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Subject: " ... The Light" Conclusion
From: Pedro Miguel Breda Pinho dos Santos
Reply To: mapug@shore.net
Date: Tue Oct 31 12:24:19 1995

Hi folks.

I come now tell you guys the conclusion of the odissey with my new
12" LX200.
The original post was titled "And Then Came The Light" and for the
interested people who didn't read it, here is a short resume:

When I started slewing with my LX200, I noticed a problem only in
the W movement. It was shaky and produced a noise like something was
hiting something. The consume indicator flied to the limit at the rythm
of each stroke, making me very upset. This problem was only dramtic
in the midle speeds and not so much noticeable in very fast or very
slow speeds.

Then I removed the power panel to see if I could spot the problem, and
as the ambient light was not enough to see clearly inside the drive
base, I pointed a flashlight. And then to my great surprise the scope
started working fine, but only when the flashlight was pointing inside.
I then considered instaling a small DC lamp inside the drive base
connected to the 18v connector :) .

As the only thing sensitive to light in the scope are the optical
encoders I imediately suspected them or their respective circuit.
At the time I reported this to the LX200 list, everybody found it
very bizarre and didn't believe light could solve the problem.
But it could ! As you will soon understand.
Scott, one of the folks from Meade on this list, kindly answered
my post and sugested me to fax directly to Meade, wich I did. Twice!

---> Now here is the place for you people who already new the story.

Meanwhile, a week after the scope arrived, the problem progressed.
The scope sometimes started to move alone (RA drive) and to not
responde to W and O keys. In about an hour it reached a permanent
state of moving continuosly and alone as soon as I turned the power on.
And it moved at a very fast speed making impossible to use the scope
with power on.
And now there was no light that could solve the problem !

Back to Meade. I sent them a fax telling all the story, including
this later progresses. No answer. After one week I sent another fax.
No answer again. Of course I prefered the fax to the phone because
of language problem and phone account.
But one week after the second fax and no answer I decided I had to phone them.
I talked to Chris who told me sometimes faxes get lost... But two ?!
As the call time was counting, I got directly to the problem, but to my
great surprise he told me that in order to use the warranty, that is, to
get free assistance, I couldn't deal with Meade directly, but should
talk to the house where I bougth it. Then I had a vision of a big
burocratic nightmare.

That is not what I have been earing from other people ont the net, and
is not definitively what is written in my warranty certificate.

I should now say that although I live in Europe I bought my LX200 in NY
and so I accept as my responsabily to take the scope to the USA whenever
I need assistance.

But back to the dealer. It was Focus Camera and I must say I was very
pleased with them as they did very well what I expected from them.
They delivered my 12" LX200 in 2 days and with the packages in excellent
condition. That is all I wanted from them. I dont want them to know
about scopes. And it is not definitively their fault if Meade sends them
a scope with mal-functioning electronics.
I don't see why Meade would need them in order to solve my problem.
In the warranty certificate or the manual, I am not sure wich, it states
clearly that all that is needed is a invoice certificate from the
dealer.

I then lost my patience, and decided to solve the problem myself.
As I suspected the optical encoders, I took the scheme of the small
circuit board that makes the interface between the encoders and the main
board. All it does is comparing the output of the 2 photo-diodes with
a reference voltage, giving a digital signal to the main board.
As the output from a photo-diode is a current and not a voltage,
its output current must cross a resistor in order to be used by the
comparator. This resistors are realy variable resistors wich can be
adjusted with a small screwdrive.
With the aid of a oscilloscope it was obvious that the signal from
one of the photo-diodes was only producing a voltage between 3 and 4 V,
not enough to triger the comparator as the reference voltage is around
2.5V.
I adjusted the respective resistor and there it was, a fine well-working
LX200 scope.
If someone ever has this problem, I can say that the circuit in the Dec
drive is exaclty equal and that both include a 3 pin conector where
groung plus the 2 photo-diodes signals are available for a easier
regulation of the variable resistors. You should adjust them in order
to get a 1 to 4 V swing.

I recommend everyone that ever gets a Dec or RA or precision problem
to check this signals, as they can be the cause of many problems.

Now is easy to explain all that hapened with the scope:

At the time the scope arrived the signal from one photo-diode had
a signal with a peak equal to the critical comparing point. So it
was loosing some teeth moves and the main board was giving some
extra motor pulses in order to compensate. That extra pulses were
the ones responsible for the noise.

As the signal was in the critical point, the light from the flashlight
was enough to shift the signal a little from this point.

With time, due to photo-diode or resistor characteristic variation,
the signal got beyond the critical point and could never triger the
comparator. As the scope does some startup moves for checkup, and
as it could never read the moves it was ordering, it continued to
order that move.

At this time not even the small offset produced by the flashlight was
enough to triger the comparator.

The intermitent moving phase refered early was of course due to the
transition of the signal between this two phases.

And there you have the all story. (Hope you liked it :) )

Now my only problem is when think what will I do when I get a Scope
problem that I can't solve myself. What a nightmare that will be.
I now live hoping that day will never come...

See you,

Pedro Santos