I have a portable clock radio that uses two 1.5v batteries, [3v total, obviously], & has an optional 3v, 300ma, [center pin -], DC adapter input on the side. I found a 3.7v, 340ma DC adapter at a thrift store, & would like to know if that difference would harm the unit. I think the 340ma part is ok, as that's just the maximum output rating for the current load on the adapter, but the 3.7v part is what concerns me. Any input would appreciated.
It would depend on the radio itself. 3.7v might not seem like alot, but it's a 25% difference, but you never know how tolerant the circuit inside would be to higher voltages.
Additionally, your adaptor might be a cheaply regulated one. It might only provide the rated voltage, at near the rate current draw.
If you can open it up, you could see if it has an internal regulator, but a 3v circuit on a cheap device, it would probably only have a diode or resistor.
Is the new adaptor also center pin Negative? The standard is normally center pin positive (have not seen any center pin negative power supplies in a decade, but they are not rare either). Having the wrong polarity would definitely cause problems.
As you stated, yes, the current rating is okay, because that is it's maximum output capacity. The radio will only draw as much as it needs/was designed for, and undoubtedly, it will draw less than the 300ma it lists (design tolerances).
(Personally, I have and would use such a minimally different wall wart. But it's not recommended, and only done it knowing that it could possibly damage the device. If you arn't willing to replace it if it breaks, don't do it).
No. The higher voltage will likely damage the clock radio, unless the clock radio has some protection circuit (unlikely since the spec is to the connector). As far as current, it's not an issue since 370mA > 300mA.
You could easily build a little circuit that regulates the 3.7V to 3.0V using a Linear Regulator. Make sure that the max output current is higher than 300mA so you have plenty of headroom (actual current will be whatever the clock radio needs, at or below 300mA).