Custom Search
|
|
  |
|
|
TROUBLESHOOTING POWER SUPPLIES Whenever you work with electricity, you must follow all the appropriate safety precautions. In the front of all electronic technical manuals, you will always find a section on safety precautions. You should also find posted on each piece of equipment a sign listing the specific precautions for that equipment. One hazardous area that is sometimes overlooked, especially on board ship, is grounding of equipment. By grounding the return side of the power transformer to the metal chassis, manufacturers can wire the cathodes of the tubes in both the power supply and the load being supplied by the power supply directly to the metal chassis. This eliminates the necessity of wiring each tube directly to the return side of the transformer, saving wire, and reducing the cost of building the equipment. While this solves one of the problems of the manufacturer, it creates a problem for you, the technician. Unless the chassis is physically grounded to the ship's ground (the hull), the chassis can be charged (or can float) several hundred volts above ship's ground. If you come in contact with the metal chassis at the same time you are in contact with the ship's hull, the current from the chassis can use your body as a low resistance path back to the ship's ac generators. At best this can be an unpleasant experience; at worst it can be fatal. For this reason Navy electronic equipment is always grounded to the ship's hull, and approved rubber mats are required in all spaces where electronic equipment is present. Therefore, before you start to work on any electronic or electrical equipment ALWAYS ENSURE THAT THE EQUIPMENT AND ANY TEST EQUIPMENT YOU ARE USING IS PROPERLY GROUNDED AND THAT THE RUBBER MAT YOU ARE STANDING ON IS IN GOOD CONDITION. As long as you follow these simple rules, you should be able to avoid the possibility of becoming an electrical conductor. TESTING There are two widely used checks in testing electronic equipment. The first is the VISUAL CHECK. Do not underestimate the importance of this check. Many technicians find defects right away simply by looking for them. A visual check does not take long; in fact you should be able to see the problem in about 2 minutes if it is the kind of problem that can be seen. You should learn the following procedure. You will find yourself using it quite often, as it is good not only for power supplies but also for any other type of electronic equipment you may be troubleshooting. 1. BEFORE YOU PLUG IN THE EQUIPMENT, LOOK FOR: LOOSE TUBES - A tube that is not properly seated in its socket may not be making proper contact with the rest of the circuit. It may very well be the source of your problem. Push the tube completely into place. SHORTS - Examine any terminal or connection that is close to the chassis or to any other terminal for the possibility of a short. A short in any part of the power supply can cause considerable damage. Look for and remove any stray drops of solder, bits of wire, nuts, or screws. It sometimes helps to shake the chassis and listen for any tell-tale rattles. Remember to correct any problem that may cause a short circuit. If it is not causing trouble now, it may cause problems in the future. DISCOLORED OR LEAKING TRANSFORMER - This is a sure sign that there is a short somewhere. Locate it. If the equipment has a fuse, find out why the fuse did not blow; too large a size may have been installed, or there may be a short across the fuse holder. LOOSE, BROKEN, OR CORRODED CONNECTIONS - Any connection that is not in good condition is a trouble spot If it is not causing problems now, it probably will in the future. Fix it. DAMAGED RESISTORS OR CAPACITORS - A resistor that is discolored or charred has been subjected to an overload. An electrolytic capacitor will show a whitish deposit at the seal around the terminals. Check for a short whenever you notice a damaged resistor or capacitor. If there is no short, the trouble may be that the power supply has been overloaded in some way. Make a note to replace the part after signal tracing. There is no sense in risking a new part until you have located the trouble. 2. PLUG IN THE POWER SUPPLY AND LOOK FOR: SMOKING PARTS - If any part smokes or if you hear any boiling or sputtering sounds, pull the plug immediately. There is a short circuit somewhere that you have missed in your first inspection. Use an ohmmeter to check the part again; begin in neighborhood of the smoking part. COLD TUBES - After allowing the equipment about two minutes for warm-up, touch all the tubes. If a tube is cold, it is either burned out or there is a break in the heater connections and the tube is not receiving proper heater voltage. Remove the tube and connect an ohmmeter across the heater terminals to see if the filament is open (reads almost infinite resistance). If the filament reads open, it is burned out. Replace the bad tube with a good one. If the filament reads a low resistance, this indicates that the filament is all right. Use an ac voltmeter to find the break between the filament and the output of the transformer. SPARKING - Tap or shake the chassis. If you see or hear sparking, you have located a loose connection or a short. Check and repair the problem. If you locate and repair any of the defects listed under the visual check, make a note of what you find and what you do to correct it. It is quite probable you have found the trouble. However, a good technician takes nothing for granted. You must prove to yourself that the equipment is operating properly and that no other troubles exist. If you find none of the defects listed under the visual check, go ahead with the signal tracing procedure. The trouble is probably of such a nature that you cannot see it directly with your eye-you must see it through the eye of the oscilloscope. The second type of testing is signal tracing. Tracing the ac signal through the equipment is the most rapid method of locating a trouble that you cannot find by a visual check. It also serves as a check on any repairs you may have made. The idea is to trace the ac voltage from the transformer, to see it change to pulsating dc at the rectifier tube filament, and then to see the pulsations smoothed out by the filter. The point where the signal stops or becomes distorted is the place to look for the trouble. Before you begin signal tracing, it is a good idea to measure the dc voltage. The dc output voltage should be in the neighborhood of 340 volts. If you have no dc output voltage, you should look for an open or a short in your signal tracing. If you have a low dc voltage, you should look for a defective part and keep your eyes open for the place where the signal becomes distorted. Signal tracing is done by observing the waveform at the input and output of each part of a circuit. It is the method used to localize trouble in a circuit. Let's review what each part of a good power supply does to the signal, as shown in figure 3-50. The ac voltage is brought in from the power line through the line cord. This voltage is connected to the primary of the transformer through the ON-OFF switch (S1). At the secondary winding of the transformer (points 1 and 2), the scope shows you a picture of the stepped-up voltage developed across each half of the secondary winding - the picture is that of a complete sine wave. Each of the two stepped-up voltages is connected between ground and one of the two plates of the rectifier tube. At the two rectifier plates (points 4 and 5) there is still no change in the shape of the stepped-up voltage the scope picture still shows a complete sine wave. Figure 3-50. - Complete power supply (without regulator).
However, when you look at the scope pattern for point 6 (the voltage at the rectifier heater), you see the wave shape for pulsating direct current. This pulsating dc is fed through the first choke (L1) and filter capacitor (C1), which remove a large part of the ripple or "hum," as shown by the waveform for point 7. Finally, the dc voltage is fed through the second choke (L2) and filter capacitor (C2), which remove nearly all of the remaining ripple. See the waveform for point 8, which shows almost no visible ripple. You now have almost pure dc. No matter what power supplies you may encounter in the future, they all do the same thing - they change ac voltage into dc voltage. The following paragraphs will give you an indication of troubles that occur with many different electronic circuit components. Tube Troubles The symptoms of tube trouble will vary with every type of circuit and each type of tube. However, the problems that can develop with a tube are common to every tube. Here are the five possible tube troubles that you should keep in mind. The meaning of each trouble will be clear by the time you end your study of vacuum tubes, even though you may not quite understand them now. The filament, after long service, may be unable to emit as many electrons as are required for proper operation.
The symptoms you will come across in signal tracing will be many and varied. You will need to combine your "know-how" of the circuit and your knowledge of these five possible tube troubles to determine if the tube could in some way be causing the symptoms. If you suspect the tube of causing trouble, either try another tube in its place or check it on a tube tester. But remember, the final check of whether or not the old tube was bad is whether or not the equipment works properly when a good tube is put in its place. Therefore, putting in a good tube and then trying out the equipment is the best check. Transformer and Choke Troubles As you should know by now, the transformer and choke are quite similar in construction. Therefore, it is no coincidence that the basic troubles they can develop are the same.
As with the tube, the symptoms of these troubles will vary with the type of circuit. However, when you have decided that one of these four possible troubles could be causing the symptoms, there are definite steps to take. If you surmise that there is an open winding or windings shorted together or to ground, an ohmmeter continuity check will locate the trouble. If the turns of a winding are shorted together, you may not be able to detect a difference in winding resistance. Therefore, you need to connect a good transformer in the place of the old one and see if the symptoms are eliminated; but keep in mind that transformers are difficult to replace. Make absolutely sure that the trouble is not elsewhere in the circuit before you change the transformer. Occasionally, shorts will appear only when operating voltages are applied to the transformer. In this case you might find the trouble with a megger - an instrument that applies a high voltage as it reads resistance. Capacitor and Resistor Troubles Only two things can happen to a capacitor:
You can check a capacitor you suspect of being open by disconnecting it from the circuit and checking it with a capacitor analyzer. You can check a capacitor you suspect of being leaky with an ohmmeter; if it reads less than 500 kilohms, it is more than likely bad. However, capacitor troubles are difficult to find since they may appear intermittently or only under operating voltages. Therefore, the best check for a faulty capacitor is to replace it with one you know to be good. If this restores proper operation, the fault was in the capacitor. Resistor troubles are the simplest; but like the rest, you must keep them in mind.
You already know how to check possible resistor troubles. Just use an ohmmeter after making sure no parallel circuit is connected across the resistor you wish to measure. When you know a parallel circuit is connected across the resistor or when you are in doubt, disconnect one end of the resistor before measuring it. The ohmmeter check will usually be adequate. However, never forget that intermittent troubles may develop in resistors as well as in any other electronic parts. Also remember that the final proof that a resistor is bad is when you replace it with another resistor and the equipment operates satisfactorily. Although you may observe problems that we have not covered specifically in this chapter, you should have gained enough knowledge to localize and repair any problem that may occur. Q.43 What is the most important thing to remember when troubleshooting? |
 
|
