Fixing Korg I3s
#1 Fading backlight on LCD Display Screen. Although the display screen works, it can be difficult to see the text without a backlight. Firstly check that the contrast control works. Sometimes the contrast level is turned all the way up/down and you can’t see the text.
If the backlight is broken, you can replace the LCD perhaps an LED type 240×64 LCD display. You can get brandnew LCDs with LED backlights there on Aliexpress.com quite cheaply about $20 USD. You need the T6963 type LCDs. Remember also that the EL backlight on the original LCD needs about 100VAC which the internal inverter takes care of. If you are replacing it with an LED display, please use 5V ;).
#1a Unit turns on, LCD goes ‘black’ and no lights or keys / sound comes on. I have found one cause of this to be the powersupply +5V not starting up quick enough. The work around solution is to place a 10uF capacitor from RESET to Ground.
Where is the RESET pin? Fine the big D70433GD Microcontroller located on the Bottom Right of the Main Processor board.
Get the link here: http://pdf1.alldatasheet.com/datasheet-pdf/view/6999/NEC/UPD70433GD-12-5BB.html
Find Pin 8, and follow it around to the Specialised KORG MB622E 15 IC. There is a place where you can easily solder a capacitor between this RESET line and ground.
#2 Bad Capacitors. Does your Korg i3 smell like fish? Because if it does, corrosive electrolyte from the capacitors has leaked allover the board! Terrible! This causes tracks to be damaged and can cause open circutis and short circuits too.
Other than a visual and nasal inspection you can probably find the damaged capacitors.
#3 Still no sound? I’m not sure why but the Headphone / Line output ports of the i3 are controlled with quite a few of these NPN transistors. (Which have a non-conventional SOT23 1-B 2-C 3-E pinout, most SOT23 NPNs have 1-B, 2-E, 3-C.. but anyway) I found that these NPN are connected in common emitter mode (Emitter is tied to ground) the audio signal is passed in the Collector and the base is controlled by this ‘interesting’ circuit.
This is the mute circuit. If the mute circuit is damage the unit will always be MUTE! Again the mute circuit relies on a working capacitor, if this capacitor is dried up like most electrolytics are, then you need to replace this.
I also found that all these NPN bases are all commoned. I found the voltage base control circuit output 1.4V and all the NPNs were fully on. Hence the loss of sound (it got grounded!)
You’ll also see that there is a 1MOhm resistor pulling the base output to the -12V Rail, but it’s too weak to have any effect.
So to get sound, simply HardGround / Shortcircuit the base output to 0V. Yay the NPN’s don’t turn on and we get sound!
#4 I also had an issue with the PCM69 DAC… it didn’t seem to be outputting any sound despite Digital audio data being put in. So I replaced it with a PCM1781. The PCM1781 output also has to go in after the first stage OPAMP otherwise you’ll get really bassy sound. I also noticed that at high volumes there is digital distortion which is not good. Does anyone else get this on their Korg i3 at MAX all volumes? The distortion is not related to headphone clipping as the master volume is an analogue control not a digital control like the rest.
Hopefully that helps you guys get things fixed!
Taking and putting back together a Korg I3 is really a pain as there are a lot of screws needing to be taken out. Especially with the Display you pretty much need to take it all out before being able to have access to the display module LCD.
You can also download the Korgi3 Service manual here:
KORG i3 Service Manual