Light Path Fail Issue

Most basic thing first;

Replaced vis lamp?

Aligned Vis lamp?

Thanks, gr. I aligned (peaked) the Vis lamp and it was off by about 46%T. Upon re-starting everything passed. Hopefully that has solved the issue. If Light Path Fail re-appears I'll update this post. Thanks again!

Well, it has been working for nearly a year, but the Light Path Error is back on every start up. I have checked the Vis alignment (spot on) and have replaced the Vis lamp. Any suggestions on what to check further?

Several issues can cause a light path failure at start up.  Just because the lamp is aligned does not mean that it has sufficient ENERGY to pass the start up sequence.  The detector for the DU640 requires 1V DC minimum at start up in order to continue on.  You cannot determine how much energy is actually being detected by using the %T value to peak the lamp.  This process only tells you that the lamp is peaked, not the actual energy value.  The only way to determine the actual energy is to use a DVM on the test point near the left front corner of the main board right in front of the connector that goes to the detector assembly.  If it is less than 1.0 Vdc then it could still be the lamp.  Are you using a Beckman lamp because not all visible lamps are created equal.  We tried for years to come up with an off the shelf replacement lamp but none are made to Beckman's specifications, especially in regards to the coating on the outside of the lamp.  The other big issue is when was the last time that your optics were cleaned?  Everything from the beam combiner,which can be rotated, all the way to the detector needs periodic cleaning and please, if you REALLY don't know what you're doing as to cleaning the optics, especially the grating assembly, then call someone in that actually does know how to clean them properly.  There's a lot of misinformation out there on how to clean the optics so be careful where and who you get the information from.  If you scratch the grating assembly as this item does not have a coating on it, then you are looking at a VERY expensive replacement part not to mention the fun, or lack thereof, of actually aligning the grating assembly.  Something else best left to a factory trained service engineer.  Hope this helps but I believe you really are in need of service on your DU.

Don

One other item that I forgot to mention in my previous post, following the instructions in the operator's manual on visible lamp replacement is not really complete.  There is an up/down alignment as well as a left to right alignment.  The up/down alignment can only be done by actually taking the Visible lamp holder out of the DU if it needs adjustment.  Do you have a target assembly for the lamp alignment?  Every Beckman trained engineer has one and it is essential to a real lamp alignment for both lamps.  Anything else is guesswork at best.

Don

Thanks, I will be moving this to a new building in a few months so may have a tech visit then. In the meantime; I replaced the Vis lamp with one from Sonntek (L835), but also found an original unused Beckman lamp, neither removed the error.  And as far as I know I do not have a target assembly.

I run this almost exclusively in the UV range (280-400nm); will this light path issue interfere within this range? I have run some know samples and the scans are nearly identical to previous ones from before the error message.

As you may have discovered, even if you get a light path failure, the DU can still be used by just closing down the error display screen.  Actually, you are running your samples in both the UV and Vis range.  The switch over point is at 325nm at which point you are getting light from both lamps illuminating your samples until around 350nm where it switches over exclusively to the Visible lamp.  The fact that your scans are nearly identical probablly indicates that you're not being adversely impacted by the visible lamp issue.  Are you using a microcell holder for your samples?  If you are then you can get a better idea as to how well your lamp is actually aligned without having a real alignment tool.  With both lamps on, select 0nm as the wavelength in the fixed wavelength mode.  That will give you a very intense white light.  The light beam should be adjusteded such that the light is hitting in the center of the cell holder window left to right and should also be flooding the slit on the cell holder.  The beam may appear to be a little diffuse when you look at it but you need to remember that the focal point for the lamps is within the center of the cell.  Goingforward, do yourself a favor and only use Beckman visible lamps.  No one else makes a bulb that is completly compatible with your DU.  Beckman did extensive testing with Osram to get visible lamps that are meant for the DU.  They are relatively inexpensive but will give you the best performance.

Don

We have a du650 and upon setting the RediRead to 0nm, the vis light beam falls short of hitting the exit slit. The only diagnostic test that fails is the light path. How do we get the grate to aim the vis beam at 0nm to hit the exit slit?

My first response is that YOU don't. The replacing or alignment of the grating is truly something that is best left to a service engineer that actually knows the correct procedure. Otherwise you just end up with more problems than you really want to deal with. The physicsl replacement of the grating is minor. The alignment is not and anyone that says it is is only lying to you. This was the most hated job on the DU600/800 Spectrophotometers for any BCI engineer. Sorry that I can't help you but this is more of a show and tell technique than a type and tell.

Then PLEASE help connect us with a serviceman for the Cleveland, OH area. I have rebuilt this du-650 from the ground up and am faced with this last issue. The mirrors or gratings were never moved or replaced, just cleaned. Also, why do the 3 Wellnut screws run so long? Are they just used in shipping and should be totally loosened before operation? If I tighten them down any, they will dig into the tabletop below. I have new Wellnuts in place. Thank you for your help!!!