Resetting Meade’s Mirror Lock

How to fix your lock if the locking knob spins freely in either direction and does not lock

The following material was lifted from Yahoo’s LX200GPS user group postings (specifically messages 10578 and 58158) and edited to further clarify this relatively simple procedure.  Development of this procedure is attributed to Buck Harley and Doc Clay, and for the most part I’m directly lifting text from the referenced postings. 

I’ve had to do this fix a couple of times on my own GPS scopes, and offer this publicly available  information freely as a service to the astronomical community.  If you are mechanically challenged or cannot follow instructions it is suggested that you find a mechanically inclined friend to do this fix for you.  Peterson Corp. assumes no liability for this freely offered procedure.   

Most damage and subsequent repair to the mirror locking assembly on the Meade LX200 ACF and GPS telescopes is due to user over tightening of the knob. The mirror lock must be used properly.   

With a properly operating mirror lock, when turning the locking knob in the clockwise direction (unlocking direction) the knob will spin freely and then hit a stop. To tighten the lock and engage the mirror for zero shift requires that the mirror lock be turned COUNTERCLOCKWISE (CCW) until it is firm (typically about 8-10 turns from the stop at the full clockwise end of travel but this will vary from telescope to telescope). When resistance is first felt, turn ONLY one to two more full turns and stop.  Do NOT tighten until it stops. Never. This is where damage will occur.

Should the assembly stop locking the mirror, or if the chrome locking knob suddenly begins
spinning freely or will not totally stop in either unlocked or locked position, adjustment or refitting is required.

Readjustment procedure is as follows:

First,  turn the knob all the way clockwise to the unlock position stop.  Then remove the 3 cap screws that hold the locking knob assembly from the backplate of the OTA.  The locking knob assembly will now be free.  You may have to tilt the knob assembly slightly as you removed it from the telescope.

Viewing through the backplate locking knob hole you will be able to see the 8" steel gear that serves to lock the collar around the mirror sled.  About 1-inch of the rim of the gear has no teeth on it.  This is the “stop”.  (For those of you familiar with machinery, the mirror lock is simply a very large collet)

Insert your right index finger into the hole and push down, rotating the gear clockwise ONLY until the part with no teeth on the gear has just passed the hole where the mirror locking knob fits.  DO NOT rotate the large geared locking disk COUNTERCLOCKWISE as you will be unscrewing it from a very short thread, and if it comes free you’re faced with a major disassembly of the telescope or a return to Meade for major repair.

Once you have rotated the locking section of the gear past the open hole, reinstall the locking knob assembly.  (See addendum)   Your mirror lock should now lock and unlock properly.  Don’t worry about how many turns you have from end stop to where you start to feel locking resistance.  All that matters is that you have some free movement in the full clockwise position and when the knob is turned counter-clockwise  you’re able to snug up the mirror.


ADDENDUM of Dec 11, 2017

Hi Pete,

Many thanks for the LX200 mirror lock guide on your website - I found it very useful.  However - there is one piece of information that is missing off that page:

On the very last step - reinserting the mirror lock knob into the scope - in fact the knob has teeth on the back of it, well those teeth are spring to slide back and forward.  If you simply reinsert the knob, the teeth do not reach far enough into the scope to engage with the scope gear teeth.  What you have to do is use a thin screwdriver to push the knob teeth out (i.e. extend them) as you are inserting the knob.  This makes them long enough to reach the scope teeth.  Hold the knob with a small bit of sideways pressure to keep the two sets of teeth together, then withdraw the scre3wdirecver and keep holding the knob while you reinstall the three capscrews.

The spring on the knob teeth pulls the teeth back and the bottom plate of the knob teeth then sits behind the scope gear teeth.

It all sounds tricky but it's very easy and is an essential step to ensure the knob teeth are properly engaged in the scope teeth.  Once done I found the clockwise (unlock) stop worked perfectly, then it was 8.5 turns counterclockwise to mirror lock position.

Regards, Phil West