6939808-PC-Power-Supply-Repair-magazine-article-1996-WW.pdf

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Its 8:00 a.m., the neighbors dog barked allnight, your coffee tastes like weak tea, and the phonemessage light blinks frantically. Full of resolve, youflip on your PCs power switch, and ... presto - noth-ing! No lights, no beep, no fan, nada. Suddenly yourealize, its gonna be a really bad hair day.

While theres nothing I can do about the earlyhour or the coffee, I can probably help you get yourPC back on its feet. The most common case ofSudden PC Death Syndrome is a defective powersupply. The problem can come from many sources,like heat, power surges, and old age. While its easy

enough to replace a power supply by swapping theold for new, its not always practical.A case in point: I have an AST 486SX that died

when a truck plowed into the corner power pole andcaused a two-hour black out. When the power cameback on, my PC didnt. A quick check showed thecause was a fried power supply. Unfortunately, a callto AST revealed, to my horror, that a replacementpower supply costs $150.00. Moreover, because of itsunique case design, theres no generic substitute.

Fortunately, its not difficult to fix PC power sup-plies. While they may look different on the outside,most PC power supplies use the same electronics onthe inside. In this ar ticle, Ill show you how easy it isto fix a dead power supply.

The Basics

The power supply is a large metal box, mountedinside the PC that provides power to the mother-board and various peripherals. Its easily identified bya warning sticker on the case that reads CAUTION!Hazardous Area (or a similar high-voltage warning).

On the back of the power supply is an AC con-nector that plugs the PC into the wall. Often theresanother AC connector thats used by some monitors.Most power supplies also have a voltage selectorswitch that lets it work with 110V or 220V powersources.

A typical PC power supply provides four DC out-put voltages: +5, +12, -5, and -12 volts. These volt-ages are available through four different types of con-nectors (Figure 1; 1-4). The color of the wire identi-

fies the voltage and its use (Table 1).

Getting Started

Alot of power supply failures are actually simpleproblems that are easy to fix. Obviously, theplace to start is at the beginning -- in other words, areyou getting power from the wall to the PC? As stupidas it sounds, the f irst thing to do is look under yourdesk and see if the PC is plugged into the wall. If it is,move the plug to a different socket (they go bad, too,you know).

That done, pull thepower cord from theback of your PC andsee if the power is get-ting that far. You cando this using a VOM ora simple neon lampcircuit tester, like partnumber 22-102 fromRadio Shack.

If theres no power,and youre pluggedinto a power strip orsurge protector, thestrip is probably theculprit. To test it, sim-ply remove the PCs

plug from the strip and

plug it into a wall socket. If the PC starts working

problem is in the strip. Generally, the problem is blown fuse or a tripped circuit breaker. Youll findboth at the cord end of the strip. The last item yoshould test before popping the hood is the powercord itself; replacing it with another cord is the faand safest method.

Under The Hood

Still nothing? Now its time to remove the coverMost covers are attached by five or six screwsthe back. Before going any further, carefully readinstructions in the section called Safety First.

The next logical place to look is at the poweswitch. Unfortunately, this may not be possible atstage of the game. Many power supplies have a bin power switch which isnt accessible until you d

Reprinted from September 1996 Nuts & Volts Magazine. All rights reserved. No duplication permitted without permission from T & L Publications, Inc.

Repairing a broken PC power supply is a lotsimpler than you might think. Nine times outof ten you can do it yourself for under $10.00.

L JPC Power Supply Repair

: :: : by TJ Bye

Figure 2. A dummy load can be made from acouple lamps that you can buy at any autoparts store and an extension cable from RadioShack. Figure 3. The low-voltage supply provides four output voltages.

The 5.25-inch drive connector is the easiest to access for testing. The motherboard connectors P8 and P9 are identical, and can be reversed. They pluginto the motherboard with the black leads together on the inside.

Figure 1.


