Optical spectrum analyzers slit failure - Page 1
Forum section: electronics / repair

author: jfphp
author_rank: Regular Contributor
timestamp: June 30, 2020, 10:59:53 am
content: Back on an old un-answered question : I have now several OSA (HP, Advantest) and they all have a permanent or intermittent slit failure. I understand that all these OSA use a very precise motor driven panel with various width slits to get the resolution of the light signal but I wonder about this very commun failure : dust in the slits, grease oxydation in the motor gears, backlash ? Any answer welcome !

author: _Wim_
author_rank: Super Contributor
timestamp: June 30, 2020, 08:54:16 pm
content: Cannot confirm the slit failure, but I had a broken Anritsu MS9710C with a grating failure (so also a motor driven component), and this was fixed by simply reseating all corresponding connectors. It seems many of these OSA were used on the road, and have been subjected to vibration during transport. So that at least could be a possible cause.It could also be that this is one of the first things that is checked during the initial self-test, and when it fails, no other tests are performed.

author: masterpj55
author_rank: Contributor
timestamp: January 15, 2021, 12:39:55 pm
content: curious about this too. it's the most common error i see on units.I dont really understand the internals of these units yet but I doubt it's many common components.

author: cosmo little
author_rank: Contributor
timestamp: November 22, 2022, 01:01:00 pm
content: I also have an HP70952B with intermittent slit failure error. After switching on and off a few times the error goes away. There is a picture of the internal construction of the 70952 family in the HP Journal Dec 1993 ( available on line)The slit wheel is driven by a small motor, possibly a stepping motor. I suspect that the lubricant used thickens over time so the motor cannot turn the slit wheel.I suggest that someone with a completely dead unit takes it apart and investigates. There is no service manual available. I am reluctant to take mine apart as the slit wheel is inside the sealed compartment.A full tear down with video would be really useful. Perhaps someone could send a dead unit to Dave?

author: cosmo little
author_rank: Contributor
timestamp: April 01, 2025, 11:52:09 am
content: I finally got round to investigating the intermittent "slit failure" fault on the HP70952B and other optical spectrum analysers.I first surmised that this could be a lubrication problem, with the grease getting thicker with age. This has turned out to be incorrect.In order to see the operation of the slit wheel. it was necessary to operate the plug-in in a mainframe without the case. I used a previously butchered main frame attached with MMS cables to the colour display. Access to the slit wheel and motor was through an inspection plate. Immediately it was clear that the wheel could be hand rotated with no friction.When the "slit fail" message was displayed, it was noted that the slit wheel rotated in one direction for a long time. It would then stop, with the error message displayed.Sometimes it would rotate for under 1 second, and would then make a number of adjustments in both directions. In this case there was no error message. It was also noted that all the time the analyser was sweeping, both wheels, the slit wheel and the mask wheel, would move about 10 degrees. This is evidently part of the auto zero which is performed on every alternate sweep.After an internet search, I managed to find a data sheet on the motor used to rotate the wheels. This was made by Computer Optical Products, the company no longer exists. The motor is a three phase brushless motor with an incremental optical encoder and index marker.I surmise that the system initially looks for the index marker. and sometimes fails to find it. Exactly what is wrong with the motor is unknown.Further investigation would mean removing the wheel motor and disassembling it. I am reluctant to do this as realignment with the wheel assembly might be difficult. The whee assembly seems to be attached to the motor shaft with two grub screws.If I do decide to proceed further. I will add another post.Cosmo Little

author: cosmo little
author_rank: Contributor
timestamp: April 01, 2025, 11:55:48 am
content: data sheet slit motor

author: _Wim_
author_rank: Super Contributor
timestamp: April 01, 2025, 06:03:19 pm
content: Did you already reseat the connector a couple of times to ensure it makes good contact? As you have the pinout of the motor, I would measure the encoder output with a scope an verify exactly if the issue is encoder feedback (signal dropping out) or motor control. Replacing the motor control driver might be relatively easy if you are sure it is the motor driver.