I lost my power adapter (transformer). It is a model 3900X. Can't seem to find the 3900X. Is there any cross reference
The 3900 is a Roku Express that uses a USB power adapter. You probably have a USB power adapter for a phone or tablet lying around that would work. Just look at the fine print next to Output for 5V 1A or something similar. They're pretty much all 5 Volts. It's the 1A that's the important part. A higher number is better, but something like 500mA is inadequate.
If you want an official Roku adapter, look here at the PW10.
The 3900 is a Roku Express that uses a USB power adapter. You probably have a USB power adapter for a phone or tablet lying around that would work. Just look at the fine print next to Output for 5V 1A or something similar. They're pretty much all 5 Volts. It's the 1A that's the important part. A higher number is better, but something like 500mA is inadequate.
If you want an official Roku adapter, look here at the PW10.