It started with this Problem: while in Guide Mode excessive play in DEC
As long as I have my (used) LX200 10" f/10 Classic I have been plagued by play in DEC when in Guide Mode.
I am a webcam imager and when shifting from North to South (or reverse) it took as long as 15 seconds before something happened on my laptop screen.
This implies that also autoguiding corrections in DEC were not or incorrectly executed and that has a bad effect on my images.

Setting the Backlash at 99 did not help, nor cleaning of the inner and outer clutches.

The DEC motor drive assembly.
From left to right: the manual knob, the reduction gearbox, the worm, the motor and the elctronics.
Note: underneath the worm you see a spring lying: when properly mounted this spring must be in vertical position

The connection between the gearbox itself and the "worm house" was OK as I saw no movement in the "O" ring area.
This called for a deeper investigation and I feared that I had to descend into the reduction gearbox ...

So I gathered all my courage and once more I dismounted the DEC motor drive assembly, consisting of some electronics, the small motor, the reduction gearbox, the hinged construction and the worm shaft.
A quick further inspection gave no clues: no side play in the worm shaft.
So I went on and removed the green cover from the top of the reduction gearbox and put an awl through the hole in the worm shaft.

The DEC motor drive assembly.
The green cover of the reduction gearbox has been peeled off thus revealing the inside.

I put a finger on the nylon worm shaft gear wheel so that I would be able to detect any movement in that wheel and then I applied a little bit of pressure on the awl and thus rotated the worm shaft a little bit.

The Diagnosis
BINGO: I could feel play between the nylon gear wheel and the worm shaft!
And that play should NOT BE THERE: I had found the cause of the play !
I noticed that the nylon gear wheel is mounted at the end of the worm shaft by a small bolt.
Because of the confined space inside the reduction gearbox I could not get at that small bolt, and I was - still am - very reluctant to open the side of the gearbox, as that is where those oh so tricky encoder masks live.
So I asked for help from the LX200 Yahoo Group

Friendly members pointed me towards Doc Greiner's website where I learned that the hole of the nylon gear wheel is D-shaped, which matches the shape of the end of the worm shaft.
The goal of the D-shape is that the gear wheel cannot rotate/slip on the worm shaft and is supposed to ensure a firm grip.

The DEC motor drive assembly: a look inside the reduction gearbox.
This closeup shows the head of the bolt which fastens the nylon gear wheel to the worm shaft.

Attacking the Problem Area
The problem was how to get at that bolt inside the reduction gearbox without a complete disassembly of the gearbox, because then I would be facing the - in my eyes - nearly impossible task of correctly re-installing the encoder mask.
To overcome this dreaded disassembly I already had adopted the idea to drill a small hole in the side of the gearbox just opposite the small bolt and thus get access to the gear wheel bolt itself.
I was happy to learn that Doc Greiner already had used the same 'drilling' approach so I did not feel so alone any more.
WARNING: stuff the inside of the gearbox with (toilet)paper or some other tissue to prevent that aluminium drilling particles migrate into the gearbox.

Here you can see where I drilled the hole.
WARNING: stuff the inside of the gearbox with (toilet)paper or some other tissue to prevent that aluminium drilling particles migrate into the gearbox.
Now I could remove the small gear wheel bolt and I looked through the hole.
And I saw the D-shaped hole in the nylon gear wheel which should prevent this gear wheel from slipping on the worm shaft.
In my case the D-connection was not tight enough and therefore the gear wheel was slipping on the worm shaft: the cause of my problem.
I do not know if my scope already had play in this area when it was still new, or if the play was caused by wear and tear.
Fact is that the play was there and that I wanted to eliminate it.

Note: I found that the bolt was VERY tight fitted in the worm shaft: make sure you have the correct sized Allen (Hex) wrench.
Warning: you need US sized tools, NOT metric!!
Using a pair of scissors I carefully clipped some of the plastic strip inside the gearbox away, so that I would have easier access.

Fixing the Problem
In my goodies box a found a locking washer of the correct size and I inserted this washer between the bolt and the nylon gear wheel and tightened the bolt: that already DID it!
I could feel no play anymore!
(But after two weeks the play came back: please read on ...)

