CS-80 Calibration Part 6: Noise, PWM, Sub Osc

After the voice board procedure has been finally finished the next board to take out is the Sub board which hosts the noise gen, the PWM curcuit and the sub oscillator. While it looks straight forward I got again into wonderland as soon as I reached the PWM part.

Sub Board

Noise Generator

A no-brainer, adjusting the two trimpots (one noise generator trimpot for each channel) was done in a few seconds.

Pulse Width Modulator

I was about to throw the CS-80 out of the window when I was working through this step. Luckily it is too heavy for that.

I was unable to set the 150ms/5ms adjustment ping pong at step 1. The closer I got to 5ms, the less I was able to keep the 150ms. The closest I came between them was 123ms/5ms. The original value I wrote down before starting was 151ms and 10.5ms at max speed. After some experimentation I was sure that the manual may have another error, respectively is giving the values for another reference level.

I did some reading in the CS series manuals and I came up with an idea what is happening here. In the pwm chapter is a table that lists the voltages/oscillation. I was running some tests, connected a frequency counter in between, and voila, setting 150ms at 5V will show a reading of 6.7Hz as in the table.


I think the 5ms is meant for 10V, but the maximum level my PWM sliders were going on max setting is between 8.79V for CH1 and around 8.90V for CH2. My theory is, that the 5ms would be correct if the slider at Max level would have gone up to 10V. It made sense when I did some testings. With help by a friend about how to handle exponential calculation (actually he did it and gave me the formula ;) )  I have a solution that works for me. For me it means at 8.79V the frequency should be around 95hz.

If you are stuck at this step as well, take out your calculator and enter the following calculation (replace V with the voltage you measured at max PWM setting):

• 0.2 * (2.0165 ^ V)

Result will be the frequency (f). To get the ms, just calculate:

• 1/f*1000 

and you get the ms for that voltage. for me at 8.79 it was 10.5ms, which actually matched the existing value.

Another problem solved, except that my PWM lever on channel 2 didn't work properly. The values changed when I slightly wiggle it, guess I need to replace it some time soon.

Sub Oscillator

I think the similar is happening here as with the pulse width modulator (just a different voltage, though). My lever on max showed 10.58V. When I moved it to 10V, the reading about 18ms (while the reading at 5V showed 150ms), which would be within specs. So it was set correct in my opinion and I didn't bother to fiddle any further with it.

Continue to Part 7: Ring Modulator