There appears to be significant common mode noise on the output of the
CAEN N1419 4 Ch Reversible 500 V/200 ľA NIM HV Power Supply Module (USB)
- the specification quotes 'Max Ripple smaller than < 5mVpp'. We configure
the CAEN N1419 outputs as floating wrt NIM module/crate chassis.

For example, attachment 1, shows the output of module #2 (Addr 01) channel #3
(unused channel) set to 10V (leakage current 0.030uA) - core (Ch 1)/braid (Ch 2).
Tektronix 3034C channels 1 & 2 are both set ac/1M.

Attachment 2 shows math function Ch 1 - Ch 2 (50mV/division cf. 200mV/division
for Ch 1 and Ch 2)

Attachment 3 shows CAEN N1419 #2 settings

Attachment 4 per attachments 1 & 2 *except* the cables from channels 0 and 1
of CAEN N1419 #2 have been disconnected from the NIM module. The structure of
the common mode noise changes but its amplitude remains significant. This
presumably indicates that there are other sources of common mode noise in
addition to that observed at the output of the CAEN N1419 HV outputs.

Attachments 5 & 6 show the NIM crate front panel +6V output wrt ground
for the lower (NIM Bin type REPIC,  BNC PB-4, Ortec 433, 2x MACB, 2x CAEN N1419,
1x CAEN 1419B) and upper (NIM Bin type OKEN, 9x MACB) AIDA NIM bins respectively.
All +/-24V, +/-12V and +/-6V front panel outputs show the same noise structure.

Attachment 7 shows equivalent of attachment 6 but using Tektronix DPO3054
which can display FFT.

Update -----------------------------------------
Tested the CAEN HV supply in T9 using the same method and observed similar results.

With help from Mos Kogimtzis a better method of measuring the HV noise was used.
The co-ax from the isolated output of the HV module was connected to one input of the 'scope. HV set to 1V.
Thus the screen of the HV signal is grounded at the 'scope amplifier input. This nearer to how the HV is used with a detector. 
The single signal input channel was set to DC 1MR with an offset of 1V.
The 'scope can now display at 4mV/division. 
The peak/peak was less than 5mV.See attachment 8 slide 1

There are features at both high and low frequency, attachments slides 2 and 3. NB 1mV/division.

If a differential measurement is required then a suitable cable with a shielded twisted pair is needed.

Update #2 -----------------------------------------
Moved the AIDA T9 HV supply to a CAEN NIM bin. Newly purchased this year.
This is the only module in the bin.
 Used the simple method of noise measuring as before with the same cables and 'scope.
Attachment 9 shows there is a different type of noise. Not regular like attachment 8 but more random and larger.

The HV is set to 1V.

Further investigation will be carried out using AC coupling and HV set to 50V.