If it came with an 18V power source it is probably an LX200 Classic. 18V is fine if the capacitor replacement has been done. Also, never turn the scope on without the hand paddle connected, or disconnect it while powered up. I have a thread concerning the older classics if you want to check it out.
My scope had a runaway issue with the Declination that was due to a poor connection with the external declination cable and the pins in the mount. Once I straightened out the pins to make better contact with the cable connector, it ran well. Sometimes it can be due to 2 pots on the motor encoder board that may need a bit of adjustment.
In my thread, there is also a link to Clearline Technology which sells some good replacement parts, as well as a small board that aids in getting the 2 pots on the motor encoder boards back in adjustment.
This is correct. There's nothing wrong with running the scopes at 18V once the caps are upgraded. There's also nothing wrong with running them at 12v or 16v. Less voltage only means slower slews. This is the reason Meade upped the voltage from 12v to 18v in the first place. The LX200 series was developed in the middle of the SCT wars, where Meade and Celestron were duking it out with full page, or back cover ads in both astro magazines. It was a war of specs. There were some reviews that mentioned the Meade was kinda' slow moving to target. Speed up the scope to be faster than the other guy, put it in the ad. A different power supply and some different silk screening for the front panel is all it took for Meade to get faster slews.
The main issue with the caps was Meade used some unregulated power supplies that could spank the caps with voltage spikes right at their rated capacity when turned on. Running the scopes at 12v left enough overhead where it wasn't a problem. It's hard to know if this wouldn't have become an issue with some scopes over time had Meade kept making running them at 12v through the line's lifetime. Dunno, that's crystal ball stuff.
Eventually the caps fail by shorting out, becoming tiny thermite bombs. The problem is, this all happens without any warning. The only somewhat common factor I've read about is it seems to happen more frequently after the scope has been sitting unused for a while. That's when my first cap failed, after sitting unused for a couple months in the winter. I flipped the power switch, the amp meter pegged for a couple seconds and went dead. That's when I saw the magic smoke coming out from behind the power panel. Cap one fried.
Even switching out to a regulated power supply isn't a guarantee because there's no way of knowing just how close to failure the cap is. One thing I suspected, and it's only a hunch, is the epoxy or the internal metals absorbs a tiny bit of moisture when sitting which causes further breakdown. I found some research that seemed to support that for early tantalum capacitors. FYI, tantalums have been redesigned so they no longer fail by shorting so nothing wrong with replacing with same, as long as a higher working voltage cap is used. 35v is good, 50v better. Back when my blew there was still a Radio Shack in town so I picked up some 35v electrolytics.
Don't get caught up in the voltage vs amps trap. These scopes were originally designed to run at 12v so all the motors are the exactly same whether the scope is marked 12v or 18v. Meade only made a couple small simple changes in later scopes, one of which was adding some small diodes to try to reduce the danger of hot plugging, which is what you did when you unplugged the handbox under power.
Bad GOTOs and runaways can also be caused by the trim pots on the motor boards drifting out of tune. One early symptom is lousy
GOTOs because the pulses the the software counts to keep track of where the scope is pointing become irregular. Another symptom is the motors when tracking get jumpy or jittery or they sound like they are skipping a beat. If its the pots, this usually happens before a random runaway. If the motors are running quiet and smooth, the pots are probably ok but if marginal, varying ambient temps can push them over the edge.
As mentioned above, sudden runaways can be caused by a poor connection at the cables. The first solution is to scrub the plug contacts by holding down the locking tab of the connector and running the plug in and out of the socket several times. Doesn't take much crud to block the signal. You can use a Q-tip and some rubbing alcohol, but beware of leaving a string of fuzz which will do the same thing.
I strongly suggest you take this opportunity to change out the caps. You'll also want to go to www.ClearLine-tech.com and get a calibration tool. You may never need it if your pots are good, but if they do drift, it's what you'll need to properly adjust them. I suggest checking the pots with the tool when you replace the caps. You can get the replacement caps there too.
If you aren't go to do all the caps at once, do the handbox, power panel, and motherboard. The handbox is the #1 priority because that $2 cap takes out a $70 ribbon cable.
Do the contact scrub and see what happens. Mark your calendar for cap replacements. Don't procrastinate like I did - TWICE! I knew about the issue, decided to get to it "later" and the first cap blew. Bought 5 replacements, fixed the 1 and got lazy, left the other four in a drawer thinking I'd to them later, that's when the one in the handbox went and melted the keypad cable. Double dumb. When I bought my second used mount - the LXD650 - that uses the same caps and power supply, I replaced the caps and power supply before ever powering up the mount.
The good news is, once the caps are replaced, and you know how to adjust the pots, these mounts are pretty reliable and robust. My 12" is a 2000 model, and my LXD650 is late '90s, and they are still going strong. Not bad for flip phone era tech. It's also funny when I'm at a starparty and doing a GOTO and someone says: "That sounds like a Meade over there!" My reply is: "Yes, it is! and how would you like your coffee beans ground? Drip or pour over?"