There are a lot of replacement LED displays on the market, however some of them have not been properly configured for some specific synthesizers. For example the stock JHD732-24064C is way too bright when installed in Yamaha SY-77. Not only it will make its life shorter but the backlight is kind of on the light side of the blue, burning too fast IMO. So if you bought and installed a LED replacement LCD for your Yamaha SY-77, TG-77 or SY-99 and noticed it is a little bit too bright, don’t worry. A simple modification is required and you will be all set. I can not confirm or deny that one of the sellers who sells these replacement displays didn’t paid attention to the voltage that supplies the LED element. The fact of the matter is, the LED backlight is set to be driven by 4.7V which is way too much for a LED based element. A classic LED diode is usually set at 3.3V while displays usually go 3.4-3.5V but should not exceed 3.6V. Else the 50,000 hours rating can not be guaranteed. It will be more like 5,000 hours LOL!
It is easy to recognize these JHD732-24064C replacement displays. They come with just a strip cable and no power cable. The power is taken from the logic power supply line. And the value of the provided current limiter on the board is simply too small. If your display is as bight as this, it’s no good:
A friend offered to sell me his TG-77 and as soon as I saw the display brightness in his offer via FB I knew something isn’t right. Since I was looking for a TG-77 anyway, i bought it, but decided to inspect that display – it was way too bright. Unfortunately there is no schematic for this display board, but some visual inspection showed two jumpers on the PCB board that someone enabled. And as soon as I’ve measured 4.7V over the Anode – Cathode I knew we are onto something.
LED backlight full manual control for JHD732-24064C and Newhaven displays
In this article we will create manual control for the LED backlight. That way we will have a control over the brightness of our screen. This will ensure the long life of our LED backlight and also help relief stress from our eyes during late night hours operating on the synth. If you have a Newhaven display, go to the bottom of this article. And please note, this modification applies only to LED based LCD displays and not to the factory installed EL Foil based LCD’s that came on stock Yamaha SY-77, SY-99 and TG-77.
The two wires labeled FROM LCD and TO LED ANODE are actually the two wires that you solder onto location shown in the picture below.
On the back of the LCD unit you will notice two jumpers. Disable the one on the Anode end. That way current can no longer approach the LED and we can install our own current limiter and a potentiometer. Also inspect that the jumper for the Cathode is enabled (connected). While there, solder a wire to the SMD resistor and another wire to the IC’s pin 8. My advice is to buy 1m of ribbon cable and simply take out 2 wires out which will become our “cable”.
You want to apply a hot glue because we are talking SMD components – they don’t like having wires soldered onto them. This is to release the stress from the wire – SMD component junction/solder point which is the weakest point of this circuit now.
You will measure the length of the new “ribbon” wires until they reach the area near the power supply socket. Because that’s where the potentiometer will go.
Now you will drill a hole there.
And install a 1k logarithmic potentiometer with a 47ohm resistor on one of its connected ends.
Setting the potentiometer in zero value and measuring the Cathode to Anode voltage you should get something in this range. If you are getting 3.4V – 3.5V it is ok, but above 3.5V it is not good, so replace 47ohm resistor with a larger value (use calculator to find correct value) for this current limiter. This completes our modification. Power up the unit and set LED brightness at desired value.
Definitely try the minimal setting! The display will turn beautiful dark blue, and in late night hours you eyes will thank you. Plus it will live much longer. As a matter of fact i always have my TG-77 at this setting. It is still bright enough.
Semi manual backlight control?
Of course it is possible. If you don’t want to mess with a potentiometer but just want a fast switch for reduced backlight operation to extend the life of your backlight i.e. when you don’t work on the unit directly but use it remotely and thus not need the display. If that is the case then you can simply add a switch to the back. At the switch Position 1 you will put a 47 ohm resistor and at the switch Position 2 you will put a 1k ohm resistor. These are the values that I have found to give a good response. The rest of the modification is just the same and in the the above schematic just replace the potentiometer with a switch.
Newhaven display modification
Essentially you will do everything as described in here: Korg Wavestation A/D LCD Upgrade
Except that you replace the current limiting resistor (100 ohm) in that article with a wire that goes to the potentiometer with a 47 ohm connected onto. From potentiometer you go to the Red wire of the Newhaven display which goes to the Anode and you are set. The hole and the potentiometer go exactly as shown above.
