Most likely a power supply needs to be replaced. We had that problem and when the power supply was exchanged the problem was gone. 
It is the power supply left on the bottom, when you open the cabinet on the right side of the instrument giving access to all the electronics. 
These are off shelf power supplies and price worthy (as I recall less than 100.- $US)


On 16 Jan 2018, at 17:02, Evan Baker <[log in to unmask]> wrote:

Hello all, can anyone help us diagnose this issue?

We're seeing cyclic noise in the signal coming from our Delta Plus XL. 

The noise is in the form of a voltage drop at regular intervals (every few seconds). The size of the voltage drop increases as the signal intensity increases. All of the cups have this issue, although the size of the voltage drop is different for each one and seems to move from cup to cup. The noise is present even with the Delta closed off from the EA.

Our setup: Carlo Erba NC2500 --> Conflo II --> Delta Plus XL

Here are pictures of the bad signal:
Reference N2 results: http://tinypic.com/r/262wnbo/9

The regularity of the noise makes us think this must be an electrical issue. However, we first saw this problem after adding a CO2 trap to the EA. We used soda lime as the absorbent and placed the CO2 trap between two water traps. The coincidence of the trap change and the noise makes us think this could be a contamination issue. The CO2 trap has since been removed, but the issue persists. To make things more confusing, the noise will occasionally disappear completely and the system will be stable for a few days at a time.

We greatly appreciate any suggestions!

-Evan Baker
Laboratory Manager
Stable Isotope Laboratory
North Carolina State University

Paul Scherrer Institut 
Dr. Rolf Siegwolf
CH-5232 Villigen PSI

Phone: +41 56 310 2786
Mobile: +41 76 543 2873
E-Mail: [log in to unmask]
Fax     : +41 56 310 4525
Internet:  www.psi.ch/lac/rolf-siegwolf