This page last modified April 13th, 1998

I decided to do some work on figuring out the LX200's internal power supply, and seeing under what conditions you could get a zapped keypad. I also include a fix for the keypad, the control panel, and how to check to see if you already have the keypad modification made by Meade, along with some other bits of information.

If your scope is still under warranty, I really wouldn't suggest making any mods; I don't know how picky Meade is about field modifications voiding the warranty, especially if you make a mistake. If you DO make the mods, be careful. This could be an expensive mistake.

All of the following comes from examining my LX200 with a mfg date of 6/97. Apparently, Meade has made changes along the way, so this might not match exactly what's in your scope.

The LX200 power circuitry:

First, the only significant source of stored energy in the LX200, discounting the battery, which is ONLY used to provide backup power to the clock and scratchpad ram, is a 2200uf electrolytic cap which is wired directly across the 18V input jack on the font panel. If you turn your scope power switch off while the power supply is on, that cap will remain charged to full supply potential and (very) slowy discharge itself through internal leakage.

If the front panel switch is OFF, there is no voltage present at the keypad connector, as measured with a Fluke DVM, regardless of whether the power supply is connected, disconnected, on or off. If you turn the scope on and the input cap is charged, it will provide power for a very short time to the LX200 electronics. As R. A. Grenier pointed out, this lasts at most for a few seconds. If it was charged to 20V, it's only storing about 0.4 joules, not enough to do much. (J = 0.5 C V^2)

I also found out this interesting (at least to me!) bit of information. The ground of the 18V plug on the front panel is NOT at ground as far as the rest of the scope is concerned. The outer part of the connector runs throught a 0.1 ohm resistor BEFORE connecting to electrical ground. This was done to allow measurement of the current being used by the scope... it's used to generate the reading on the bar graph display. 99% of the time, this is irrelevant. However, if you have any external equipment that uses the same ground as the 18V connector as its signal ground and also connects to one of the ports on the scope (CCD guider, for instance) you can have signal noise problems, and your bar graph won't display properly. You won't hurt anything, however.

There are two ways of killing your keypad/main board electronics that are relevant:

1) reverse polarity

2) connecting the keypad when the contacts don't mate in the proper sequence and there's power applied

Item 1 is of no concern unless you make your own keypad extension cable. The connectors Meade uses CANNOT be inserted wrong; you can't reverse polarity unless the cable is miswired.

Item 2 is of real concern, and this is what causes the hot-plugging problem. Basically, if the power and signal lines make contact before the ground line in the keypad connector, the IC's signal lines get reverse-biased far beyond their survivable levels. Poof, dead IC.

Diagnostic procedure:

You can isolate failures in your LX200 a bit. Remember all the reports of the dreaded 'Meade' message on the keypad, and then nothing else? The keypad itself displays this message on power-up. I ran my keypad without it even being connected to the scope! So, if you see the 'Meade' message, and your led's on the keypad flash, it's working. Either your main board in the LX200 is dead, or the interface chips that connect the keypad to the scope are dead. Listen when you power up. If the scope does its index mark seeking, indicated by the motors running for a few seconds, then the main board is probably working, and you've suffered hot-plugging failure or some other interface IC failure, most likely a BIG static discharge to the keypad cable.

Making the modifications:

First, if you want to prevent the 2200uf cap from remaining charged, just wire a 1000 ohm 1 watt resistor across its two leads. The value isn't critical; use anything you can find with a value near 1K. This will give a discharge time constant of about 2 seconds, which means that for all practical purposes, the cap will be discharged in about 5 seconds. The downside of this is that it adds an additional 18 milliamps of current drain, and that drain will be present regardless of whether the power switch is on or off. (Battery users take note!)

Protecting the keypad is only slightly more difficult. You may not even need to do this; Meade added diodes to solve the problem somewhere along the line.

You can check your keypad to see if you need the mod quite easily. Take out the 4 screws holding the back cover on and remove the cover. You should be holding the keypad so the back of the circuit board is facing you; the display is facing away and should be at the top. Now tilt the keypad so you're looking at it from the left side and can see the components on the front side of the circuit board. Look for a (in my scope) small, orange capacitor. The label on the board is 'C3'. If there is a diode wired across C3's leads, Meade has already taken care of the problem. This is the mod to protect from reverse polarity. Look past C3 at an IC near the middle of the board. This is the interface chip that connects the keypad and scope. If you see two diodes wired to the socket pins and sort of hanging in space, this is the mod to protect from hot-plugging failure.

If you need to add this mod to the keypad, you can either do it the Meade way, which is a pain, or my way, which is effectively the same but doesn't require removing the circuit board. My mod ends up being exactly the same as the factory change, just put in a different location.

