This week I started a project in which I want to replace the LCD screens in some of my synths. Most of them suffer from the well known decaying Electroluminescence (EL) backlight problem, resulting in a barely visible screen, and a annoying high-pitched sound from the transformer powering the backlight.
I decided to start with the Wavestation A/D. It contains a 240×64 pixels dot matrix display with T6963 compatible controller. This type of display is used in A LOT of synths from that era. There are some equivalent LED-backlit displays available from various (Chinese) vendors. A lot of people replacing their displays use the white-on-blue version, which looks really awesome. After some searching I decided to go with a white on black display, available from ARTRONIC (http://www.artronic.eu/pl/p/LCD-AG-240064A-DIW-WKK-E6/1035). I find that it better matches the faceplates of most synths, (white on black), and it less bright in the studio without trading in readability.
I used this guide (http://www.tellun.com/wavestation/wavestation.html) as a starting point. I did however want to cut as little cables and add additional components as possible. First I desoldered the flatcable PCB connector from the old Optrex display and re-soldered it to the new LED display. Using an external power supply i powered the backlight LED as a test. The pinouts of both displays were identical so I was pretty convinced the new display wouldn’t burn out. Powering the unit on I found the contrast signal range was way too high (as mentioned in the tellun.com guide), and the font size was off (also mentioned in the guide, suggesting Pin 19 / font selection should be actively pulled high). The latter I find strange as the original Optrex display also has font selection on Pin 19. The pin is open on the mainboard, maybe it is internally pulled high on the Optrex. I bridged pin 3 (+5V) and 19 on the display.
Next issue is contrast. The tellun guide suggest adding an additional resistors as a voltage divider to lower the contrast control voltage range. Doing a little reverse-engineering on the contrast control of the Wavestation reveals it is build around a potentiometer and two resistors (VR1, R2 and R3), a sort of voltage divider fed by -12V. Sort of, because there is no connection to ground, making it just a variable resistor in the range of 0,7k to 2k5 between the LCD contrast pin and the -12V rail.
I found that (while adding additional resistors in series) a resistance of 3.6kOhm gives ideal contrast. So ideally the contrast control circuit should work in the range of 2k5 and 4k5. I determined (using some obscure math) values of 6k8 and 3k9 for resp the resistors R2 and R3 give me approximately this range. While most of the electronics on the mainboard are surface mount, the contrast control is located on the front-panel PCB which is fully through-hole. So, replacing the resistors is an easy job and voids the need of cutting a wire or adding resistors to the display.
All whats left to do is remove the transformer feeding the EL backlight on the old Optrex display, to eliminate the high pitched noise. I reused the wires from the EL feed to power the LED backlight from the mainboard, adding a 100 Ohm resistor from the +5V rail, as described in tellun’s guide.
Excellent result with little costs and a few minor alterations. All steps I performed summarized:
- make photo’s of the screws mounting components and boards, these will be useful when screwing everything back together 🙂
- desolder 20-pin ribbon cable PCB connector and EL backlight power leads from old Optrex display
- solder them back on the new display
- connect connector pin 19 to pin 3 on the display
- remove the front PCB, and desolder resistors R2 and R3
- replace resitors R2 and R3 with new resitors valued 3k9 and 6k8
- remove mainboard and desolder the EL transformer TR1
- solder an 100Ohm resistor from the + lead of C49 to the pin feeding the EL backlight (see http://www.tellun.com/wavestation/WS_R3.jpg)
- replace the battery if possible 🙂
- put everything together and close the unit up
I have a Kurzweil K2000R, Kawai K5, Korg 01/W, Yamaha TG/77 and a Roland D70 which all have the same type of (failing) 240×64 display, wating for a overhaul 🙂 The D70 is in the worst shape with the annoying whine audible in the output signal.. so I´ll think this one will be next in line. Also I have an alpha Juno-2 with an 1×16 charachter LCD displayfor which I found a really nice OLED replacement 🙂 Stay tuned for the report on these!