Voltcraft DPS4005 DC Power supply constant current mode - high output ripple

[Momchilo]

Hi,
I have an old cheap Voltcraft DPS4005 lab power supply for DC output. It's split into four boards.
1. power output (AC power in - main DC out)
2. aux power (AC power in - +/-15V etc. out )
3. linear control board
4. digital board

If it's in constant current mode the output ripple reaches up to 1.3Vpp @0.4VDC with a 6.75 Resistor as load (ripple1.png).
With exact the same settings, the ripple almost disappears whenever the power board is lifted few centimetres above the bottom of the case (ripple2.png).
If the power board is insulated with a sheet of cardboard from the case the output ripple is ~300mVpp.
The voltage across the isolated negative DC output and earth is 10Vpp @50Hz. The case is connected to earth.
In constant voltage mode the ripple is below 3mVrms.

So I guess the ripple comes from a capacitive coupling between the power output board and the case. But even when I connect the isolated DC output to Earth, the ripple stays the same. And why it's only in CC mode? I don't know if it's by design or a fault. I found the schematics, maybe someone else has an idea what's wrong.
Thanks for your help.

Best regards
Momchilo

[TheMG]

Check caps in the circuit around U3 (current sense amplifier) and U4 (current limit comparator). Also carefully inspect for any broken solder joints or connections. Bad capacitors or poor connections could allow noise to be introduced into the current amplifier and comparator circuit, which would cause the kind of thing you're seeing.

You could also use your scope to probe the input pins of the two opamps and trace back where the ripple is being introduced into the current feedback loop.

As for the 10Vpp from earth to the isolated DC output (as measured with a high impedance DMM), that's pretty much par for the course, it's capacitive coupling resulting from interference suppression capacitors CY4 and the capacitance between windings in the transformers.