Sharp microwave - dim display (sometimes)

[Peabody]

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?

[dansabr...@yahoo.com]

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

[tabb...@gmail.com]

On Tuesday, 5 December 2017 23:44:16 UTC, Peabody wrote:

> 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.

yes

> 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?

no

> 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.

it isn't

> Any suggestions for other things I should check?

Most likely it's a bad connection somewhere. You could tap around with a plastic 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 tappers, and those volts are conducted about the place too.


NT

[whit3rd]

On Tuesday, December 5, 2017 at 3:44:16 PM UTC-8, Peabody wrote:
> 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:
>

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).

[Peabody]

whit3rd says...

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.