The Inspection
I used instant glue to put the green cover back in place and put a piece of duct tape over the hole I had drilled.
After mounting the whole motor drive assembly on my scope again I selected "Guide mode" and I looked closely at the worm shaft: even when reversing the direction (N <-> S) the worm immediately obediently rotated into the desired direction. Happy !
Hint: later I got the idea to insert a knitting needle through the hole in the worm shaft, which makes the reaction of the worm much easier to see.
Please note that so far I refrained from using any Locktite or epoxy stuff (which might be hard to remove/handle later.)
Should the need arise then I can always buy it and apply it later ....
(But after two weeks the play came back: please read on ...)

The Final Proof
A test 'under the stars' showed that indeed the excessive play has gone: the star on my laptop screen immediately starts moving when I use the South/North keys in Guide Mode.
My current Backlash setting is 50.

The 'Knitting Needle' trick ...
Watching the rotation - if any - of the worm of my DEC assy in Guide mode was hard, even with a magnifying glass: it goes VERY slow. So to verify if the DEC drive system really behaved I needed clear skies and I had to wait for that.
After a few days I got an idea when I saw my wife's knitting needles.
I 'borrowed' one and put it through the hole of the DEC worm (in the RA drive system this hole is inhabited by a magnet, for PEC management).
While in Guide Mode I used the North/South buttons of the keypad: the tip of the knitting needle nicely showed the motion of the worm. Simple and effective.
Nothing spectacular, but I thought it was worth mentioning ....
PS Oh yes: I cleaned the knitting needle and put it back where I found it :o)

Two weeks later ....
To my dismay the GoTo was not accurate anymore: the DEC was slipping again: bah!
So I dismounted the DEC drive and found that the locking washer I had used indeed had lost it's grip on the nylon wheel.
AFAIK locking washers come in 2 types: those that look like a normal washers but have been cut and twisted a bit, and the other type that have teeth.
This time I went to the shops for a teethed locking washer, but they were hard to find.
Luckily the local bike shop had them in stock and I got one of the right size for free.
I also used expoxy for extra grip: I hope this time it lasts (almost) forever.

Not too long after the above ....
Again problems: now the DEC "stammered" when in 'Ctrl' speed; at higher speeds the DEC seemed to be OK, but the GoTo missed the objects I wanted to aim at.
I started a repair project under my own steam, but soon found out that the 2.5A fuse in the Power Panel was blown (with hind sight: most likely because I had hot plugged the hand set), so I went to the shop for some spare fuses.
Alas: they gave me 25A fuses instead of 2.5A and I trusted the guys in the shop, which was a mistake ...
And when I powered up my scope and resumed my repairs it momentarily lit up and then died: no more power.
So I gave up and was relieved when my 'electronics' friend came over to help.
My friend found out that the 5A fuse in the Power Supply was blown and he temporarily soldered a wire - as a bridge - where this fuse had been (obviously my friend knew what he was doing and that makes all the difference).
After a day of fiddling we decided to transport my scope to his home where he has full repair and testing facilities
Here is what my friend did:

HP AC/DC Adapter: Mains to 18Volt DC My friend had a 18V Stabilized Power Supply that once fed a HP printer: this Power Supply now feeds my LX200 :o)

When 'under the stars' I noticed that N/S movements in Control and Guide speed did not work satisfactorily: a minute tweaking of one of the comparators (the one closest to the 6-pin plug) cured this.

More information here:
Repairing the most common problems on the LX200 Dec/RA drive
LX200 Classic Schematics and technical information
Stock Faults on the LX200 Classic
Doc Greiner's LX200 repair pages

Also see: LX200 RA problems and repair.

Reference:
LX200 Motor Assembly Sh11.pdf
LX200 Motor Assembly Sh21.pdf
More LX200 Schematics

If all this is beyond your capabilities [it is beyond mine !!] then you can turn to Ron Sampson [USA] or Alan Sickling [UK] for help.
On the Yahoo LX200 Forum Ron Sampson writes:
"If you need any assistance, advice or repair service I am happy to assist you!
I repair LX200 Classics in the States (hundreds of them) but I have repaired numerous systems abroad, so international shipping is not an issue.
Please feel free to contact me if you wish"

Of course I would be happy to add more "Techies" here !!!!