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My phase 90 is not working - MXR Phase 90 Pedal

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  • nikos Jan 18, 2013

    A friend of mine by mistake gave power to the pedal instead of 9volts 24volts!I change the two diodes in the power section of the pedal and now I have sound from the pedal but I don't have the phase effect!I change the first tranzistor and afterwards the 2n5952.But still nothing you think the problem is the tl 062 or the tl 064 tranzistors?I want to be sure because It's a bit tricky to change them and I'm afraid I might damage the pcb.Help!

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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.

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I have the MXR EVH Phase 90. When plugged in, and unit is off my sound is fine. When unit is activated, the light comes on but there is no sound. The same thing is going on with my Maxon AD999 Delay pedal....


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.

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Just recently had a friend give me a broken mxr phase 90. said that he couldnt get it to power on with a 9v batter and had never tried a power supply. so i just figured that it just needed a new 9v battery...


This is likely a problem very near the power input connector. It could be a blown filter cap (electrolytic) or a shorted protection diode. You can troubleshoot this with only an ohmmeter.

A broken power input connector could also be causing a short.

Often the input audio connector is used to switch the power on so trace the power circuitry looking for the problem.

Suggestion test seperate from other pedals. Be careful to not burn out your power supply during testing... we put a lamp in series with teh 120 volts to the power supply to limit current to a safe value while troubleshooting.

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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

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1 Answer

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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

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1 Answer

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

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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.

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If you are using battery power, just because the LED lights doesn't mean the battery is adequate. Check the battery voltage.

If you are on adapter power, make sure any bypass switch is in right state. Also occassionally someone gets the in's and out's cables backward... that can do it!

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1 Answer

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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.

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