Telescope Repair Saga
Foreword:
I became an Amateur Radio Operator in 1957 when I
was 14. I did not have any money in those days. People gave
me old radios and I cut the parts out of them and used the parts to
build new things. I picked beans to get $34.95 to buy a Heathkit
DX-20 transmitter. My receiver was a Zenith Trans-world. I
found that by sharply tuning the IF strips, I could get one of the IF
amplifiers to oscillate. With a piece of clay holding a wire to
the tuning screw (which was a "hot" capacitor) I could adjust the
feedback and create a "Beat Frequency Oscillator" essential for
listening to Morse code. From scratch, I built a Variable
Frequency Oscillator which I used for years, a grid modulator, a
cathode modulator, and finally a plate modulator which blew out my
power supply since plate modulation doubles the voltage required and I
had not realized that. Was given a power supply which I ran
"outboard" for years. The plate modulator allowed me to talk
using a microphone using Amplitude Modulation which was gradually being
replaced by Single SideBand. I also built a primitive A-Scope
Sonar set, an Oscilloscope from an 5FP7 radar tube that I acquired in a
trade, and other odds and ends. The Transceiver that I use
currently is a Heathkit SB-101 Single SideBand vacuum tube job which I
built. I recently designed and build a stepper motor system to
replace the Meade Focuser so that I have absolute resetability of
focus. It gets its control signal from the meade focusser port so
is electronically compatible. I included a 555 timer and a switch
so that when I'm controlling focus at the scope, I can run the stepper
rapidly to do hand focussing.
So why do I mention all of this? Primarily
to indicate I am not a neophyte with respect to electronics. The
story below outlines the trials and tribulations involved in repairing
one of these scopes. I am totally happy with
http://www.telescopeservice.com
and Tim Prowten's work. THANK YOU
TIM!
Here is what happened to my Meade LX-200 "clasic"
scope:
- I was using an advanced version of my
HandyAvi software to track Jupiter and take webcam images.
HandyAvi communicates through the telescope’s serial port to
the PC’s serial port. Just standard RS-232
telescope command signals through a standard cable.
- A drip-line hose developed a crack and
sprayed the telescope with water. I was
inside and the telescope was outside so I did not discover it for
probably 10 minutes.
- I left everything running and wiped
down the telescope. No water got to the mirror
fortunately...
- Everything continued to track properly. (I was sending control signals via RS-232 to
automatically keep Jupiter centered.)
- After about half an hour, I suddenly
noticed on the webcam display that Jupiter was not centered in the
field and was not even in the field!
- Went outside and discovered that DEC
had drifted, at guide speed, to a point several degrees above Jupiter.
- I assumed that there must still be a
water problem.
- Suddenly realized that the RS-232
socket and the keypad socket and the DEC socket might have water in
them.
- At that point, I may have
hot-unplugged the keypad which I have always been extremely careful not
to do…
- Removed
remaining water, yes, there was water there.
- Plugged
everything back in.
- Don’t
recall exactly the sequence of power-up/power-down.
- Subsequent to that time though, the
telescope could not get past the “Meade” display.
- This meant that the telescope was not
passing self-test.
- Took everything in the house.
- Checked everywhere for water. No water had made it into where any circuit
boards were.
- Powered the scope on.
RA motor turned one way then the other then stopped which
apparently is normal.
- Still failed self-test though.
- Researched internet extensively.
- Called Meade.
They wanted me to send the WHOLE SCOPE. That is very
off-putting. Why risk the OTA? And the mirror? And
the corrector plate? I just could not bring myself to do that
when it is only the boards that need repair and they would be simple to
disassemble and ship.
- Found Tim Prowten's circuit diagrams
(http://www.telescopeservice.com)
- Thought that perhaps 74LS14s might
have been fried.
- Replaced them. Put
a socket in for the one in the keypad but there was not enough
clearance for a socket on the motherboard due to the heat sink. (Considered modifying the heat sink but
finally just soldered the new 74LS14 in…)
- Still failed self-test.
- Information on the internet said that
if U12 was pulled, then when power was restored to the scope that both
the DEC and RA motors would run thereby establishing whether they were
OK.
- Pulled U12 with a PLCC puller that I
bought. Turned power on.
Both motors ran.
- Replaced U12. Was
not aligned. Heard tiny snap presumably current from capacitor in
Keypad.
- Replaced U12.
