Repairing the Dec/RA drive.

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Fixing the problems

Fixing the first problem

Okay! We've found our first problem! We've found that there is a wire broken in the bundle of wires on the left side of the board where the external components connect to the board. How do we best repair this particular problem?

The first thing to do is to remove the board from its mounting by removing the single allen screw holding it into place. (Be sure to watch for the small circular insulator under the screw.) Flip the board over and you have access to where the ends of the wires pass through the board. Then, using some 'Desoldering Braid'.. (Radio Shack P/N 64-2090) .. press the very end of the braid against the solder holding the remaining wire piece in place. Heat the braid with your soldering iron and, as the braid heats, it will begin to draw the melting solder from the hole and into the braid.

You may well have to move the braid a bit as it absorbs solder, but continue to do so until you can at last manage to remove the excess wire from the hole with some needle nose pliers.

In case I've gotten ahead of myself and you aren't familiar with desoldering braid, it is a very finely braided copper wire that when held against melted solder, will soak it up like a candle wick. The braid is quite inexpensive, and in general, will do a much better job than will a more expensive 'solder sucking tool', that is used to 'suck' the solder away from an area.

Back to the repair: With the hole now cleaned of excess old wire, you can strip a small portion of wire from the remaining wire. Then, apply a small amount of solder to the exposed end to give it some strength, and insert it into the hole you've just cleaned. Apply a small amount more of the solder from the bottom side of the board and you've fixed your runaway problem!

You'll notice that we didn't even identify which wire it was that was broken. That's because common sense tells us that we aren't likely to have two wires broken at the same time. Even if we do, it needs to be fixed anyway, so repair that one before looking for more.

This approach is used for any of the wires in the bundle of wires on the left side of the board. Find the broken wire, remove the board and flip it over, suck away any excess solder so as to get a nice, clean, open hole for the wire, then strip a small portion off of the original wire, 'tin' it and insert it into the newly cleaned hole, and solder it into place.

You might find that while making the repair, you accidentally broke off too much of the original wire to be able to get the slack in the wire that you need, in order to re-mount the board. If that's the case, simply use some extra wire and solder, then tape, some wire to the original wire and go from there. Personally, I suggest you use a smaller, more flexible wire if you need to extend the original. That may well be a part of why the wire broke in the first place; it was too stiff. For repairs on these units, I generally just buy a four wire telephone cord and strip out one of the individual wires. But that's a trivial matter as to what kind of wire to use.

Fixing the second problem

Rats!!! As luck would have it, your next problem is the first in a series of problems that strikes fear in the heart of every LX200 owner!! You've found that a wire is broken again, but this time, it's a wire that attaches to one of the light sensors!!!!! Worse yet, the wire is broken off right at the sensor!!! NO WAY can this be soldered!!! The situation is hopeless!!

Fear not, because with a little bit of luck, we can fix the problem with a minimum of hassle and be running again in no time! (Even with absolutely NO luck on our side, we can still fix it. It just takes one or two extra steps, is all.)

What you need at this time, is a good replacement part for the photo diode, or light sensor, that is broken. We'll use a more modern replacement part for the photo diode. We'll use an infrared photo transistor to replace the old part. It's even the proper size, too; 3mm in diameter. Just to standardize the parts in the repaired unit, replace both of the sensors at the same time. They're inexpensive enough; $0.49 each, American. (Radio Shack P/N 900-6133).

Here, we can see what we need to know about the new sensors, in order to use them. First, we can see that each sensor has a longer and a shorter leg. The longer leg is the 'emitter', and both of the emitters will be soldered together. The remaining leg is the 'collector' and will be mounted in the mounting holes as shown. That is, the left sensor will have its collector mounted to the left side of the sensor, and the right sensor will have its collector mounted to the right side of the right hole.

With the original parts, they, being diodes, had a 'cathode'.... the equivelant of our new 'emitter'... and an 'anode', which is now being replaced by the 'collector' on the new units. For all practical purposes, the two units will be connected up, and will function identically, as the old units. What's in a name, after all?

Before you do anything else, follow the wires from the old components, up to where they connect on the board. We see that, like the components that will replace them, they have a common lead. That is, their 'cathodes' are soldered together.

The wire that runs from the junction of these two leads, presently connects to the second hole down on the left side of wires on the small board. The left-most component has it's left leg connected to a wire that connects to the 7th, or next-to-last connector on the board. The right-most component has it's right leg connected to a wire that connects to the 8th, or last, connection on the board. You can't see the board in this picture, but I thought I'd be nice and tell you, in case you had problems following the original wires out.

When you're done, the wires and the mounting of the new components will look just like the original did. Same wires going to the same holes, components fitted into the same holes on the housing as before. Also when done, the components will even look the same as far as how they're glued into place.

