You have to be careful when purchasing an AC adapter online especially when the specs of the adapter are not listed anywhere. For example there is a "Made In China" AC adapter being advertised on eBay right now specifically for the Radio Shack PRO-197 scanner for $12.94 (
Wall AC adapter FOR Radio Shack PRO-197 Receiver Scanner | eBay). Nowhere is the output voltage, output current or polarity of the center conductor of the connector listed.
You might get lucky that the seller correctly listed the adapter for your device and the adapter may work OK with your device. If there are no specs listed (as in my example above) you really need to test the adapter for proper voltage, current and polarity with a test meter before hooking it up to your device. If you don't, you run the risk of causing permanent damage to your equipment with reverse polarity, under/over voltage or less than the required current to run the device.