Question: My guitar sounds pretty good just plugged in to the amp, but unfortunately my new pedal is starting to sound off; it sounds really strained and the effect is barely audible. It's less than a week old and sounded perfect out of the box when I first hooked it up. However, yesterday when I tried to play it again, dials and everything still in same place, it was almost as if it wasn't hooked up at all. Just have this sinking feeling that it's broken.

The only thing I can think of that I might have done wrong was hook it up to an external generator since I'm still waiting for my ISO bric to arrive; I wanted to play out in the summer a bit. At first glance it doesn't seem like a bad thing, the power generator is for general outdoor use and matches exactly my wall socket specs in terms of polarity, voltage and current -- erego I can't see how it would be mechanically any different than just playing in my bedroom.

Question
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Given my pedal's issues of sounding weak and totally unlike when I first plugged it in, what would be a likely explanation and/or the worst case scenario? I'm wondering if it's possible to underpower a pedal and if so, I guess it's just scrap metal now? Not sure what I should do if that's the case.

**Note:**

* Working with EHX Mainframe
* I realize this pedal sounds glitchy to begin with, but there truly is
a big difference from when I unboxed it. Fairly scientific about
concluding something seems wrong
* I have reached out to the vendor, not resolved yet, but I think I'm
within my rights. Either way, I want to learn more about these kind
of pitfalls just in case
* No battery slot

Edit
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I returned it to the vendor. Then I noticed another pedal I bought last year seems to be off. But I took it to a guitar shop and it played just fine on their amp. So the joke's on my amp; never would have thought. Systems built on systems....
Thanks to all who gave input; helped greatly.

Answer: 
The Mainframe is such an odd effect, most folks reckon that if you change some settings it is almost impossible to get back to the sound you had before.

That said, your outdoor power generator may not be well regulated, and it could have produced current spikes that could destroy circuitry.

The usual troubleshooting guidance applies - can you compare with someone else's pedal? But without an audio file, or more usefully a video file showing what settings you are using while playing through it, there is not much we can do from here. It sounds like getting EHX to look at it is the best course of action.
I don't understand "..hook it up to an external generator since I'm still waiting for my ISO bric to arrive..."

The Mainframe manual states that a power supply - presumably a 'wall-wart' type - is included. So I HOPE that's what you were using!

"WARNING: Your Mainframe comes equipped with an Electro-Harmonix 9.6DC-200BI power supply. The Mainframe requires 90mA at 9VDC with a center negative plug. Use of the wrong adapter or a plug with the wrong polarity may damage your Mainframe and void the warranty. Do not exceed 10.5VDC on the power plug. Power supplies rated for less than 90mA will cause the Mainframe to act unreliably."

If your generator DID damage anything, with any luck it was just the power supply.