

Tom