[sdiy] LEDs dying in LCD alphanumeric display

[Adam Inglis]

Thanks all and thanks Jeffrey for all that good info - spending a lot  
of time in a dark studio I understand how the ambience is affected by  
the gear's displays, although I'm not sure I'm ready to start colour  
matching!

It's  LED-lit (?side ?back), not EL. I've found a cheap replacement  
display in Aus, but I thought I would desolder and dismantle the unit  
to see how it worked and if the LED replacement was a simple fix.
Well I'm damned if I can see anything to replace - it's just a block  
of translucent plastic!
Looks like the $15 drop-in is the way to go...

cheers
Adam
On 07/11/2011, at 7:34 AM, J.D. McEachin wrote:

> On 6 Nov 2011, at 03:43, Adam Inglis wrote:
>
>> Why would the LEDs that backlight my Proteus' 2x16 LCD display
>> slowly die from left to right over the course of a couple of weeks?
>> Seems like an odd failure mode.
>
> It's remarkable how electronics manufactured at the same time can  
> reach end-of-life within weeks or even days of each other.
>
> It's odd that the LEDs would die, though, unless the unit has been  
> left on for many years, or the LEDs were overdriven.
>
>
>> The LCD works and responds to
>> contrast control.
>
> The contrast control has nothing to do with the lighting.
>
>
>> The 5 volt supply for the LED circuit is separate
>> from the LCD supply, but they both measure up fine.
>>
>> It's one of these...
>> http://www.densitron.com/displays/Alphanumeric_LCD.aspx
>
> But which one - edge LED, SMD LED, or EL?  Big differences...
>
>
>> Can you crack these open and replace the LEDs or do you replace the
>> whole unit?
>
> Maybe.  But I find it easier to just replace the unit.  Especially  
> when the same model is available.
>
> I've now replaced almost all the LCDs in my studio.  I started the  
> project because my EL backlights were failing.  Then I decided that  
> I hated the putrid yellowish-green LCDs that most studio gear  
> sports, so I started changing out working LCDs so the colors matched  
> the gear's paint scheme.  Call me crazy for spending the time &  
> money; I laugh at myself about it sometimes.  :)  Here are some tips:
>
> * Pretty much all character LCDs are compatible with the Hitachi  
> HD44780 controller.  So far I haven't found any 80s gear that won't  
> work with the CrystalFontz and Matrix Orbital LCDs I've been using.
>
> * If the original display isn't available (or wanted), ALWAYS open  
> the gear up and take some measurements before selecting a  
> replacement LCD.
>
> * LCDs w/ LED backlights are almost always thicker than the ones w/  
> ELs.  Usually this can be accommodated by changing out the standoffs  
> or bending the mounting tabs, but sometimes requires more radical  
> surgery.  The worst was my Prophet VS rack - I had to partially cut  
> the pcb and let it flex back to get the .5mm clearance needed (it's  
> ugly on the inside, but beautiful on the outside).
>
> * Make sure the LCD is the right size for the window on the front  
> panel.
>
> * If the contrast circuit has a panel-mounted pot, it will probably  
> work just fine.  If it's a fixed resistor, probably not.  If it's a  
> CPU-controlled PWM circuit, forget it.  For the latter 2 cases, I've  
> been drilling a hole in the back panel and mounting a potentiometer  
> so the VO voltage (pin 3) can be adjusted.
>
> * If there's a high voltage inverter for the old EL backlight, this  
> is a great opportunity to remove it and get rid of a potential noise  
> source.
>
> * Make sure you get the current-limiting resistor for the LEDs  
> right!  I usually calculate the value according to the datasheet,  
> insert a trim pot in the circuit, and dial up the resistance until I  
> can see the display is getting dimmer, then back off a little until  
> it's back to full brightness.  There's usually no need to run at max  
> current, and it will increase the LED life.
>
> Hope this helps.  Good luck with your repair!
>
> Jeffrey
>
>