Hello all. I recently bought a mxr blue box on ebay at a reduced price because the seller said there is an issue with it. The problem is that sometimes it will turn on and sometimes it doesn't. I bought it because i thought i may be able to fix the issue but I don't know too much about electronics and I was hoping someone would know what could be wrong with it. Thanks.
Could be a bad input jack or foot switch or a bad connection on one of those.
Have you tried it with batteries or a power supply?
Imagination is more important than knowledge...
It is a miracle that curiosity survives formal education.
- Albert Einstein -
When I try it with a battery I get no signal when I turn it on and no LED. I also tried it with a power supply and when I turn it on I get an LED light but no signal. Im starting to suspect the input jack now because with a power supply you get led light whether or not the input cable is plugged in. How can I test it though?
A digital multimeter will test for voltage at the ring. If yours has continuity test, you can use that too to make sure everything has continuity.

You need to determine if it's a power issue or a signal issue or both. There definitely seems to be something wrong with the battery clip and that should connect through the ring of the input jack so that is a good place to start.