Noisy DAD @280 nm baseline during run.  Changed lamp to no avail.

What model DAD?

Without knowing what model of DAD you have, what I can say is if it is noisy, I would try another flow cell if I have one. Make new mobile phase (if you just dump mobile phase in the bottle every time, change solvent bottle and make fresh mobile phase. I would also look at my pressure to make sure my pressure is stable or if it fluctuates a lot. If it fluctuates, look into your check valves maybe or your degaser.

1260 Diode Array Detector VL+

Pump pressure is something I routinely monitor - pressure has been solid.  Will try the cell.

Go back to known good conditions first, so check the noise with water and a restriction capillary. Use the LabAdvisor program.

I assume you use an Agilent D2 lamp?

1260 VL+ is a G1315C.

Which model DAD- the G1315X and G7115X?   then you can look at the baseline with the flow cell out. And run a cell test.

If you have a G4212X or G7117X then what happens if you stop the flow.

And did you do an intensity test with Lab Advisor?  

Based on your latest results, I would say that nothing is fundamentally wrong with the optical unit in terms of noise, so I believe this is more an applicational problem - your passed quick noise test points to that direction.

The error 1050 with the failed Holmium oxide test is a problem though and needs to be fixed. Make sure the flow cell is seated correctly and has no contamination. If the HOX test still fails, you need to call an Agilent service.

- Yes, its a model G1315C. 

- Deuterium lamp was replaced.

- Intensity test passed.  Cell, Dark Current and Quick Noise tests passed as well.

- Holmium Oxide test failed; Instrument Error Occurred EE 01050 with parameter 0.

Rechecked seating of flow cell and also swapped cells from a different detector.  Still failing holmium oxide test. Looks like there is nothing else to do but call Agilent.  In the interim, I may swap out the faulty 1260 DAD with an 1100...  Not sure if the older model will be readily configurable.