The replacement process is quite simple. First, remove the small electronics board and flip it over, as in the previous example. Using the desoldering wick, unsolder and remove the three wires that feed down to the sensors. Next, remove the existing sensors.

NOTE*: If at all possible, you want to remove all of the remains of the sensors and glue without having to disassemble the gear box. Depending on the type of glue that was used, you'll likely find that alcohol or acetone applied with a q-tip will aid in loosening the glue and allow for complete glue removal. Take the extra couple of minutes to make sure you've got all of any broken pieces of sensor removed. Even use a straight pin or a small crochet hook to dig out all of the excess sensor and glue. You don't want any of it sitting inside the hole between the actual sensor and the mask that covers the hole on the inside.

Temporarily mount the sensors into the holes for them and confirm that you have plenty of room to solder the three new wires to them. Be especially sure that you have the two 'emitter' leads soldered together before you solder the wire to it.

As you mount the sensors into the holes and after having already soldered the wires into place, coat the 'wire' ends of the sensor blocks liberally with epoxy, hot melt glue, or whatever type of bonding agent you wish to use. Insert the sensors only about 1/2 the length of each sensor, into its mounting hole and let the glue dry. You only want to insert about one-half the length of the sensor into the hole because the sensor body itself, is long enough to be pushed a bit too far into the hole, and make contact with the light mask. That, you don't want.

Be gentle when first inserting the transistors into their holes. The metal mask inside is thin, and it wouldn't take much stress on the sensor against the mask, to deform it.

When the glue has dried, it should look much like the original layout, shown in the previous drawing, with lots of glue covering the metal portions of the contacts.

As before, I suggest you add a bit more hot melt glue than is necessary, for holding the components into place. That way, if one of the leads should ever break again, you'll be able to peel back some of the hot melt glue and be able to access the broken metal tab directly, without having to replace it.

There is just one more thing that has to be done in order to be up and running, but let's kill two birds with one stone. Let's see about replacing another part first, since this 'minor' little detail has to be done in that case, too.

Fixing the third problem

Once again, your troubleshooting for motor runaway has led you to discover yet another broken wire. And as was the case for the previous problem, this wire again, is broken off much too close to the component to allow for soldering a wire directly to it.

In this case, the wire was soldered to one of the LED's. They are mounted, you recall, right beneath the drive motor for the unit.

As before, we'll fall back to Radio Shack to find a suitable replacement part, and also as before, we replace both of the parts at the same time, just to conserve uniformity in the parts. A quick search shows us that a 3 millimeter infrared LED is available to us (Radio Shack P/N 900-1573) at a reasonable price ..... $0.69 each, American. Perfect. It not only meets our optical and electronic requirements, but again, is the correct size for fitting into the mounting holes.

The components in this problem, and the drawing for them, looks very much like the previous problem with the photo transistors. There are only a couple of differences between them.

For one thing, there are only two leads coming from the small board down to the components. Each of the wires connects to the OUTER wires on the components after the central two are soldered together. That's correct; there is no wire connecting to the junction of where the two components are soldered together.

The second major difference is in what the lead lengths on the components represent. On this component, the short lead is the 'cathode' and the long one is the 'anode'. Now, I realize that these names aren't the same names as those used on the photo transistors, but on the photo transistors, I said that the 'emitter' was the equivelant of a 'cathode' and the 'collector' was the equivelant to an 'anode'. The primary point here is, watch closely as to which connector... short one or long one... connects to where.

The third difference is, the 'anode' of the LEFT component connects directly to the 'cathode' of the RIGHT one! Since we agree that the name difference isn't of concern to us, we must be certain in this case, to connect the proper wire to the proper component connection and back to the proper place on the board. As in the previous example, the safe thing to do is to write down which wire on which component, connects to which connection!! (Whew!)

All of that having been said, this should require no more work for replacing the components that it did for the previous trouble.

Adjusting the comparators

You've replaced some components for the previous two troubles, so what you must now do is to adjust the outputs of the comparators to give you that nice, square wave signal that was discussed earlier on. This is true any time you make a significant change to any of the components, so I'm going to go over the 'new technique' for doing so, right now.

But let me digress just long enough to expand on an earlier comment. I had said that having bad connections on the input/output cable was one of the primary causes of intermittent motor runaway. The operative word here is 'intermittent'. The SECOND-most often cause of intermittent motor runaway for no apparent reason, is because this adjustment is incorrect, and the comparators aren't delivering the good, clean pulses back to the main board as they should.

