Electrical Problems with a Microkorg
- Hi all,
may i introduce myself - i´m Stefan from Germany and i am a "more or less" long-time user of the MS2K, got my first one when it came fresh out.
Well, after selling my last one i´ve obtained a death Microkorg (old Version). It has the "usual" CPH602/S8520 power problem.
Now the previous owner removed the faulty parts and attached three wires, one for 9+ Volts, GMD and 3.3 Volts. As i have a swiching power supply, which already provides these voltages, i´ve used it.
So the Microkorg powers up, plays, buttons + rotray encoders work, even midi in and out. But i cannot get the knobs working. I have to say that i only have the single board, no keyboard, pitchbender, modwheel etc.
Now the technical details: I´ve done some measurements on the CPU and components around The CPU gets for its ADC converter on AREF and AVCC nearly 3.3 Volts and also GND. The resistors, which are connected up to the knobs, lebeled R94-R98, show on the input side up to 2.8 Volts, but on the opposite side as a maximum Value 0.7 Volts. These Value changes as soon as i turn a knob, but the CPU doesn t take any action. So it proably "could be clear", that the ADC (so the CPU) is defect. But the question for me is, if AREF/AVCC could be proably too high (what i doubt), or if the values from the resistors to the analog inputs of the CPU are too low, -or- if the PSU is proably too noisy or delivers some voltages simply "too late", so that the CPU takes on its initialisiation a wrong AREF value. Or could it be a problem that the analog inputs of the pitchbender and modwheel are not connected (running free, producing proably noise to the ADC)?
In Test mode the CPU complains about somewhat "C9B" and in operation about a "low battery", which could indicate an ADC failure, since the power detection circuit also ends up in the ADC (R114 is ok and outputs 0.89 Volts). I have to say that i feed the Input with 12 Volts instead of nine, but since it goes through the 5VDC converter, i see no problem. 5 Volt and 3.3 Volts are stable for at least three digits behind the decimal point. I also measure no noise (in terms of Hertz) between the analog inputs and GND.
Does anyone know how much AREF / AVCC could be and the values of the analog inputs, when the knobs are turned to the maximum? At least, is there anywhere a firmware/operating System out there for the Microkorg and is the machine at least able to load firmware upgrades?
I know, it´s a very hard problem, but it would be great if anyone could have some tips.
- Well, seems to be clearly an ADC fault. If i take 3.3 Volts into one of the analog input pins, the CPU starts to get warm (not hot!) in the Area of the pin. Then the ADC starts to react "somehow", it doesn´t care about values, but more or less on "trigger signals" of the 3.3 Volts on its input. Means to me: The ADC tries to obtain its AVCC through the analog input pin through the internal multiplexer and that causes the overheat. This could also explain why the ADC itself does not work at all, cause AVCC seems to be internally disconnected, maybe caused by a voltage peak of the failing 3.3 Volt components. And that also causes the "low battery"-messages.
Well, if there is no way to get hands on a H8S/2350 or something similar, it could be again something from the trash.