30 April 2008
8" LX90-ACF - Second Bad One - More Tests
I have been receiving some feedback on the "thumping" problems that cause severe vibration in azimuth and/or altitude when viewing objects.
Subject: Vibrations LX90 Sent: Monday, April 28, 2008 01:12:37 From: Henrik van Holthoon (email@example.com) I still believe there something wrong with the motor drive boards, unstable servo loop?, causing these oscillations. Anyway nothing you can solve yourself so back to Meade. I must say I do not admire Meade's quality control. Regards Henrik
Subject: LX90 Sent: Monday, April 28, 2008 05:39:37 From: mingo (firstname.lastname@example.org) I have had the same problem for the last 2 weeks. I have had my 8" lx 90 for a yearand a half now and not had any real problems. Now thou it has started this vibration problem. I put in 4.3eg a week or so after it come out. Not sure that is the problem, is it possible to go back to a pre 4.3eg software update to find out. Because it makes it almost in possible to do AP. frustrated!! Tim
In extensive email discussions with Dr. Clay Sherrod, Dick Seymour, and Andrew Johansen, about the thumping, it appears that there is a definite problem but whether that problem is due to a mechanical problem (loose or too tight worm gear housings or loose or bad wiring connectors), a PIC firmware bug on the motor cards, or something else remains unclear. I have ruled out the AutoStar and AutoStar cable being the culprit by testing a second AutoStar on the LX90. Both AutoStars have the current version, 4.3Eg. Since I did not want to invalidate the warranty I have not opened up the base or right fork arm. I did remove the altitude axis locking knob and plate, and reattched the knob. When the altitude thumping would occur I could feel a definite vibration in the altitude worm gear housing at the left (rearward) end of the housing. I could feel no vibration on the right end.
Dr. Clay doesn't like the out-of-balance condition of the LX90 design. I had checked this as reported on my "Report #3" and noted that with the standard 1.25" diagonal and 26mm eyepiece, the OTA is very rear heavy. I attached my Nikon D70 DSLR to the Meade Off-Axis Guider, which really increases the out-of-balance situation. I noticed that there is interference in Alt/Az with the D70 camera that occurs when the OTA is raised to 65 degrees altitude. I then attached the a 2" diagonal and a 2" eyepiece. No interference problem but of course, the OTA is still rear heavy. But since this rear-heaviness is normal on the LX90 it would not seem to be the cause of the thumpings/vibrations.
Andrew Johansen had wondered if there was some pattern to the thumpings/vibrations. He had seen in my earlier reports that in some orientations there were no problems and in other orientations it would be severe. He suggested checking for the thumping sound at 15 degree increments in azimuth. If there was a correlation with altitude tracking rates as the azimuth changed, it might show up. He used this formula to calculate altitude slewing rates in Alt/Az mounting mode:
Rate = 15 * cos(Lat) * sin(Az)
I created an Excel spreadsheet with this formula (the "expected" thumping rate shown in the table below) and then recorded the thumps I heard at each azimuth position. Please note that this is not a scientific test as I recorded only one rotation in azimuth and I listened for the thumps from each axis. There is a lot variance in the frequency of the thumping occurrence at any orientation. For example, earlier I might have commented that the thumping would occur 3-5 seconds at 165°s azimuth but that during this test it was 5-15 seconds. This variation means that sometimes the thumping would repeat within 5 seconds at times and then not repeat for 15 seconds at other times during the testing. Definitely odd!
While there seems to be no direct correlation to the altitude tracking rates, there is a definite pattern that appears, with no thumping occurring at some orientations and severe thumping at other orientations. There is also the oddity that occurred as I slewed to 180° azimuth.
I am packing up this bad 8" LX90-ACF for return to Meade. UPS tried to pick up the first bad LX90 this morning so it will go back to Meade tomorrow. I hope the 2nd bad one will be picked up and replaced soon with a good one.
Stay tuned for more on this continuing saga...
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