If you really want to reduce it for test purposes, you could use 3 100uF or so capacitors, one from point 39 to ground, one from 6 to ground and one from S to ground, then break the link between 39 and 6 and put in a resistor. I assume the current is quite low, so use a resistor that only drops as much voltage as you're sure won't be missed, so for 10mA, 100R for 1 volt drop. This should drastically reduce the ripple that you measure. As Benta said, this is likely not your issue, but it will allow you to rule it out, as you wished. Keep in mind that ripple can be a function of load as well as input, what I've described will address both issues.