Stock Faults on the LX200 Classic.

Electrical

These are standard faults found on various sections of the LX200 Classic series telescope systems.

This list is not complete by any means but serves as a starter pack for anybody looking for a problem. It will be added to as time goes on. (For your interest, these are not faults on my own LX200, but data gathered together from the number of return systems I receive for servicing).

All servicing should not be undertaken lightly, and the correct tools are essential for diagnosis. A good Digital Voltmeter and Oscilloscope form part of that toolkit, as well as antistatic precautions.

Symptom:- General checks.

Voltage checks on the mother board. 9volts, or half Vcc across C27 (dependent on supply voltage). This is the centre rail/reference of the motor driver IC's. 2.5volts across C17. This is the mid-rail reference for the analogue amplifiers. Lastly, the output of the 5volt regulator, which should never be below 4.8volts, and does get warm.

Symptom:- Fuse blowing, either externally or the fuse mounted behind the Power Panel.

This is most probably caused by the failure of one of five components, all of which should be replaced to effect a lasting repair. It is the result of component failure of C1 on the Power Panel and/or C8 on the Processor Card (near the 5v regulator) and/or C2 in the hand controller and/or C1 on the DEC/RA motor control board. These are Tantalum Capacitors, normally fawn in colour, and are polarity conscious. Replace with 10uF 35volt working Tantalum or Electrolytic. (Please note that the capacitor in the hand controller has a tendency to burn through the keypad membrane when it goes faulty, rendering the handset useless and unrepairable...see further notes here).

While you have the units stripped down, check the Lithium Battery BAT1, to the rear of the Power Panel. It should be >2.9volts. If not, replace.

Also add additional decoupling, (100nF Layer capacitor), to each of the DEC and RA motor driver chips on pins 2 and 5. Solder the extra component to the underside of the PCB directly to the IC pins.

Another decoupling point, (100nF Layer Capacitor), is the RA motor control board to pins 3 and 12 of IC U2.

The addition of these capacitors cleans up the triggering point of the main PCB timing circuits.

Symptom:- Motor runaway, or not turning at all.

Essentially, the Dec and RA motor units are identical in circuitry apart from the Mag Pulse on the RA unit.

To prove the motor driver chips and circuitry, and the motor mechanism itself, remove U12 on the Processor Board. Make Sure The Drive Clutches Are Released Before Applying Power. Both motors will now run. If not suspect either the large white power resistors for open circuit, (never seen one yet), or U18 for the Dec motor or U17 for the RA motor.

With an oscilloscope, check for RA direction pulses at pins 5 and 6 on J2 of the Processor Board. These should be squarewaves with similar timings for mark/space ratio, 2volt pk/pk. The circuitry near the motor controls these pulses and is linked to the opto couplers and slicer mounted in the gearbox.. This circuitry seems to deteriorate over time, and adjustment of the two resistors on the motor board will restore the original pulse shape. Check the middle connection of the opto emitters for 1.2v DC. Never had a problem with the Mag Pulse, but that can be checked on pin 7 of J2.

With an oscilloscope, check for Dec direction pulses at pins 3 and 13 of U13. These should be the same as the RA drive and are adjusted the same. Again, this deteriorates in the same manner.

The Tantalum capacitor C1 on both the DEC and RA motor control board can add noise into the system if this capacitor is going faulty and should be changed. Another fault this component can induce is intermittent fuse blowing.

Symptom:- No N. E. S. W. operation from the handset. Powers up OK, and GoTo works.

Usually a fault in U12 which will need replacement. These are supplied by various manufacturers, Lucent and Exilinx being the main source. This failure is probably linked to the hot-plugging scenario with the handset.

Symptom:- Dec Motor Runaway. Powers up OK, then DEC motor starts up at full speed.

This fault has occurred a few times now and it's always the same thing. It is associated with the Power Board. The PCB through-via between the top and bottom track leading from Pin 18 of J8 to Pin 3 of J1 goes open circuit. Re-work with a jumper between these two pins, dont attempt to repair the via.