A warning. It would be a REAL GOOD IDEA to check with a ohmmeter to confirm that GND is really GND if you're going to make the mods. You can do this by measuring from the GND pad in both the keypad and the control panel to the metal nut that holds the 'reticle' jack onto the control panel. The scope power must be OFF, and the keypad cable connected. You should see near 0 ohms; anything less than 10 is fine. I've traced things carefully, and rechecked all of this twice, but I still might have mislabled the diagrams or some such. It would be great if someone else could independently confirm that GND is really GND!

You'll need 3, 50 volt 1 amp diodes. This isn't critical; you can use any voltage rating you want, and any current rating that's at least 1 amp. However, space is tight, so be sure what you use will allow you to put the back cover on later.

Look at the back of the keypad, just as you did when you first opened it up. Near the bottom center edge, there are 4 solder dots where the keypad connector is attached to the board. It should look like this:
0 0
0 [] <- note square pad

These pads correspond to:
GND SIGNAL
SIGNAL +18V

If you want to confirm this (and you should), use a voltmeter with the scope on, keypd connected. Put the black lead on the GND pad. CAREFULLY! probe the square pad with the red lead. You should see about 18V. It might very well be off by a volt or so; the external power supply isn't very accurate. Now probe each SIGNAL pad. You should see something between near 0V to near 5V; the exact value isn't critical.

Ok, you now have to wire the 3 diodes to these solder pads. The diodes are marked with a band at one end. It is VERY IMPORTANT not to install them backwards!

Wire one diode so that the BAND end connects to +18V and the other end connects to GND. This is the reverse polarity protection. The way this works is to short out the power supply if polarity is reversed and (hopefully) blow the fuse. Crude, but that's the way it needs to be done.

Now wire one diode so that the BAND end connects to one SIGNAL pad, and the other end connects to GND. Repeat for the other signal pad. This provides the IC reverse-bias protection.

So, you should now have the NON-BAND end of three diodes connected to GND, and the BAND end of each diode going to one of the other three pads. Plug it in, stand back, and turn the power on. If it doesn't go up in a puff of smoke, you now have a protected keypad. (Just a little humor; if you wire it right, it'll work. If you don't, you probably won't kill anything UNLESS you've shorted +18V to a SIGNAL line, in which case you can kiss your electronics goodbye. CHECK for solder shorts! If you just reversed a diode, you might blow a fuse, and your keypad won't work until you wire things correctly, but the electronics should survive.)

This is only half the fix. You now have a protected keypad, but not a protected main board.

I assume Meade has made this change along with the keypad mod, but if so, they did it on the main board, not the control panel board. I would suggest doing this mod ONLY if you have older electronics that don't have the keypad mod. The purpose of this mod is to be sure the voltage on the signal lines doesn't exceed their maximum rating. The voltage will be 'clamped' to no greater than 5 volts.

Take off the control panel by removing 4 screws, just like you would do to change the battery. Unplug the panel from the ribbon cable that connects it to the rest of the scope.

Hold the board so you're looking at the back of it and the bottom edge is down. Look near the center bottom where the keypad connector is attached. You'll see 4 solder pads in this configuration:
O O
O [] <- square pad

These correspond to:
+18 SIGNAL
SIGNAL GND

Note that the square pad is GND, unlike the square pad in the keypad. The two SIGNAL pads and the GND pad are the ones you will attach to.

You need 2, 5V 1/2 watt Zener diodes. These MUST BE Zener diodes; there's a critical difference between Zener and regular diodes. The wattage rating is not important; anything 1/2 watt or greater will do.

Connect the BAND end of one diode to either of the SIGNAL pads. Connect the other end to the GND pad. Do the same for the other SIGNAL pad. You should now have the BAND end of each diode connected to one SIGNAL pad, and the non-band end of both connected to GND.

This is a fairly safe mod, but if you connect the diodes backwards, your keypad won't work. Prolonged operation with reversed diodes could cause the interface IC's to die, so if your keypad doesn't display the 'LX200' message in a few seconds, turn the scope off and recheck your work. HOWEVER, if you connect the diodes to +18 instead of GND, you WILL fry both your main board and keypad interface IC's, so BE CAREFUL and check for solder shorts and such.

Reconnect the panel to the ribbon cable. Be VERY CAREFUL to be sure the connector is attached correctly, and not shifted over by a position. If you plug it in wrong, you'll probably fry your main board.

You're all done.

NOTE that just because you made this mod doesn't mean it's a good idea to hotplug. It STILL stresses the IC to do this, but it does keep it within its rated values. This is primarily to protect from the occasional mistake.


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