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mantle the unit. If you have a tower computer case,though, the switch is located on the front panel, andconnected to the power supply via four wires. All youhave to do is unplug the wires from the switch -- withthe computer unplugged from the wall, of course --and test the switch with an ohmmeter. If you want todo a hot test of the switch (that is, bypass theswitch), you can short the power wires together usingtwo insulated jumper wires and plug the computerback into the wall. Just be careful that the jumpersdont touch anything.

Lets now look at the DC voltages. (If youremoved the AC wires from the front-panel powerswitch, replace them first.) With the main switch off,locate a free power connector (the 5-1/4 inch version,Figure 1d, is preferred) or unplug a f loppy drive tofree up one. Dont unplug the hard disk; youll need itfor the entire duration of this test. Power up the PC,and measure the +5-volt (red) and +12-volt (yellow)lines using a VOM (black is ground). Make sure theyfall within the voltage range specified in Table 2.

If they are out of range, power off the systemand disconnect the mechanical drives one at a time,

beginning with thefloppies. Measure the+5- and +12-volt linesat each step. This willtell you whether or notthe power problem isspecific to a device.Dont forget to poweroff the system eachtime you disconnect a

device. With the hard disk(s)still connected, remove plugs

P8 and P9 (Figure 1) from themotherboard.Finally, its time to deal

with the unlikely possibility ofa shorted hard disk. If youhave more than one harddisk, start shedding them oneat a time. When youre downto your last hard disk, unplugit and connect its power plugto the dummy load shown inFigure 2. (I dont recommendrunning a PC power supplywithout a load.) If the powersupply is still dead, its off tothe drawing board.

The Drawing Board

Now that weve done all that we can do with thepower supply inside the cabinet, its time toremove the unit and place it on the workbench. Sinceweve already disconnected all the power connectors,its a simple matter of removing the mounting screwsand sliding the power supply out of the cabinet,right? Well, hopefully.

Unless you have a tall tower, youll probably runinto obstacles, like adapter boards, disk drive signalcables, and support brackets. If yourelucky enough to have a detailed usersmanual, it shows you the procedure.Otherwise youre on your own. In eithercase, make notes of where everything is,how theyre connected, and keep thescrews with the items they came from.

WARNING: MAKE SURE THE PC IS

DISCONNECTED FROM THE WALL

BEFORE STARTING DISASSEMBLY!

If the supply was powered from the AC line win the last few minutes, the large electrolytic capators in the high-voltage section will most likely stihave a charge in them that could give you a shocing awakening. If so, let the power supply rest forwhile before you crack the case.

Each case has its own method of constructiobut generally two sides of the enclosure are what tect the inside electronics. Remove the cover scretaking care to watch out for attached leads, switcand sharp edges. If you have to disconnect any le(typically fan wires) or mechanical parts, note ca

fully how they go back together.Give the electronics a good looking over, payattention to any scorched or burned parts that mpoint to a failure. If you have a built-in power switnow is the time to check it. Next, check the fuse.blown? If in doubt, use the VOM to test for contin(use the X100 range). If the fuse is blown, replacwith one of the same type and rating before goinany further. Its possible the trouble is the result ometal fatigue or mechanical failure of the fuse itsTo see if this solved the problem, connect thedummy load to one of the drive connectors andapply power.

If nothing happens, remove the dummy loaand proceed to the resistance checks procedure.the fuse blows with an explosion, go to the high-vage repair section.

Resistance Checks

Referring to Table 2, perform a resistance measurement test. Keep the VOMs polarity correthat is red to ground when testing a negative souand wait for the filter capacitors to charge beforeing a reading. The resistance values listed in Tablare only representative (the figures were gatheredfrom actual measurements of several power suppusing a cheap VOM), so dont worry if your valueare different from those listed.

However, if a resistance value is abnormallhigh or low, you have a problem. As a rule of thua reading of 50 ohms or higher on the 5-volt andvolt lines means the output is probably okay. A rtance value of 40 ohms or less indicates a short, erally in the rectifier diodes. The five-volt line is thmost prone to failure because it carries the heaviload (typically 20 amps). An extraordinarily highresistance reading indicates an open, probably azapped board trace or a burned resistor. Both cotions are often harbingers of problems in the highvoltage section, but not necessarily. It depends ohow fast the shutdown circuit reacted. But before face that possibility, we first need to find the extethe low-voltage damage.

