The failure was due to a cold solder on the ram socket and a bent pin on the ic MOS 8276.

I have disabled the mod to make the screen in reverse (white chars on black background), i have also made the composite mod with the right resistor value for the best quality video output.

Replacement:

12►

Simple steps to replace the keyboard Membrane of the Sinclair ZX Spectrum 128k (heatsink).

• Buy a new Sinclair Spectrum 128k Keyboard membrane.
• Unscrew the eight screws on the bottom.
• Kindly disconnect the keyboard from the connectors on the main pcb.
• Remove the back of the keyboard.
• Unscrew the block that holds the cable.
• Remove the old membrane.
• Clean the keyboard with air-spray and a soft brush.
• Install the new membrane.
• Close the back of the keyboard.
• Very kindly put the block of the flat cable and screw in.
• The keyboard is ready to be connected.
• Very kindly insert the flat cable into the connector.
• Your Sinclair ZX Spectrum 128k is like new.

source: rwapsoftware.co.uk