John

I had a very similar sounding problem just a few months ago. It turned out
that the RS232 driver chip on my mainboard had failed. I diagnosed it by:

1. looking at the RS232 signal coming from the LX with a scope. It was not
correct.

2. I next looked at U9 on the main board which is a MAX232CPE. A very common
RS232 Driver/Receiver. With the power off there should have been 5KOhms
across pins 13 and 15 and between pins 8 and 15. Pins 8 and 13 are the
RS232 inputs and pin 15 is ground. I had about 20K across pins 14 and
ground proving that at least on the input side the chip was defective. If
your chip has blown, then it may well have a different set of readings.

You can see the specs for the MAX232CPE at
http://209.1.238.250/arpdf/1798.pdf
or
http://www.maxim-ic.com

To replace it involved unsoldering U9. Not easy, but with care it can be
done. I cut it off first and then unsoldered each pin individually.

To make life easier in the future I soldered a carrier into place and then
simply pressed the new chip into place. Now if it happens again I can
simply plug in a new chip.

Obviously my scope was well out of warranty and therefore I didn't worry too
much. Clearly anyone with their scope in warranty should send it back for
replacement. Also it would be very easy to do some serious damage to other
components as I found out later.

I very much suspect that the failure is all tied up with the odd grounding
system on the LX. As you know the main board ground is not at the case
ground due to the circuit which is used to measure and display the current.
This could cause odd currents to flow between the grounds of the PC and the
LX as you start them up.

One complication I had was that I must have got a little too much heat from
the solder gun near one of the two crystals which are close to U9.
Interestingly the scope tracked an object in RA just fine, but would not
perform an accurate GOTO. I looked at the RA readout on the hand unit and
noticed that to my surprise it was increasing second for second in real
time. (obviously the RA should have been fixed whilst tracking). I
replaced the crystal with one from an old PC motherboard (its a standard
clock crystal) and all was well.

It appears that one of the crystals controls the date, time and real time
driving of RA, while the other simply controls the RA readout used to
calculate GOTOs. Very odd.

Lastly I could not have done this work without the LX200 circuit diagrams
that
DocG very kindly put on his site.

Hope this helps
Paul

----- Original Message -----
> Date: Mon, 21 Aug 2000 08:15:13 -0500
> Subject: [M]: LX200 comm failures
>
> Just wondering...has anyone ever fixed by themselves, an LX200 serial
> comm failure? I just wonder if there is a UART or another component
> that is the weak link in the comm system. For those that are faced with
> the $200 cost and time lost in shipping components back to Meade, it
> seems that, if one is comfortable in replacing components on a PC board,
> that there might be a cheaper and faster repair that can be done at
> home.
>
> John

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