Sharp microwave - dim display (sometimes)

Dec 05, 2017 4 Replies

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Peabody 8 years ago

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?

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dansabrservices 8 years ago

As with most displays, the likely cause is bad grounds. Check all of the solder joints at the display itself. Look specifically at the connections at each end of the display.

Keep us posted.

Dan

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tabbypurr 8 years ago

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Most likely it's a bad connection somewhere. You could tap around with a pl astic rod to see if you can locate it, but the capacitor that supplies the magnetron is filled with enough volts to fry a queue of circuit board tappe rs, and those volts are conducted about the place too.

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whit3rd 8 years ago

Well, C1 and C2 are intended to keep the lamp voltage near-constant, I'd check those (or replace, they're inexpensive parts). You might also consider your AC socket, at least hold your hand on it to determine if it's getting warm (this has happened to me). Be safe, if you open up that socket (live wires even AFTER turning the breaker off is not unknown).

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Peabody 8 years ago

I tried it on a different socket, but no change. Also exercised every connection I could get to, but wasn't able to get any further improvement. It's bright enough to be useable for the time being, so I'm going to declare victory.

Thanks for the comments.

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