Hello,
I have an older direct drive turntable that has a trim pot adjustment for sinewave voltage that drives the motor. The turntable was made before DSO's were available. The service manual tells which controller test point to use for the (analog) oscilloscope tip and where to connect the ground lead. SM instructions say to (with platter in stop mode, no sine wave to motor) adjust the baseline of the scope signal (horizontal trace) to the center of the graticule. Then with platter spinning adjust trim pot so that the max and min voltages of the sinewave are equal. Turning the trim pot only affects the negative portion of the sine wave. Turning the trim pot clockwise results in a lower negative voltage of the sine wave, but positive voltage stays the same. Sinewave generator is not perfect, slight variations of voltages between each cycle. Hence trying to get a better result with DSO.
I thought that with a DSO, if I set the trim pot so the mean voltage was zero over a large number of cycles, that would mean average positive voltage would equal average negative voltage, and that would be a more perfect way to set sine wave voltages with the trim pot. Measuring done with a 2000 series Tek DSO, 20K measurement points setting, DC coupling, edge triggering, somewhere around 100 cycles per measurement gate.
So when I get the trim pot set so that the DSO reads a mean of zero volts (DSO meaurement math function reading) for the sine wave and then I check on the analog scope using the SM recommended instructions, the negative max voltage of the sine wave is about a box lower than the positive max voltage of the sine wave on the analog scope.
I was using 0V offset on the DSO for measurements. Maybe I should have put a DMM on the test point and measured the voltage in stop mode and set the offset negative to that on the DSO?
Not sure how to do this correctly on the DSO. I don't understand the discrepancy between what I am seeing on the DSO and on analog scope measuring the same signal. I am assuming that I am doing something incorrectly with the DSO.
Thank you.