@jlcastemtz I am not certain how the determination was made that this is an issue with the "principal circuit" (not sure what is meant by that). I would start with a new LCD and check the flex cables to ensure that they are properly seated. Start with the easy stuff first.
I recognize that the on screen menu is still displaying, but the possibility of a bad LCD is still there.
Before anything else, you do want to try a different memory card. You also want to repalce your main battery. It is possible that it can't deliver max power demanded and the voltage will drop too low, causing a mis-operation. It may recover to quickly for the camera to register it as a power error. If that does not do anything,there is a small battery (BH3101) on the mainboard.
Replace that and see if that will help. Check for corrosion around the battery case on that board.If nothing happened to the camera, there must be a relative simple explanation, but since the viewfinder and the LCD display the same thing there is always the chance it is the image sensor or a failed shutter. Remember to start with the easy things. New battery, new memory card and then a hard reset
Reset your camera to the default settings:
1.Select Menu -> Setup -> Initialize on the camera.
2.Select Reset Default.
4.Wait more than 10 seconds. (Do not care if the LCD screen of the camera turns off, or not)
5.Remove the power source ( battery or AC adapter ) from the camera.
6.Attach the power source ( battery or AC adapter ) back to the camera. Let me know if the schematic etc. would help you with the repair.