Dec Axis, Backlash, & Motor Issues, page 3

       

Subject: LX200 DEC Cable Repair

From: Doug Carroll <voxrattbi.com> Date: May 2001

I had the Clips break on my DEC cable last night so now it will not stay put. Since I had seen recently a discussion on correct Pin-Out for the Dec cable I put my repair info on my website showing the correct arrangement of the pins on both sides and the for those who want to order a cable the correct path number to order from Digikey. The page is at:
    <http://www.ccdguy.com/projects/deccable.htm>

and the cable has been tested and is functioning on my scope now.

Subject: Loose Dec Clutch Fix

From: Bruce Johnson April 2004

Walt, when this 'thump' happens, does the OTA just kind of flop around up and down? Or to put it simpler, when you have the Dec clutch locked, can you *easily* move the OTA North and South a short distance, after which the OTA is firmly stopped? If this is the case, then what I'd say is the most likely problem, is that the two screws holding the clutch face on, have become loose. When this happens, the amount of 'slop' within the holes with the screws allows for maybe 15 degrees or so of very easy movement of the OTA.

If that indeed is the case, then what you want to do is to take the housing off of the Dec side of the scope and then completely unscrew the Dec clutch knob and remove it. You can then grab the middle of the clutch ..... the 'circle' part of the unit that fills the inside hole of the worm gear.. and wiggle it off. When you get it off, you'll see the two screws that I'm speaking of. Usually they're held in by Loktite, but if the unit has ever been apart before, the screws can and will have a tendency to loosen.

Again, assuming all of this is true, tighten the two screws up very tightly and put it all back together and you're all set.

Subject: LX200 DEC Cable Source

From: Bill Wood <wrwoodqwest.net> Date: Jun 2001

I just ordered a replacement DEC cable from DigiKey.com, part no H1882-07C. It is a modular 8 wire phone type cable with RJ45 connectors on each end wired just like the stock Meade cable. The DigiKey cable is coiled flat black wire about 4' long at rest. Works perfectly. I attached one of those plastic stick-on hooks on the outside of the right fork arm to hang 6-8" of the cable from before it plugs into the fork connector which stands it off from the tripod and keeps it from hanging up on the tripod legs. It allows the OTA to rotate 360 degrees RA in either direction with no binding or tangling. Also the DigiKey cable does not stick up and hit the underside of the fork arms. Quite an improvement over the Meade cable which is too short and sticks up too high from the control panel connector.

Cost $7.28 plus $5.00 handling (handling charge applies only to orders under $25) plus shipping.

Subject: Reading the Dec Scale

From: Darrell, Date: June 2001

>From: Stan Rife <srifeswbell.net>
> Well, I looked at that but couldn't figure out how to interpret/read that
>scale. I suspicion that this scale was used for that. There is, of course,
>nothing in the manual about how to read or use it. Can you enlighten me
>further Darrell? I'll study it some more in the mean time.

I'll see if I can explain this technique of reading these types of scales. It is very easy to do, but a little hard to explain, so bear with me.

To start with we have two scales, or two halfs of one scale depending on how you look at it, what I will call the Dec scale and the Dec marker scale. The Dec scale is the markings on the Dec setting circle, the major (tallest) marks are every 10 degrees, the mid (half height) marks are every 5 degrees, and the minor (shortest) are every 1 degree. This scale most people see and can interpret easily and relatively instinctively. Now the other half of the scale, the Dec marker scale. The Dec marker scale is on the fork arm, and this is what you line up the Dec setting circle with. The Dec marker scale has three different height markings on it also. The center tallest mark (with a dot at the end) is the 'on' mark, the mid height marks are at 0.5 degree and 1.0 degree off the 'on' mark, the minor marks are 0.1 degree marks.

Now how do you read these two scales together to get resolution of 0.1 degrees or better? For my examples I will work all in positive numbers, but it works regardless of sign. I'll describe the part most people probably understand almost instinctively first, just to establish terminology a bit further. Then I'll get into making 0.1 degrees from it.

