I finished examining the keypad/control panel interface, and now have mods
for protecting the main board as well as the keypad. The additional mods
can be made right on the control panel; no need to touch the main board.
I also discovered some interesting (well, at least to me!) things out about
the scope in general, and a procedure you can use to do some very rough
diagnostics.

First, a correction. In my original post, I said to look for diodes hanging
in space connected to a big IC in the middle of the keypad board. Everything's
correct except for the 'big'. The big IC is the microcontroller for the
keypad, but the diodes connect to the smaller 14 pin IC, which is the line
interface chip. This doesn't really change anything, but it might eliminate
some confusion. It's hard to see this without removing the circuit board
from both sides of the case; I don't recommend doing that.

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 you have some led's illuminated on the keypad, 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.

Interesting tidbit:

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.

Main board hot-plugging protection:

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. My scope's
finally back on its new pier and it's not coming off for a while, so I
can't confirm this. 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 5.7 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 are the ones you will attach to.

Now look diagionally up and left a bit for 5 pads layed out as:

O O <- these two are +5V
O
[] []

The three in the right column correspond to the three pins of the VREG1
IC on the front of the circuit board. BE SURE you have identified the
correct pads. The only purpose of this IC is to provide +5volts to the
AUX connector, but we need it for the mod. (How convenient!)

You need 2, 50V 1amp diodes. Just as for the keypad mod, the actual values
aren't critical; anything of at least 50V is fine. You can get these at
your local Radio Shack.

Connect the BAND end of one diode to either of the +5V pads. Connect the
other end to one of the SIGNAL pads on the keypad connector. Do the same
for the other SIGNAL pad. You should now have the BAND end of two diodes
connected to +5V, and the non-band end of each connected to one of the
SIGNAL pads. This is a fairly safe mod; you would have to make 2 mistakes
to do serious damage, 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. Be aware that
the two square pads at the VREG1 location are connected to +18V. Be very
sure you don't use these by mistake!

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.

Ok, that's it for my scope examinations for a while; the sky's clear today,
I drift-aligned last night, and tonight I'm actually going to look THROUGH
the thing for a change.

______________________________
| Bill Ezell |
| http://www.duckpond.mv.com |
------------------------------

The box said 'Win95, NT, or better', so I used Linux

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