fix Yamaha DGX-640 LCD backlight?

[greenpossum]

I've got a Yamaha DGX-640 digital piano which works fine in every respect, except that the LCD backlight has failed. The LCD is working and can be read by shining a light on it.

Once in a blue moon, the backlight will fire up and it will stay lit for a while, then randomly turn off hours or even days later, or until the piano is turned off. This leads me to think some voltage converter or dimmer control has failed.

I found a copy of the service manual online, but I am loath to take the instrument apart because I play it all the time. But if someone can say for example, yeah this happens if Q13 has failed, replace it, then I would give it a go. Otherwise I'll wait until I upgrade my piano and it becomes a spare.

So, is it Q13 or ...? 

[james_s]

Unfortunately I'm not familiar with that piano, however the fault sounds a lot like either a cold solder joint or a worn out CCFL tube. How old is the unit? Can you tell what sort of backlight it uses?

[greenpossum]

It's from 2010 according to the manual and started without warning or dimming a couple of years ago. I'll look into those, thanks.

[james_s]

Does it respond to gently tapping on or around the display? Is it difficult to open it up just far enough that you can access the PCB driving the display to poke at that?

[greenpossum]

Does it respond to gently tapping on or around the display? Is it difficult to open it up just far enough that you can access the PCB driving the display to poke at that?

No, "percussive maintenance" doesn't get any results. There is no rhyme or reason why once in a blue moon (and I really mean rarely, like once a month) it will come on. I think perhaps there is some voltage converter that drives the backlight which only starts to oscillate once in a while, but the manual has no circuit diagrams, only block diagrams.

I'm not inclined to do anything since it's only a nuisance. I leave it at Grand Piano setting and never use the voices and rhythms anyway. If I want to I can shine a torch on the display. I was just hoping on the off-chance that this was a very well known problem with a simple solution. The LCD unit may be integrated and impossible to service by itself.

Thanks!

[james_s]

Well it's possible that someone who has fixed one of these will chime in with a solution, you might also look for more musical instrument or synth related forums and see if you find anything there.

I suspect the problem would be fairly trivial to diagnose once you got in there to take some measurements but if you want to work around it that's fine too.

[Till Matte]

I suspect the problem was from the welding connection for the backlight pins, the poor welding connection between backlight pins and lcd voltage input, you could open it and check if it is.

[Haenk]

There is a replacement panel available on Aliexpress (less than 50 EUR). As the backlighting is integrated everything is connected with only one flex strip, it's most likely not a power supply problem - replacing the panel/modul should fix the problem.
(Of Course one could still disassemble the module and check for the failure, that's a matter of how much your time is worth to you…)

[coromonadalix]

Hi  youll need to do some checks to see if you have a problem

You have a cotrol knob for the backlight adjustment,  i think it will go from blank / total dark to a white square on the lcd ? does it work ?

And yes youll have to dig in it ??   saw lot of the same lcd problems on many yamaha models ??

For the backlight voltage  you should have between 21 to 27 volts, a 3.3v supply and a 5 volt for the backlight voltage, supposed to be on a 2 pin connector ??


The lcd its an 320x240 pixels,  it should be easy to source and find the data sheets  its a standard 14 pins header like i have on my 320x240 lcd i have
Nope   seems a poprietary one ?

service manual link to help others to answer you
https://elektrotanya.com/yamaha_dgx-640.pdf/download.html#dl

But  its incoomplete you dont have the DM board schematics  ...


Youtube video




Other similar problems on the dgx-620
https://www.instructables.com/id/YAMAHA-DGX-620-LCD-Fix-With-NO-Cost/

https://www.yamaha-keyboard-guide.com/yamaha-ypg-keyboard-screen-display-problem.html


Lcd sold on Aliexpress around 50$ usd      much higher on Ebay ...
https://www.aliexpress.com/item/32830468421.html

https://www.aliexpress.com/item/32810594958.html

[greenpossum]

Thanks for the suggestions and links.

The LCD display works fine, as does the contrast pot, so it's just the backlight. Tapping doesn't make it light up, so it's probably not a solder joint issue. Only once in a blue moon will it light up at power up, and it's completely unpredictable when that will happen, might be tomorrow, might be next month. So I suspect an intermittent component in the backlight circuit.

Getting a replacement for about 80 AUD probably is the best solution. But I am loath to dismantle it because I play it all the time. I don't use the accompaniment features since I learnt to play two-handed. So it's more important to me to have a working piano than to read a display that I don't use at the moment. I can shine a torch on it if I need to change anything.

So I'll take note of the various suggestions and maybe someday when I get a better piano I might fix this one. Thanks again all.