Prehistory: Fluorescent lamp gone and I didn't find replacement in local shops.
I could throw lamp to the trash but I decided to upgrade it to LED lamp.
Old lamp sockets (holders) have been removed as electronic block.
I opened OpenSCAD software and started design new things.
Meanwhile I purchased a 12V 24W power supply unit. I have some LED strips and wires on hands. So, I took out PSU from it's enclosure and made just an insulating holder. PSU board has been hot glued inside holder as holder has been hot glued to lamp. Soldered several wires and one connector between PSU and LEDs. Nothing magic.
Three lines of LEDs (21 LEDs each) used in upgrade. See photos.
You need 2 LED holders and 6 clamps. You can see it on photo.