Proscan was the premium nameplate of RCA's Indianapolis headquarters in the early 1990s. It was created to compete with Sony and Mitsubishi. In 2005, Thomson SA in France sold the brands to a Chinese manufacturer. Only the nameplates remain of the vaunted RCA reputation.
Fixing a Proscan HDTV is similar to fixing any flat screen television. Power supplies can be replaced; on-board software can be reset. Generally, the best "fix" is having an extended warranty and a receipt from Costco or Walmart, where the Proscan HDTV can be returned.
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Reset the audio back to factory defaults if the Proscan TV has no sound. Unplug the TV from the outlet for an hour. This resets the TV's defaults. As long as the main tuner's signal board is working, the sound will be restored.
If pictures appear black and white, check that video connections are going to the component video inputs (red/green/blue) RCA jacks. Be sure red, white or black plug RCA audio cables are not in video ports. Preferably, use HDMI cables for best picture quality and sound in a single cable.
For power on-off issues, try all on-off buttons on the set and the remote. If none work, replace the capacitors. Use the Proscan manual to locate and identify them. The download link is below.
Replace the power supply. Power supplies are at least generic. This is the easiest DIY repair, but more costly than capacitors.
Proscan is no longer offered by Costco. That is a red flag. In addition, remember that Proscan is nothing more than a name on a now third party-owned HDTV.