Friday, May 31, 2013
K2000 Repair, LCD contrast capacitor added; similar to XP50 contrast issue
This Kurzweil primarily had an issue with the 220 uF capacitor right near the fet transistor which regulated the 5 volt rail. That was causing the fet to heat up. Once that capacitor was replaced, there were still issues with the LCD contrast. Using the old trick of holding enter and turning the alpha wheel didn't really help; what ultimately fixed the issue was adding a 10 uF electrolytic capacitor between the LCD contrast voltage and ground (see photo). I found that this greatly stabilized the LCD and made the writing legible. Although there is a capacitor that is supposed to filter this voltage already, I couldn't easily find it, and adding another capacitor fixed the issue.
This was a situation where the customer did not want to spend too many resources on repairing the issue; so although this repair is not one" right out of the textbook" , it did fix the problem and the unit is working.
In my opinion, the idea of using the microcontroller and PWM to create a voltage for the LCD is a scheme which is prone to failure. When the user data containing the contrast parameter is lost or corrupted, or the PWM circuit for the LCD contrast fails (as it did here), the user is left with a blank screen. I think it is far more sensible to use a good old potentiometer to provide the LCD contrast voltage.
A similar issue occurs on the Roland XP50, another synth which uses the microprocessor to store the LCD contrast parameter. When you change the battery on an XP50, you may wind up with a blank screen In that case I used the info kindly listed here, which tells you how to ground the contrast pin in order to bypass the PWM controlled LCD contrast voltage.
Posted by chris rowland