Low-Voltage Repair

The low-voltage section of the power supply is very simple rectifier, L-section filter design (F3). Key to the success of this design is a multipleondary power transformer. There is a 5-volt windand a 12-volt winding. In high-power supplies (25watts and larger), there are usually two five-volt wings that are paralleled for higher output current yet treated as a single winding.


Table 1. Power Supply Color CodesWire Color Voltage UseRed +5V Motherboard, adapter cards, disk drivesWhite -5V Logic circuits (rarely used in modern PCs)Yellow +12V Disk drive motors, RS-232 serial port, fans,

adapter cardsBlue -12V RS-232 serial port, fansOrange n/a Power OK signalBlack 0V Ground (GND)

L:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:JL:

Figure 5. The switcher section is themost common to fail. The power transis-tors have to have a breakdown voltageof 600 volts or more, and the damperdiodes have to be fast recovery (a1N4005 wont work).

Safety First!Would you put a hairpin in an AC outlet socket? N

hardly! So why would you consider putting your fingea power supply that is clearly labeled CAUTION!? Alwunplug your PC before going under the hood. Once thepay attention to my WARNING! signs. Ive done my beto make the troubleshooting processing as shock free possible, but power has to be provided at various stagof the game. Be alert, dont be stupid, and if you dontknow what to do next, stop now!

Figure 4.

You can gen-erally identi-fy the semi-conductorsby theirshapes.From left toright, thefirst threeare diodes,+12V rectifi-er, +5V recti-fier, andswitchingtransistor.


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Each winding has a grounded center tap to per-mit fullwave rectification using just two diodes (full-wave bridge rectifiers need four diodes). The direc-tion of the rectifiers determines the polarity of theoutput voltage. Common cathodes are positive, and

common anodesare negative.Because of its high-current require-ments, the +5-voltrectifier is usually anarray of parallelSchottky diodes in asingle package(Figure 4) thatmounts on a heatsink. The -5-volt out-put is often derivedfrom the-12-volt rectifier viaan IC regulator (typ-ically an LM7905

equivalent) rather than from the five-volttransformer winding. However, Ive seen itdone both ways.

The output of the rectifiers is filtered first byan inductor, called a choke, then by a heavy-duty electrolytic capacitor. In some designs,the five-volt line is double-filtered to reduceripple by cascading two L-section filters on theoutput. Invariably, a bleeder resistor is placed

across the output to discharge the capacitors afterpower off.

The most common cause of low-voltage failure isa shorted rectifier. If one blows, so does its compan-ion, which forces you to replace them as a packagedeal. Second on the hit list is a shorted capacitor,which usually does less overall damage. Most of thetime, the failure is limited to just one output line, buttheres no guarantee.

The first step is to locate the shorted compo-nents. For this operation you need access to the bot-tom side of the printed circuit board. This is the hardpart, because no two supplies are alike. Use yourimagination, and be care-ful not to damage othercomponents in theprocess. For example,twisting and turning theboard too many timescan cause attached wiresto break loose.

Now comes the tricky

part, because you have to first locate the affectedparts on the circuit board. Use the road map, HTo Find Waldo, to help you in your quest. An ohmeter is a good way to probe suspected areas foshorted devices. Once the area is located, the reawork begins because its virtually impossible to tthe difference between a shorted diode and a shocapacitor without removing one or the other. Sinthe rectifier is the most likely culprit and the easiremove (the electrolytics are glued in place), Id there.

The +5-volt and +12-volt diodes are most like

nestled inside a transistor case mounted on a hesink. The bigger one (Figure 4e) is the +5-volt reer, and the smaller one (Figure 4d) is the +12-vorectifier. The negative-voltage rectifiers are individdiodes typically in a DO-41 case.