The location of the PCB track-via is between the two mounting lugs of the Keyboard Jack.

 

Symptom:- Dec Motor Runaway. Powers up OK, GoTo works but slows the DEC Motor.

Again, this fault has occurred a few times. Check the capacitor mod on IC15 pin 11. The lead of the capacitor has always been in contact, intermittently or full, with pin 10 of the same IC, unbalancing the Dec null detector.

Symptom:- RA Motor Runaway. Appears to powers up OK.

Normally a symptom of a faulty U17.

Watch out for the top of R3 on the motherboard touching the heatsink of U17. Intermittent. Motor drives hard then stops only to start again.

Symptom:- Mains PSU 18V Cable Restraint.

The cable restraint on the Mains to 18V switch mode power supply is very rigid and can cause a fracture of the 18V cable insulation. This in turn leaves the + and - cables bare and can touch causing a short to the main PSU. There are no fuses at this point apart from the mains plug and PSU internal input fuse, so catastophic failure could result. No problems have been encountered so far except the identification of the cable insulation.

Symtom:- Dec Runaway or not responding.

A number of instances of an open circuit between Pin 1 of the I/C U2 on the Dec motor control board to pin 1 of the RJ45 connector have been found, thereby not allowing the comparitor pulse chain to be fed to the uP board.

Todate, all these problems have been traced to an open circuit in the DEC RJ45 cable itself. A change of cable cures the problem, but be careful when removing the cable as there are two wires directly attached to the motor. (Normally blue and black). These need to be removed before replacement. Note the colour code when dismantling as these wires are DC connections and therefore control the motor direction.

 

Mechanical

The main problems encountered with the mechanics of the telescope centre around the mechanical swarf, grease compound, the liberal use of grease and the quality of machining of the main DEC and RA gears.

1. Remove the motors from the telescope. Be careful when removing either motor as there is a compression spring between the motor mounting frame and the worm shaft assembly.

The RA motor is located under the telescope body, removing the 4 screws and 2 Allen bolts will allow the bottom cover to be removed, thereby allowing access the the 2 Allen bolts holding the motor in place. Unplug the motor from the main PCB before removal.

The DEC motor is in the the fork arm containing the clutch and fine DEC adjustment control. (Never to be used with power applied to the scope). Remove the 3 small Allen screws holding the plastic cover plate in place and the DEC clutch knob to allow access to the motor. Remove the RJ45 connector located inside the telescope fork arm. Then remove the motor by unscrewing the 2 Allen bolts holding the motor in place.

2. Remove all grease from the motor worms, especially around the RA motor's magnetic pulse detector and the permanent magnet built into the worm shaft.

3. Remove all grease from the main DEC and RA gears, making sure to clean into the gear teeth.

4. Lightly re-grease with a Molybdenum Disulphide based grease (Castrol MS3 is great for this application if you can get it)

5. Assemble in reverse order. But before doing so, carry out the electrical adjustment above then, (a) relocate the the RA and DEC control PCB's in the vertical plane thereby gaining access to their respective backlash Allen screws. Adjust for 20thou max clearance when in final position. Note:- the DEC adjustment screw has to be replaced for a longer unit complete with a thin bolthead as this adjustment has to be carried out with a spanner....there is no room for an Allen key adjustment here.

6. A drop of Loctite must be applied after adjustment.

(Historical. The above RA adjustment modification was originally proposed by an Italian astronomer, but I can't remember his name. I think the details may be in the MAPUG Archives, not sure. Contact me if you know of the gentleman, then I can credit him for his work).

7. The final part of the process is to run in the gearing. Apply 12 volts to the motor wires, either at their respective plugs or to the motor direct. Allow to run for 2 to 3 hours in each direction by reversing the battery polarity to bed the teeth engagement. Remember to release the clutches.

 

 

 

 

 

 

 

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