This particular unit I acquired really cheap. It's a digital guitar effects processor. The listing said that no data showed up on the screen. That was a gamble, but I felt it was the power supply. The listing said it was on "all the time", which probably meant the filter caps were worn out.
Right away, I found a low 5 volt rail. A bad filter cap feeds ac into the rail, and makes it show up as less than 5 volts. Filter cap replaced.
Now it works, kind of. It turns on, and it consistently says on the screen 01 No DSP. I felt for sure that meant the digital signal processor components were not registering with the micro controller.
I suspected software. I reloaded the firmware, and suddenly, it turned on! It worked.
Being a thorough individual, I let it sit for an hour or so, and went to try it again. "No DSP". Back to that problem.
Now at this point I began to suspect the connections on a socketed IC. I cleaned it and tried again. No change.
I did battle with it for an hour, focusing on reflowing the solder around the various IC's. I'd seen issues like this, intermittent digital problems, almost always caused by a necessary reflow of solder around surface mount components, especially when lead free solder may have been used.
No real success.
This was one that was better left unfixed. This happens; at times we have to realize that a repair is not feasible.
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