With the suspect rectifier or diodes in hand, resistance check of the defective voltage output again. If the reading is within the normal range, tthe old part or parts and replace with new. (HelpHint: If the new diodes come in an axial-lead pac

age, typically DO-41, solder them on the trace sidthe circuit board instead of the component side. lot easier.) If the output still shows a short, yank electrolytic and check again. If the output is stillshorted, make sure youre pulling the right teeth.

Exact replacement parts always cost more thgenerics, so go with the generic. You can get unsal replacements from GE, RCA, and Philips ECUnfortunately, theyre almost as expensive as theoriginal. For the +5-volt rectifier, I recommend theMBR series from General Instruments and Motoro(available from Digi-Key and Allied Electronics,respectively). The +12-volt rectifier is a dual Schodevice thats available from several vendors, and erally sells for a buck or two. The negative voltagrectifiers must be fast recovery diodes, like a1N4933. Replacement electrolytic capacitors are close as your local Radio Shack.

When the voltage line has a three-terminal ICvoltage regulator, check the resistance between bthe input and the output (Figure 4) to ground. If the output pin is shorted, the output capacitor is


Table 2. Output Voltage and Resistance

Nominal Voltage Voltage Range Resistance Wire Co+5V +4.75V to +5.25V >100 ohms Red-5V -4.75V to -5.25V >100 ohms White+12V +9V to +15V >250 ohms Yellow-12V -9V to -15V >250 ohms Bluen/a 0V or +5V ~1000 ohms Orange0V 0V 0 ohms Black


Figure 6. The high-voltage supply is a simple voltagedoubler circuit.

Figure 7. A cheap VOM is the best way to check tran-sistors and diodes. Why? Because the test voltagehas to be enough to breach the barrier voltage of asilicon diode, typically 0.7 volts, and a lot of DVMshave a probe voltage of 0.3 volts and less.

HowToFindWaldo

SOURCES

Allied Electronics800-433-5700

Digi-Key800-344-4539

Marshall Electronics800-877-9839

Newark Electronics800-344-4539Radio Shack

800-843-7422Wyle Laboratories

ElectronicMarketing Group

800-672-3475


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If only the input pin is shorted, the rectifiers are bad.If both are shorted, the chance is both the diodes andthe IC are shorted. To verify this theory, remove theIC and check the resistance again. If it reads okay,replace the semiconductors. The re-placement for the-5-volt IC voltage regulator is an LM7905.

High-Voltage Repair

If the new fuse blows when you apply power, theresa problem in the high-voltage section. We know this

because the low-voltage section has an automaticshutdown circuit that reacts a lot faster than the fuse;that is, a low-voltage problem disables the power sup-ply long before the fuse has time to blow. That does-nt necessarily mean the low-voltage outputs areokay, because failure of the -12-volt line can causecascading damage that goes all the way back to thehigh-voltage section.

The high-voltage section is divided into twoparts: the high-voltage power supply and the switch-ing circuit. Most high-voltage failures occur in theswitching circuit.

WARNING: COMPLETELY DISCHARGE

THE INPUT CAPACITORS BEFORE WORKING

IN THE HIGH-VOLTAGE SECTION!

If the fuse has a mirrored look to it, you canbet the farm that at least one of the two switchingtransistors is shorted (Figure 5). Typically they perishas a couple. These transistors are mounted on theheat sink(s) closest to the two largest electrolyticcapacitors (see How To Find Waldo). With the redprobe of a VOM on the collector of the first transistor,

check the collector-to-emitter resistance, then tcollector-to-base resistance (Figure 4). If a shorfound, replace both the transistor and the damdiode thats across its emitter-collector. I normause a Motorola MJE13009 for the power transisand a 1N4937 for the damper diode.

You should also replace the low-value resistothats in series with the transistors base. This rtor is often used as a fusible link that goes puffwhen the switcher fails. Its purpose is to protecother components in the chain from harm. If thresistor is burned beyond recognition, you can

replace it with any 1/4-watt resistor with a valu1 to 10 ohms (the exact value isnt important).Sometimes, though, even the fusible isnt fastenough to prevent damage. So before installingnew parts, its wise to check out the pulse shapnetwork (typically a resistor-diode-capacitor conation) associated with the base circuit, too. Aquick way to test all three components at onceto treat the network like a single diode, checkinas a whole for shorts and opens (Figure 7). Norepeat the procedure for the second switching sistor.

