Unplug. Open the box and check for damaged caps.
They look like this:
(Notice the one on the left is good)
If you are competent, you can replace them.
If it's not the caps, or if you can't replace the damaged components, You can get a new one. Try this:
Mediasonic HW-150PVR HomeWorx ATSC Digital TV Converter Box with Media Player and Recording PVR Function/HDMI Out (Black)
It will not only convert to analog, but function as a basic recorder if you add a USB drive.
Or, just bite the bullet and buy a new HDTV. prices are really good now.
They look like this:

(Notice the one on the left is good)
If you are competent, you can replace them.
If it's not the caps, or if you can't replace the damaged components, You can get a new one. Try this:
Mediasonic HW-150PVR HomeWorx ATSC Digital TV Converter Box with Media Player and Recording PVR Function/HDMI Out (Black)
It will not only convert to analog, but function as a basic recorder if you add a USB drive.
Or, just bite the bullet and buy a new HDTV. prices are really good now.