I have a Sharp Carousel countertop microwave oven, model R-303CW, vintage
2001 - barely broken in. It works fine, but the LCD display has been gradually getting dimmer. So I found the schematic:archivos.diagramas.mx/otros/R303CW.pdf
and I took the cover off and looked around. It turns out that that control board and display are not readily accessible, and in any case it's all the smallest SMD stuff, so I figured my chances of actually repairing it weren't too good. And I put the cover back on.
However, that process produced a change in the display. It is now somewhat brighter when the door is closed and the oven isn't doing anything. But if I open the door, or start the oven cooking, the display goes dim.
The display appears to be backlit by an LED panel, shown in the schematic as four LEDs in series. From the symptoms, it seems that there's a power problem somewhere that I inadvertently partially fixed. So of course I will go back in and remake all the connectors, and will re-socket the fuses. But I wondered what else I should check for. In particular, do fuses increase in resistance as they age? I've heard that fuses age, but I don't know if that means they become more resistive, or simply become more fragile at the same resistance.
The other thing I need to check is the compartment lamp, which is ON when the display goes dim. Maybe its resistance is decreased. And that's the only thing that changes when I open the door - that lamp comes on.
Any suggestions for other things I should check?