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Dimming LCD panel.

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 23 March, 2010 » Dimming LCD panel. « Previous Next »

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ant
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Username: ant

Post Number: 594
Registered: 05-2006

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Posted on Thursday, 11 March, 2010 - 11:34 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello all,

Our Kenwood microwave, two years old; the LCD, black numerals on a whiteish background illumination, is growing dim. The depth of the blackness, as it were.

Is the density of the figures dependent on drive voltage, is there likely to be a resistor going high? And yes, I'm very well aware of the hazards in a microwave oven...

If this thing gets much worse it'll only be good for destructive testing of CDs or whatever! P'raps it'll be no bad thing, it's not the most user-friendly appliance ever.

Regards Ant
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phonoplug
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Username: phonoplug

Post Number: 48
Registered: 08-2009

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Posted on Saturday, 13 March, 2010 - 11:49 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Sorry for asking the obvious, but is it the LCD or its backlight thats fading? I couldn't be sure from your description. Is the backlight LED or filament bulb? If the LCD glass is being kept hot by a bulb, that could degrade it over time.

Alternatively it could be LEDs fading with age is if they are backlighting.
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ant
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Username: ant

Post Number: 595
Registered: 05-2006

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Posted on Saturday, 13 March, 2010 - 12:04 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Helloee,

I _think_ it's the black numerals that aren't as dense as they were but I suppose that there'd be a similar effect if the backgrond grew less bright?

I don't think it's a bulb, certainly the display isn't warm and the colour's wrong, it's a sort of dirtyxxx yellowy white colour. No hint of it being discrete lightsources behind a diffuser either. Electro-luminescent? I've not played with these things but I'm aware of them...

It just seems stupid to need a new microwave for such a piffling but uneconomic-to-repair reason!

Regards Ant
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phonoplug
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Username: phonoplug

Post Number: 50
Registered: 08-2009

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Posted on Saturday, 13 March, 2010 - 04:02 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If its Electro-luminescent its quite likely that it would 'wear out' 2 years. Is it on continuously? They don't have that long an 'on' life but if are only used occasionally thats fine. LEDs can fade of course, especially cheap ones, and the colour could also change if they are white, as the phosphor on the LED ages.

I reckon you need to take it apart and do some investigating!

An EL panel will be no thicker than a piece of laminated paper, and look very similar too.
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microguy
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Username: microguy

Post Number: 19
Registered: 01-2010

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Posted on Saturday, 13 March, 2010 - 08:37 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Does it appear to be the contrast of the LCD itself? Sometimes there's a pot for that, sometimes it's just a voltage divider network.

I think with the EL panels, it's not so much the panel itself, as I've had some plugged in and they stay the same, after years, but rather the high voltage driver.
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ant
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Username: ant

Post Number: 596
Registered: 05-2006

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Posted on Saturday, 13 March, 2010 - 10:37 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello all,

I _think_ it's the actual black numerical display that's losing its contrast, I was trying to find out what to expect before I open it up as I have enough odd jobs on the go already!

I'll have a delve and let you know...

Regards Ant
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echase
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Username: echase

Post Number: 310
Registered: 07-2007

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Posted on Tuesday, 16 March, 2010 - 03:09 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I'd agree wth microguy. Try the LCD contrast resistors. Or maybe the voltage (probably 5V) that supplies these resistors has drifted.
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azayles
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Username: azayles

Post Number: 1
Registered: 03-2010


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Posted on Tuesday, 16 March, 2010 - 07:07 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The contrast on LCD panels is controlled by a resistor. Over time, this resistor may have degraded, developed a dry joint or gone completely open circuit.
I know what you mean about "losing contrast", and the back lighting itself will either enhance or diminish the effect depending on viewing angle and ambient light.
If it's a simple alpha numeric Text only LCD with a HD44780 or HD44100 controller chip, you'll be able to find a replacement. For example here http://www.maplin.co.uk/Module.aspx?ModuleNo=36389http://www.maplin.co.uk/Module.aspx?ModuleNo=36389
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ant
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Username: ant

Post Number: 598
Registered: 05-2006

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Posted on Tuesday, 16 March, 2010 - 10:05 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thankyou all,

Hopefully I shall be able to have a look tomorrow -

Regards Ant

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