- RA motor no longer cycled during
power-up. Probably U12 blew? Damn!
- Checked internet for availability of
U12. No problem as long as you want 10 of
them at prices ranging from $10 to $45 each!
- I did order some TMP68301AF16
Microprocessors which is the "brain" of the scope. However,
they are in 100 pin surface-mount packages and I chickened out with
respect to replacing it myself. There are "desoldering stations"
sold for the purpose of desoldering such items. A "desoldering
station" suitable for this work runs at least $600. An
alternative is to cut the leads with an exacto-knife, desolder them and
brush them away, then resolder if you have iron nerves, a really tiny
soldering tip, and the steady hand of a master brain-surgeon.
Mess up your board traces and you are dead! You cannot buy
just one of these particular microprocessors these days. They
come from "parts brokers" who bought up whatever was left when Toshiba
quit manufacturing them. The parts brokers want to sell you at
least 5 and they set the price to whatever they think they can
get. In this case, $35 each. The "freight charge" was
$37! (Chips weigh almost nothing... Don't know whose
"freight" I was paying for...)
- Also bought an oscilloscope to be able
to check the RA and DEC motor pulses. $289. I had always
wanted one anyway...
- Finally called Tim Prowten
http://www.telescopeservice.com. Should have called him in the
first place. He does not want you to ship the whole scope, just
the boards... Perfect...
- Removed boards and motors (Power
board, Motherboard, Dec Motor, RA motor, hand controller).
- Shipped boards and motors to Tim by
UPS on 18 May 05.
- Tim received them on 20 May 05.
- Received boards back from Tim on or
about 27 May 05.
- Carefully installed everything.
- RA motor did not turn. Hand
controller could not get past "Meade" display. Self-test still
failing somehow.
- Talked with Tim.
- Shipped boards back to Tim on 28 May
05.
- Tim received boards on 3 Jun 05.
- I received boards back from Tim on or
about 11 Jun 05.
- Carefully installed everything.
- RA motor did not turn. Hand
controller could not get past "Meade" display. Self-test still
failing somehow.
- Talked with Tim. He said to ship
just the motherboard back to him this time.
- He suggested that it might be the
Xilinx part. Implication was that they are sometimes
flakey.
- Shipped motherboard back to Tim on 11
Jun 05.
- Ordered 10 Xilinx XC3030-70s (parts
brokers would not sell me fewer) at $10 each. Just insurance
because this part seems to be strongly implicated in the problems being
experienced.
- Got a note from Tim where he said he
had run the motherboard all weekend. (This would constitute
"burn-in" which used to be necessary for many boards. There used
to be component failures that would show up after running the board for
a few hours. Wondering if the Xilinx part is a "marginal" part
where burn-in is required to sort out the good ones from the bad ones?)
- Received motherboard back from Tim on
23 Jun 05.
- Carefully installed all boards.
- This time it worked! RA motor
ran, self-test completed, hand controller controlled the motors, RS-232
control using advanced version of HandyAvi worked so that complete
control of the telescope via the software is again possible.
- THANK YOU TIM! It was a tough
case and we both wondered about each other I'm sure but I am EXTREMELY
happy to have my scope back in working order!
Cost:
- Five
TMP68301AF16s $207
- Ten
XC3030-70s
$100
- Oscilloscope
$290
- Telescope
Service $250
- Board shipment
#1 $ 37
- Board shipment
#2 $ 23
- Board shipment
#3 $ 38
- Chip
puller
$ 7
- Diodes, Hex Inverter, etc. $ 10
- TOTAL
$962
I did not make a lot of brilliant choices clearly. I REALLY
wanted my scope to work again so was hedging bets more than I should
have. The low-cost solution would have been to immediately send
the boards and motors to Tim Prowten at
http://www.telescopeservice.com. That did not occur to me
up-front because I am used to fixing everything myself. The
problem is that the parts are expensive. You have to buy a bunch
of them. Desoldering equipment is very expensive and you need
such equipment to even THINK about doing some of what may need to be
done.
The upshot is that if you have LOTS of money, you can probably acquire
the parts and equipment to effect your own repairs. It won't be
cost-effective though. If you were fixing a LOT of scopes, then
the investment would be worthwhile but if you are only fixing one, you
are better off letting someone who specializes in repairing scopes
handle the job.
Copyright 2005
Howard C. Anderson
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