Quick update:
I found Sony 4.5V/ 500 mA adapter from my collection and luckily the polarity and plug matches with the scanner's needs. I noticed some clipping audio when the switch was in "charge" mode so I turned it back to "alkaline" mode. I am guessing this is due to low voltage and/or current. The voltage is showing 3.85V which is about the same I get when running off the batteries. Would you guys recommend running the radio this way? Thanks.