In the past, proper adjustment of the comparators has always been one that has required the use of a dual trace oscilloscope. However, due to the nature of how the comparitor circuit is designed when it isn't hooked up to drive an actual circuit, I've found a very simple way of making the adjustment, using nothing but your multimeter, set on the "DC VOLTAGE" setting. (It's been tested pretty thoroughly and compared to the results of when using an oscilloscope, so I'd say you're pretty safe here and shouldn't feel as if you're being used as a sacrificial lamb.)

In order to make this adjustment, you MUST be powering the board and the motor using our method #3. That is, there must be a +1.5 volt or +3.0 volt battery powering the motor, and there MUST be a 9 volt battery powering the small logic board! The unit can NOT be connected to telescope power, and all leads other than the power leads must NOT be connected to anything!

We really don't care which direction the motor turns, and that's why I haven't specified any that particular motor pin be connected to a specific side of the battery.

I am assuming that when this adjustment is being made, that the multimeter you are using, is an 'auto ranging' meter. That means that in order to measure DC voltage, for instance, there is but one setting for this operation and that you don't have to switch to different voltage scales as the voltages change. It isn't absolutely necessary to have this feature, but it makes things a lot easier, and todays multimeters are almost all auto-ranging types; even the very inexpensive ones.

Let's set things up for making the adjustment before we connect the batteries. First, connect one side of the multimeter to test point #7, which is zero volts. Second, put the other connector from the multimeter onto pin #1 of the input/output connector. Easy enough. (Don't worry if your meter is hooked up backward somehow. If it is, your voltage readings will just be 'minus' voltage readings, and we don't care about that. All we care about is the absolute voltage value.)

Connect the motor battery up, then the 9 volt board power battery. Next, watch your voltage reading on the multimeter. It's bound to be quite off because we've just changed some components. Now, with a small screwdriver, turn the potentiometer for comparitor #1. Watch to see if it goes up or down in value. You're adjusting to get a reading of 1.21 volts. The adjustment may seem a little sensitive, but no problem; just slowly close in on the voltage setting until you get the 1.21 volt reading.

Next, disconnect the 9 volt battery and move your meter lead from pin 1 of the input/output cable, to pin 2 of the same cable. Leave the other end connected to test point 7, the 'ground'. Connect the battery once again and now adjust the potentiometer for comparitor #2 until you again get a reading of 1.21 volts. Once you've got it, you're all set!

Disconnect the batteries and their leads and you're all set!

What if ??? .........

If you can't seem to get a voltage reading, or if you get a reading that won't adjust, obviously there is something wrong. The first thing to do is to look at whatever it is that you've just repaired. Are the wires attached correctly? By any chance, when you soldered a wire, either onto a component or onto the board, did you short out another point with the solder? This would most likely happen for a connection on the small board since the connections are quite close together.

If all looks well, connect everything back up, but instead of putting your second meter lead on one of the comparitor outputs, attach it to the test point near the ground point, on the pin that represents the output for light sensor #1. Again adjust the potentiometer for comparitor #1 and see if the voltage changes. It should, since you're now reading the output from the photo transistor.. or photo diode, depending on what was replaced... as it is fed into the comparitor. It should be able to be varied from relatively low up to fairly high.... 3 volts or so.

Also check the same situation for photo transistor/diode #2, to see if that one will give a reading. If BOTH transistors show no reading, then it's likely that there is no light being emitted by the LEDs. If they both continuously read 3 or 4 volts approximately, then you have to suspect that there is a wiring error.

If worse comes to worse and there seems to be no output from the photo diodes or photo transistors, you may have to dismount them from their mounting holes and disconnect the wires. You can then put your multimeter across the two pins of the photo diode/transistor, and with the meter set on the 'resistance' (Ohms) scale, you should be able to measure a resistance that varies as you point it at a bright light. If not, switch the meter leads around to the opposite connections. The diodes/transistors will only act like variable resistors when the meter is hooked up in one of the two directions.

Those tips should help you to isolate down what the problem may be. But also remember; the odds of you having done anything seriously wrong is very low. Chances are, your 'fix' and the adjustment, will work right from the start.

Technical note: For slightly more advanced technicians, I want to explain why the 'meter' method works as it does. If this part doesn't matter to you or is confusing, then just skip it. The circuit works, and that's what's important.

The output from the comparitor circuit is an open collector circuit, that is fed by two transistors. The voltage dropped across a typical silicon emitter-collector is approximately 1.2 volts. Thus, the total voltage dropped across the two transistors is slightly over 2.4 volts.

The output square wave of the comparitor swings between 2.4 volts and just slightly above 0 volts as the signal toggles, when the component is not loaded. When the duty cycle is 50%, the average DC voltage reading will be half way between the two extremes, or 1.21 volts. This results in a symmetrical, 50% duty cycle square wave, which is what the desired wave shape should be.