To set 70 degrees of Dec you line up the 70 degree mark of the Dec scale with the 'on' mark of the Dec marker scale. To set 71 degrees of Dec you line up the first 1 degree mark positive of the 70 degree mark on the Dec scale with the 'on' mark of the Dec marker scale. To set 72 degrees of Dec you line up the second 1 degree mark positive of the 70 degree mark on the Dec scale with the 'on' mark of the Dec marker scale.

If the Dec is set on 70 degrees then the 'on' mark of the Dec marker scale is lined up with the 70 degree mark. Note that on each side of the 'on' mark are the minor marks, on both scales, none of which should line up with anything on the other scale. Yet. But several of them should be very close to lined up, the closer to the center of the scale, the closer to lined up they should be. Now I am assuming the scope is locked down at 70 degrees and powered off. If you use the manual slow motion control knob to rotate the scope very, very slowly from 70 degrees towards 71 degrees you will see that the first minor marks on both the Dec scale and the Dec marker scale will line up, stop there. The scope is now set to 70.1 degrees. No other marks on either scale should exactly line up. Continue the motion in the positive direction and you will then see the second minor marks on each scale line up. Stop there. No other marks on either scale should exactly line up. The scope is now set to 70.2 degrees.

Subject: Classic Dec Drive Test Points

From: Doc G, Date: June 2001

The small circuit board on the drives has a set of test points. They are together so that a plug can be inserted and leads made to connect a dual channel oscilloscope. The test points are the Schmitt triggered signals from the photocells at the encoder disk. There are two pots on the board that set the triggering levels.

These adjustments are rather touchy. At slow speeds of the motor shaft, note it only turns once every 4 seconds at sidereal rate, you will see a rounded square wave at each of the terminals. At high speeds, 15000 RPM at full slew, these wave forms become sinusoidal looking.

These two wave forms are the two components of a bi-quad speeed/direction signal. Ideally they should be of similar size and 90 degrees out of phase. (bi-quad) These signals go to the computer where they are converted with appropriate logic to speed, pulses per second, and direction. The computer uses this information to determine how far the scope has moved and in which direction. This is a very common control scheme.

The two pots should be set so that the two signals look similar and as close to 90 degrees apart as possible. I assume that Meade uses a jig to hold the drive, runs the motor and sets the pots for some optimum. The final quality of the signals generated will depend not only on the setting of the pots, but exactly how well the encoder mask has been placed.

It looks to me like they position the mask by looking at the raw signals as well. The mask is positioned and then clamped by tightening the gear box housing. The tab is then cut off. It is very difficult to redo this adjustment, but it can be done. I can describe how to do this in some detail if you need to know. Post me privately please.

A good service manual would of course have all of this information in it. But there is no service manual available that I know about. I have a good one in my mind.

Subject: Dec Problem -- Melting Tantalum Capacitor -- part 1 of 2

From: Alan Sickling <alanthesicklings.force9.co.uk> Date: June 2005

The current surge you see registered on the LED bars is consistent with the drain created by the melting tantalum capacitor. If you had been using an unregulated PSU it would have taken enough current to blow the fuse in the control panel. Because (I guess) you were using a regulated pack, the current limit kicked in and saved the fuse. Unfortunately, this may have allowed enough current for a sufficient time to possibly damage the wiring between the base and the Dec drive PCB in the fork arm. This wiring is not particularly thick, but my judgement would be that it will have survived this little trauma unscathed.

Trust me! I have seen more exploding tantalums than you can imagine. The damage to this component should not have upset anything else. You have been very lucky. Dark, smelly smoke normally means "goodbye electronics", but not in this case.

Do as I suggest - remove the PCB, change the capacitor, replace the PCB and I believe you will find all will return to an operational state. E-mail me direct if you need step-by-step instructions as I don't want to clutter the forum archive space with graphics files. It really is a simple repair job.

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Subject: Dec Problem -- Melting Tantalum Capacitor -- part 2 of 2

From: John Mahony

Here's a picture of the circuit board from Bruce Johnston's website: <http://www.mapug-astronomy.net/ccdastro/decfix_3.htm> (the picture at the bottom of the page is clearer than the one at top). This is the Dec board, but the RA is essentially the same.

From what I've heard of the problem with the capacitors in the LX200 Classic, the one on the motor board that usually blows is "C1", at far right in the picture, pressed up against the cable connector.

Dec Axis: