Telescope Repair Saga


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 and Tim Prowten's work.  THANK YOU TIM!

Here is what happened to my Meade LX-200 "clasic" scope:

  1. 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.
  2. 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.
  3. I left everything running and wiped down the telescope.  No water got to the mirror fortunately...
  4. Everything continued to track properly.  (I was sending control signals via RS-232 to automatically keep Jupiter centered.)
  5. 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!
  6. Went outside and discovered that DEC had drifted, at guide speed, to a point several degrees above Jupiter. 
  7. I assumed that there must still be a water problem. 
  8. Suddenly realized that the RS-232 socket and the keypad socket and the DEC socket might have water in them.
  9. At that point, I may have hot-unplugged the keypad which I have always been extremely careful not to do…
  10.  Removed remaining water, yes, there was water there.
  11.  Plugged everything back in.
  12.  Don’t recall exactly the sequence of power-up/power-down.
  13. Subsequent to that time though, the telescope could not get past the “Meade” display.
  14. This meant that the telescope was not passing self-test. 
  15. Took everything in the house.
  16. Checked everywhere for water.  No water had made it into where any circuit boards were.
  17. Powered the scope on.  RA motor turned one way then the other then stopped which apparently is normal.
  18. Still failed self-test though.
  19. Researched internet extensively.
  20. 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.
  21. Found Tim Prowten's circuit diagrams ( 
  22. Thought that perhaps 74LS14s might have been fried.
  23. 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…)
  24. Still failed self-test. 
  25. 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.
  26. Pulled U12 with a PLCC puller that I bought.  Turned power on.  Both motors ran.
  27. Replaced U12.  Was not aligned. Heard tiny snap presumably current from capacitor in Keypad. 
  28. Replaced U12.
  29. RA motor no longer cycled during power-up.  Probably U12 blew?  Damn!
  30. Checked internet for availability of U12.  No problem as long as you want 10 of them at prices ranging from $10 to $45 each!
  31. 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...)
  32. Also bought an oscilloscope to be able to check the RA and DEC motor pulses.  $289.  I had always wanted one anyway...
  33. Finally called Tim Prowten  Should have called him in the first place.  He does not want you to ship the whole scope, just the boards...  Perfect...
  34. Removed boards and motors (Power board, Motherboard, Dec Motor, RA motor, hand controller).
  35. Shipped boards and motors to Tim by UPS on 18 May 05.
  36. Tim received them on 20 May 05.
  37. Received boards back from Tim on or about 27 May 05.
  38. Carefully installed everything. 
  39. RA motor did not turn.  Hand controller could not get past "Meade" display.  Self-test still failing somehow.
  40. Talked with Tim.
  41. Shipped boards back to Tim on 28 May 05.
  42. Tim received boards on 3 Jun 05.
  43. I received boards back from Tim on or about 11 Jun 05.
  44. Carefully installed everything. 
  45. RA motor did not turn.  Hand controller could not get past "Meade" display.  Self-test still failing somehow.
  46. Talked with Tim.  He said to ship just the motherboard back to him this time.
  47. He suggested that it might be the Xilinx part.  Implication was that they are sometimes flakey. 
  48. Shipped motherboard back to Tim on 11 Jun 05.
  49. 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.
  50. 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?)
  51. Received motherboard back from Tim on 23 Jun 05.
  52. Carefully installed all boards.
  53. 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.
  54. 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!
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  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