20 Most Recent MXR Phase 90 Pedal Questions & Answers

I would check the difference in sound between using an AC adapter and a battery. These things lose the effect when the batteries get low.

Make sure power voltage is adequate. If using a battery, the LED will light LONG after the batteery is beyond usability.

If it released it's majic smoke- likely an electrolytic blew out. Easy to replace those. It may just have been all that blew. If something else blew, it may have been a diode, especially zener diodes. But I would start at the Capacitor that blew. I would replace the other electrolytics just to make sure no future problem developed.

First thing to check is the battery or the adapterr.

The battery is being shorted by something. It is impossible for me to guess what without looking at it. If the pedal is brand new, take it back for warranty repair/replacement. If you bought it from somebody, you bought THEIR problem... they likely knew about it. If this is the case, look for a pinched wire from the battery lead... make sure it is not backward. If you can't see anything and cannot get a refund, open the pedal. Look for a problem where the 9 volt adapter plugss in. Broken jack, etc. Look for evidence that somebody has taampered with the insides. Of course look for foreign debris.

The fact that it happens with 2 different units may point to something in common. You just first eleminate the cables as a possibility by swapping. Also, most pedal problems can be traced to a battery issue, so make sure it is a good one. Make sure you are using mono, not stereo cables.

The footswitch could be faulty, try holding it in different ways to see if it makes contact.

If using another pedal in the loop, change the orrder

Have you tried experimenting with the order of your signal chain? Have a look at this link:http://www.zimbio.com/Guitar+Boomer/articles/36/How+Choose+Correct+Signal+Chain+Order+Effects

sounds like a switch contact problem, try some surgical alcohol. (for the MXR not for you!!!)

My first guess would be that the zener diode is probably shorted. Also check the 2N4125 transistor for a short. Start there and update this with your results.

Dan

One needs to consider the history of the unit. Could someone have used a reversed power adapter? If so they probably blew those diodes.

Input power protection diodes are often in the 1N4004 family.

Do not change any diodes without knowing the type. There may be Zeners as part of the effect shaping.

Check the power getting to the Op amps... pin 8 and 4 are the power pins, 8 being positive. Don't get fooled as sometimes the circuitry has the positive as signal ground.

ONLY SHOTGUN a device as last resort.

The posts on this site will help:http://music-electronics-forum.com/t10712/ I had trouble finding the correct schematic but the one here:http://www.tonepad.com/getFile.asp?id=42 should give you an idea of what I am talking about. Using a volt ohm meter on the D.C. volts scale ,hook up the black probe to the circuit ground. Make sure to use the correct adapter as a variation in the current output. ma will blow your pedal up. The diode in the schematic with one end hooked to ground protects the circuit from adapters that are wired backwards. It is the first thing you want to check. You will have to perform a diode test. Hopefully your meter has this function. If the diode is O.K. and you can get the correct voltage ( polarity, milliamps etc.) into the circuit, have a look at the schematic. Test each I.C. for voltage at pin 8. Depending on what I.C.s are in your pedal you can find the defective one by testing for voltage at the supply pin and then the other pins of each I.C. If you find voltage at a pin that should only be carrying signal you probably have found a defective one.For instance, in the sample schematic, pin 2 of IC 1a should have no voltage present such as 9 volts for instance. Test all the diodes, test all the ICs. If they are all fine you will have to test each transistor. The info on how to do all this as well as the pin outs are all available on the internet. After you find the defective part or parts test any parts that are hooked to them in the circuit as well as they may have failed or caused the other part to fail. Before I got an oscilloscope I had to unsolder a lot of parts to test them with a VOM, a transistor tester and a capacitor tester. The repairs paid for the scope. Let me know how you make out. I don't work on effects that much any more as my eyesight is failing but I might be able to steer you in the right direction. If you decided at this point that you don't want to try the repair, take it to a good musical instrument repair place and have it fixed. Vintage pedals are getting to be worth a lot of money and they usually sound better than the reissues.

These break and SOMETIMES the circuit board gets cracked.

Open up and inspect for damage. If board is cracked, bridge broken traces by soldering wires around the break.

If the solder is broken, resolder it.

If the connector is damaged or weak spring, replace the connector.