pedal works on 9v battery but not power adapter

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I recently accidentally plugged a reversed polarity 12v power adapter into my Tube Screamer clone pedal, which damaged the polarity protection diode and the IC. So I removed the diode, and I swapped the opamp for a new one ..... the pedal now works like normal again using a 9v battery, but will not work when I plug in the standard boss 9v power supply I was using before. This seems strange. I did measure the power adapter connector terminals with a multimeter though and was not getting a 9v voltage, more around a couple hundred millivolts. Any ideas on why this would have happened? Or what I could do to solve the issue. I don't really understand how I could have damaged the power adapter in any way in the process even though my multimeter says otherwise. Could I have possibly damaged the DC connector jack itself and its extra positive pin (pin 2) that goes to the board? I am still sorting my way through learning all this circuit theory and electrical engineering stuff that would be useful for me to know so any help or insight would be very much appreciated.