I know it's nearly been a month since your post - but I will try to answer your questions. My posts are typically long - so bear with me as I cover your questions and a few others you might have down the road.
Power supply. Meade designed the electronics around an 18 volt DC supply (center pin positive). While it technically can work at 12 volts, it isn't recommended at all. Others will argue with me -- but in the end, don't do it.
Missing spreader. It is essential to have this when mounting the drive. 1) It stabilizes the tripod. 2) It limits the length of the thread which can enter the drive base.
The drive base is cast aluminum and each casting has different flaws and their own characteristics. Interestingly, as I've torn one drive base entirely apart during a rebuild -- I've found that the design is pure crap, in my opinion, of course. One small allen screw holds the entire OTA and fork on the drive base. This is threaded into the center cone where the bearings sit. While it does work and has worked for decades, it's not how I'd go about doing it.
3 allen screws. These were probably left over from a wedge. They serve no purpose whatsoever when mounted without the wedge.
DEC cable. Make sure it's a rollover cable and not straight through. A rollover cable is one that is wired 1-8, 2-7 etc...instead of 1-1, 2-2. What could be happening is you're shorting something out.
*Other known issues with the Classic LX200 line*
If for any reason, your scope just says "meade" when it is powered on - one of the biggest causes of this is the hall effect sensor. The sensor is glued to the R.A. housing and detects a pulse when a magnet embedded in the R.A. worm gear goes by. It's a very easy fix and it's super cheap. What's happening here is, during power on, the R.A. drive starts moving. The software on the main board is looking for this pulse as it provides a sort of "home detection". Without this pulse, the drive will just keep running slowly.
DEC issues, runaway - etc. Some DEC drives can just startup and never stop. This can be caused by a few things. 1) Faulty wire somewhere in the cable (either DEC cable or the actual cable that runs to the DEC motor board. 2) Broken IR LED lead. One of the two IR emitters or one of the two IR receivers leads has broken off the back of the die. It can be soldered back on if care is taken. 3) Encoder mask alignment. This can only be repaired by someone qualified - and I do know one guy who's great at it. Basically, if you decided to pull the DEC gearbox apart, chances are you shifted the position of the encoder mask. The alignment of this is critical for it to work properly. Repairs are typically well under the cost of a replacement part.
I've had my share of LX200's. I've owned 1 8", 1 10" and three 12". I will never buy a fork mounted OTA again. It is much better to just pull the OTA off and mount it on a equatorial drive. If you decided to go this route, removal of the OTA is easy.
If you have no plans to use or sell the fork once you're done...lay the telescope on some carpet, entirely stretched out, remove 4 of the allen bolts on the right side of the fork near the drive base, underneath the fork arm. Remove the three allen screws near the ota. This will free up one side. Now just remove the remaining three screws on the DEC side. If you want to sell the mount later, put a piece of masking tape along the seam between the drive base and fork arm. Cut where the division between the two parts are with a razor and make a few pen marks across the piece of tape. This will help the next person align the fork when putting an OTA on.
Feel free to contact me here if you have any questions. I don't check my messages very often, so there may be a delay in my response.
Best of luck!
St. Louis, MO