This article applies to the Platinum Penta
With the compressor engaged and the compression and ratio knob turned fully clockwise you should see the maximum amount of gain reduction (all LEDs on the gain reduction meter lit when the signal is present). Make sure that you have a good input level and see if you can hear a difference when switching the compressor in and out with these settings. If so then see if you can hear a difference when you reduce the 'compression' knob. You should hear the sound get louder and should see the amount of gain reduction decreasing (that is with a constant make-up gain). If you do not see this then it may be that the compression knob is not having an effect. If you leave the compression knob fully clockwise and reduce the ratio knob you should see the same effect as when rotating the compression knob but possibly to a lesser extent.