"ERRATIC LASER SIGNAL" ON OLDER PERKIN-ELMER FTIR,
RX-1 & 1600

SYMPTOM: during power-up, the test program gives an ERRATIC LASER SIGNAL error. Low laser signal is usually responsible, and the cause can be: laser's drifting mechanical alignment, aging laser tube or HV power supply, or even humidity-damaged KBr in the windows or beamsplitter.

See Youtube vid from Tesla500: Perkin Elmer 1600 teardown

These old FTIR units employ an actual HeNe laser tube as their interferometer geometry reference. Old-school technology. "Vintage." But replacements still available from Perkin-Elmer for the Spectrum RX1, or direct from Melles-Griot for the Model 1600, see below.

TEST: inside the instrument sample well, verify red output beam by holding a white card in the well when the instrument is up and running. (Switch out the energy probe assy, or just remove it from the well.) Note the small red dot; usually somewhat dim in normal room lighting, but *not* near-invisible. The beam comes in from the left side of the well.

If a dim red spot is absent in the well, open the instrument case, pull up the interlock switch to apply power, and after the laser tube lights up pink, hold a white card in the gap between the right end of the laser housing and the plastic opto-bench cover. When working correctly, the beam should appear as a piercingly- intense red dot, similar to a 1mW red laser pointer. If the beam is dim or absent, either the kilovolt laser supply has failed, or the 12VDC to the laser supply is missing, or perhaps it may be time for a new laser tube. (With the housing open, does the laser tube have some blue plasma, or distinct purple highlights around the pink glow? If so, then it's "gassy," and N2/O2 has got through the seals.

Over years of operation these gas-discharge tubes always degrade and eventually fail. Nitrogen slowly leaks in through the seals for the laser mirrors, while Helium slowly becomes "gettered" by the laser's metal electrodes. Eventually the tube's output milliwatts drops too low to run the unit's interferometer. In this case the tube will either stop lasing entirely, that or the direct output will be a dim red spot on a white card. During such old-age tube failure, in the PE 1600 you'll still see a bright glow in the laser tube. With the RX-1, if you release the springs and lift the working tube from its bracket, you should still see a pink or violet glow down inside the output orfice. In either case, a bright blue/violet glow can indicate N2/O2 in the gas mix.

If the laser's output beam is still there and plenty bright, then probably the laser tube is just misaligned. This is a fairly common failure, since over several years, any slight vibration of your lab bench can wiggle the laser around in its bracket. The laser bracket has an X/Y adjustment via long 3/32" hex-wrench accessible through two holes in the top of the laser box. While in test-mode, use a hex driver to maximize the laser reading. Note that sometimes the X/Y adjustment isn't enough, and a working-but-old laser tube must be rotated or shimmed to give best aim. The beam both must strike that tiny 3mm mirror-chip on the optical bench, but also it must follow the entire IR path to reach the visible-light detector at the end. Sometimes the hex-nut X/Y adjustments don't attain the actual maximum available, but trimming the laser tube can do it.

Much more rarely the kilovolt tube-driver supply will fail. In this case there will be no red output beam, but also the PE-1600 laser tube remains dark, and for the RX-1, no glow is seen when looking into the laser output orfice.

PARTS:

L1181245 laser module for Spectrum RX1 FTIR, enclosed HeNe tube, approx $2000 (in 2013) [order from PE field service]

 

3121H-P-02-73 HeNe laser tube for 1600 FTIR, bare tube, Melles-Griot, approx $650 (in 2012) 800-645-2737 x3115 [INSURE FOR FULL VALUE, or ups will run a forklift tire over the box.]

LASER GAIN ADJUST, RX-1, PRESS THE FOLLOWING CONSOLE BUTTONS

[SETUP]
[OTHERS]    <-- note, not the adjacent [EXIT] button
[OTHERS]
[TEST]
[ADJUST]
[LASER]

See also: