We live in a "disposable society." Disposable razors, lighters, pens and a myriad of other items are a part of our daily life. Many operators consider power supplies to be disposable as well. Their rumored inability to be repaired and relatively low cost create the impression of disposability.
This is not the case, however. I repair lots of power supplies and it's really quite easy and practical. In fact, a repaired supply may end up having a longer life expectancy than a new one, as you'll see.
Switching Power Supply Operation
Modern power supplies are known as "switching regulator power supplies." In most switching supplies, the 110 volt AC input is first rectified by two diodes and filtered by a pair of capacitors. This creates two high-
voltage sources; one positive and the other negative.
A pair of transistors is then used to switch these high voltage supplies across the primary winding of a ransformer. This switching action is very fast. A typical switching speed is around 40,000 cycles per second
or 40 kilohertz. An integrated circuit is commonly used to control the transistors. This IC not only controls the speed at which the transistors are switched, but also controls the amount of time that each transistor is energized. The output voltage of the power supply is determined by the "on" time of the transistors. If the ransistors are keep on for a longer period of time, the output voltage of the supply will rise, while shorter times lower the output voltage. This is known as "pulse-width modulation."
The output of the transformer (which is now alternating current) is then rectified by special high-speed diodes to change it back to direct current. This output is not pure DC however, and requires extensive filtering to remove the high-frequency "noise" that is generated by the rapid switching action of the transistors. Filtering is accomplished by using a combination of coils (also known as "chokes") and capacitors.
The output voltage of the power supply is regulated by feeding some of the output back to the integrated circuit that controls the switching transistors. If the output voltage is too low, the IC allows the transistors to remain energized for a longer period of time, raising the voltage. An output voltage that is too high signals the IC to cut back on the transistors, lowering the output voltage.
Power Supply Failures
I have found that there are only a small handful of components that fail in switching regulator power supplies. The most common failure is the switching transistors themselves. The transistors short-circuit, causing massive amounts of current to be drawn across the transformer and blowing the fuse.
Transistor failure is often caused by bad capacitors. It is extremely common to find output filter capacitors that are swollen or leaking. Any capacitor that appears to be bad should be replaced. To prevent a recurrence of this all-to-common failure, output filter capacitors should be replaced with special "low ESR" (Equivalent Series Resistance) capacitors. These capacitors are specifically designed to handle the rigors of filtering in a switching supply. Most power supply manufacturers do not install low ESR capacitors as original equipment because they are somewhat more expensive that conventional capacitors. However, it is well worth the money to use them as replacement components as they will greatly extend the life of the power supply in the field. When I work on a power supply, I replace all the output filter capacitors with low ESR caps regardless of whether they appear to be good or bad. Since a service call costs far more than the capacitors, it's a prudent thing to do.
Diode failure is another common problem. There are quite a few diodes in a switching supply and failure of any one of them will cause the supply to blow the fuse or shut down. The most common diode failures are
shorted +12 volt or -5 volt output rectifiers. Failure of these diodes will not blow the fuse. The supply simply detects the short and shuts itself down. Some of these failures may be caused by using the +12 or -5 volt outputs to power coin door lamps. The -5 volt output is not over- current protected in all power supplies. A shorted lamp socket may blow the diode by drawing too much current from the supply. The +12 volt diodes may be blown if 6 volt bulbs are inadvertently used instead of 12 volt bulbs. The high-voltage input diodes may also short-circuit. This is often accompanied by shorted switching transistors and will blow the fuse.
Testing and Repair
All testing is done with the power off. Start by testing the pair of switching transistors. These will be mounted on a heatsink that helps them run cooler. Test them by using an ohmeter or a digital multimeter set to the diode test range. Check each transistor for a short between emitter and collector. Replace any transistors you find to be bad. Although some technicians claim that you should replace them both even if just one is bad, I have not found this to be necessary.
By the way, these transistors will always seem to test shorted between base and emitter when tested "in-circuit." I generally don't bother testing the base-emitter junction of the transistors. When the switching transistors fail, they always short between emitter and collector. If you're in doubt, pull the transistors out of circuit to test them. If the transistors are shorted, the fuse will have blown. Be sure to test the high-voltage diodes as well. The high-voltage diodes are usually part of a bridge rectifier, although they may be individual diodes.
Next, test the output rectifiers. There are three pairs of diodes to test. One pair is for the -5 volt output. These will be fairly small; approximately the same size as the ubiquitous 1N4004 with which we are all familiar. The +12 volt diodes are usually somewhat larger. The two +5 volt output diodes are housed together in a "dual-diode" package that looks very much like a transistor. Like the switching transistors, this
diode package is mounted on a heatsink. It will generally have the diode schematic symbols printed on it. This diode will usually not test properly in-circuit. Testing can be simplified by unsoldering it with a "solder sucker" instead of removing it completely from the printed circuit board. I have seen very few failures of the +5 volt output diodes. All diodes must be replaced with high-speed diodes or the power supply will generate excessive noise.
Follow these tests by replacing all the output capacitors with low ESR caps and fire up the power supply. The supply should be tested under load. Use a 1 ohm, 50 watt resistor or equivalent as a "dummy load",
connected between the +5 volt output and ground (DC COM). This will draw 5 amps from the supply, which is adequate for test purposes. If the supply is still inoperative, the integrated circuit may be bad. Test the IC by removing it from the printed circuit board and installing it in a power supply that you know to be good. I have a spare power supply with a socket in it that I use exclusively to test integrated circuits. Just
about all the supplies use the same IC; a type 494. Equivalent integrated circuits are: TL494CN, uA494, uPC494C, IR3MO2, and MB3759. The over-the-counter replacement for these is ECG1729.
Obtaining Replacement Components
Capacitors can be obtained from TTI. TTI is the nations largest distributor of passive components such as resistors and capacitors. The capacitors you want to order are made by Nichicon. Order 3300 microfarad
at 16 volts (part number UVX1C332M) and 1000 microfarad at 25 volts (part number UVX1E102M.) These will be suitable as replacements for output filter capacitors in virtually all makes and models of power supplies. Remember, you can always substitute a capacitor of higher voltage when replacing filter capacitors. E.G. A 1000 microfarad, 16 volt capacitor can be replaced with a 1000 microfarad, 25 volt. TTI has 20 offices nationwide. Contact them at (800) CALL TTI.
Minus 5 Volt Output Too High
Most switching regulator power supplies have three DC outputs. One is the main +5 volt DC output that powers the computer system. The others are the +12 and -5 volt outputs. These DC outputs are often used to power the sound generating system and the audio amplifier itself. When you're testing a power supply, it's important to check all three of the outputs. This is especially true when you have a game that basically works okay but has distorted or missing audio.
When a switching regulator power supply fails, all three outputs will usually drop to zero volts (see Play Meter, June, 1989, page 107.) Sometimes, however, the output voltage may rise. If you find that the +5
VDC and +12 VDC outputs are normal but the -5 VDC output is too high (more than -6 VDC), try replacing the -5 output filter choke.
It's easy to locate the -5 volt filter choke, even without a schematic diagram. Just follow the trace on the printed circuit board back from the -5 VDC output of the power supply. You will eventually come to a
component that may look something like a capacitor but will be clearly labeled "L" on the board and will generally be accompanied by the schematic symbol for a coil as well. The coil is wound on a ferrite coil
and is covered with a plastic sleeve that has been heat-shrinked over it. Examine the coil. If the heat-shrinked cover has been melted or is missing entirely, the coil may be bad.
This is not a component that you can boogie down to Radio Shack for a replacement. There are a couple of options for obtaining a replacement coil. The preferred method is to take the coil off a junk power supply.
Alternately, you can pull the burned wire off the ferrite core and rewind the choke yourself using the appropriate gauge wire. There aren't that many turns of wire on it that you can't rewind a new coil in five minutes.
Switching Regulator Power Supplies : Output Capacitor Replacements
I have received a number of calls and letters from operators and technicians that are having trouble obtaining replacement capacitors for switching regulator power supplies. I recommend using Nichicon brand
capacitors. I have been using them for almost two years and to date I have not seen a repeat capacitor failure.
Nichicon brand capacitors can be obtained from a company called TTI. They have a score of offices across the United States of America, all of which can be reached by calling (800) CALL TTI. I recommend that you
order just two different capacitors for use as replacements for the output filter capacitors in the "Peter Chou" style power supply. It helps a great deal when you have the part numbers. For the +5 VDC output, use
3300 microfarad, 16 VDC capacitors. The Nichicon part number is UVX1C332M. Each power supply requires two of these.
To make ordering and stocking easier, I use the same capacitor for both the +12 VDC and the -5 VDC outputs. It's a 1000 microfarad, 25 volt capacitor. The Nichicon part number is UVX1E102M. Although some power supplies use a 2200 microfarad capacitor for the +12 VDC output, I have found the 1000 microfarad to be perfectly satisfactory. Most power supplies use one capacitor each for the +12 VDC and -5 VDC outputs so order the same number of 1000 microfarad capacitors as you do the 3300 microfarad capacitors. When you replace the output filter capacitors, it's a good idea to change them all at once. Switching Regulator Power Supplies ("Peter Chou" Style) Output Diode Replacements
Output diodes are a common failure item in the switching regulator power supply. I would say that around twenty-five to thirty percent of them have bad output diodes.
High Speed Diodes
There are three pairs of output diodes; one pair for each of the outputs: +5 VDC, +12 VDC, and -5 VDC. These are not ordinary diodes. They are special, high-speed, "fast-recovery" diodes. High speed diodes are made to handle the very fast switching action (around 40 thousand cycles per second) of the power supply.
I have rarely replaced the +5 volt diode assembly in a switching regulator power supply. The +12 and -5 volt output diodes are the most common failures. It is normal for these diodes to test bad when checking them "in-circuit." There is usually a low ohm resistor (normally around 100 ohms) across the output of the power supply that causes a very low reading when checking the +12 or -5 volt output diodes. Most people unsolder and remove one end of each diode to test it but you can usually bypass this step. When these diodes fail they will generally short completely. Instead of reading around 100 ohms, you will get a reading of around zero ohms; a dead short!
Substitute Diodes
The +12 volt output diodes will usually carry an original part number like PXPR302 or FR302. These are 3 amp diodes. The -5 volt output diodes will often be type PXPR1502 or similar. Good engineering practice dictates that high speed, "fast-recovery" diodes be used in this circuit. I have found normal diodes will fail prematurely and as such are unacceptable as substitutions.
That's all there is to it. The more you work on repairing power supplies, the easier it gets. When you consider that many power supply repairs are effected with the replacement of a single diode, you can see that they are anything but disposable!