The high-voltage supply is a simple voltagedoubler with an output of about 300 volts (Figu6). While this section rarely fails on its own, ashorted switching transistor can wipe out thebridge rectifier in an instant. Check the AC inp

for shorts, and replace the entire bridge if a shofound. Bridges can be either discrete diodes or large, rectangular module, and you can find suable replacements from Radio Shack. Theres pably a one-ohm resister in line with the AC inputhat needs to be checked, too. On the outsidechance that one of the doubler capacitors is shed, do a resistance check of each.

When powered from a 220-volt AC power southe capacitors serve as voltage dividers to provan artif icial ground. Consequently, the capacitaand ESR (equivalent series resistance) values othe capacitors are critical when operating from220-volt line and have to be evenly matched, owise the switching voltages will be uneven. As etrolytics age, both the capacitance and ESRchanges. If the mismatch is too great, one voltacould exceed the limits of the switching transiswhich can start parts a-poppin. You can checkvoltage balance with a VOM. Always replace bocapacitors, not just one, and use a good gradecapacitor, like the Panasonic TSU series.

Its Showtime

If youve made it this far, you probably have a wing power supply. But before you apply power, lmake sure weve covered everything.

-- You did a final resistance check on the outvoltage lines, and all are within the specificationsTable 2, right?

-- You checked the resistance across the ACinput (with the power switch on) and it measuresmegohm or better, check?

-- You checked the fuse.-- Any broken wires or burned parts?

Good! Then its showtime. Re-assemble thepower supply. Plug the dummy load into one of tdisk drive connectors. Apply power.

If both lights light, congratulations! Youve goyourself a working power supply, because the powsupply itself needs the -5- and -12-volt lines to opate. Consequently, you dont have to test them,unless youre as curious as I would be. Now all yohave to do is put everything back together and ena more peaceful day - except for the coffee. Hesuggest ... NV



Nine times out of ten, the troubleshooting techniques presented in this article will solve your PC power supplyproblems. But what if the power supply still doesnt work? There can be lots of reasons, ranging from afaulty transformer (good luck finding a replacement) to a bad solder connection. In most situations, Id cutmy losses and find a substitute power supply or try to salvage the motherboard for use in another system.

But if youre really dead set on reviving the system, there is one more stage we havent discussed - the PWM (pulse-width modulator). But put your seat belt on, cause this is gonna be short and fast. Its not for everybody.

The PWM (Figure 8) is what drives the switching transistors, and when it doesnt work, nothing works. Considerit the brains of the power supply. The PWM is generally a single IC chip, most likely a Motorola TL 494. But beforeyou replace the chip, lets see if its working or not. For this youll need an oscilloscope and a power supply.

The simplest way to test the PWM chip is to grab a disk drive connector and pump +12 volts into its yellow wirefrom an independent power supply. This can be done using another PC power supply or any other DC source (batter-ies work, too). Once power is applied to the PWM chip, observe the output waveforms on pins 8 and 11. Both out-puts should be active squarewaves. If at first you dont succeed, ground pin 4 and try again. If the scope still showsnothing, replace the LT 494 chip. If the scope shows waveforms, the most likely culprit is the LM339 comparator.The LM339 is cheap, about a buck, and readily available, so its worth a shot.

My method of replacing an IC is to clip the leads as close as possible to the body of the IC, leaving 14 or sometal pegs standing upright from the main board. Paying attention to direction, slip the replacement IC alongsidethe pegs and solder the new component in place.

If by now the power supply still doesnt work - chuck it.

Still Dont Work, Huh?

Figure 8.

MANUALS

First Principles of PC Power SuppliesGrafnet Technology

11120 Tattersall Tr., Oakton, VA 22124Price: $19.95

PC Power Supply Troubleshooting GuideJim

P.O. Box 5123, Tucson, AZ 85703Price: $40.00

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