@Rain66
First, it does not cost anything to activate your RokuTV. Just make sure you are dealing directly with Roku. Any other entity that asks for payment to activate a Roku device/RokuTV is a scammer. Roku does ask for a credit card, but you can skip this step, or remove the payment method later after activation. (this is just to have payment info for any subscriptions you may purchase through Roku, but the Roku platform itself is free, as is setup and activation.)
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As for your desktop issue, you may just have to check your desktop settings (which is hard without a monitor so may need to swap your existing monitor to this desktop to check the Video Out and configure the graphics card), or try a different video adapter to connect to the RokuTV HDMI port. I have an older desktop that I always used with a TV that had a VGA Input for a monitor. When I upgraded to a newer TV that only had HDMI inputs, I discovered my HDMI output on my graphics card was not working (not sure if it ever worked) and received the No Signal message on the new TV. I just used an Insignia VGA to HDMI converter/adapter and the TV recognized the desktop signal. (this adapter comes with an integrated USB cable for audio delivery).
Not sure how you are trying to send the video and audio signal from desktop to the TV. (HDMI, VGA, DVI, DisplayPort). I assume you already tried using the HDMI connection of the other working devices (cable box, PS3/PS4)?
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102