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Solving Capacitor Troubles With Your Z METER™

With all the ICs being used today, it mayseem that fewer discrete componentswould be used. It might surprise you tolearn that more capacitors are used intoday’s modern solid-state chassis thanwere used in the old tube sets. SencoreEngineers counted the capacitors and coilsused in three comparable color televisions,a newer solid-state set, a tube set, and anRCA CTC131. The tube set had 219 capa-citors; the new solid-state set had 263capacitors, and the RCA CTC131 had 665capacitors!

The Electronic Industr ies Associat ionreported, in their 1987 Electronic MarketData Book, that factory sales of capacitors

increased from $438 million in 1972 to$1.217 billion in 1986. Also, reported wasa 23 .5% inc rease in the number o fcapacitors being used between 1985 and1986. Natura l l y capac i to rs , and theproblems associated with them, are here tostay.

Capacitors fail in-circuit, and they also failjust lying in the parts bin. The failure canbe traced to one of four things; valuechange, excessive leakage, excessived ie lec t r i c absorp t ion (D/A) , o r h ighEffective Series Resistance (ESR). AZ Meter is the only capacitor analyzer on Fig. 2: A television with ripple in thethe market that lets you make all four of picture, such as the set shown here,these important tests. In this Tech Tip, usually has a problem with one of its power

supplies. Is the problem a loaded downsupply, a bad rectifier diode, or a bad filtercap?

we’re going to take a look at some actualcase histories of capacitor related problems.Soon you’ll see why you need a Z Meter.

Capacitors Change Value

How often have you seen a set that has the“bends” or 60 Hz r ipple, l ike the setshown in Figure 2? What is causing theproblem? A bad recti f ier, a bad f i l terchoke, a bad filter capacitor, a circuitloading a power supply, or a combinationof several things could be the cause.

In the set shown here, we found that a 900µF, 350 VDC filter capacitor had decreasedin value; it only measured 500 µF.

Only about 25 % of the problems associatedwith capacitors are value related. Three outo f four fa i lu res a re caused by o therproblems. A “value only” capacitance

fig. 1: The RCA CTC131 chassis contains 665 capacitors compared to the 1981 CTC108 meter isn’t enough to find most of thechassis with only 263 capacitors. capacitor problems in today’s circuits.

Dielectric Absorption - The HiddenFailure

One common problem associated withcapacitors is dielectric absorption (D/A).D/A is one of those “mystery” problems.You won’t find it with a value only tester,Dielectric absorption can cause very realc i rcu i t p rob lems. Le t ’s cons ider theexperience of one of our customers to seethe effect of D/A.

A customer was convinced that the Z Meterwhich he had recently purchased was notfinding bad capacitors. The capacitorswere 22 µF and were located in the verticalstage of a television. The customer knewthat the capacitors were bad becauser e p l a c i n g t h e m c u r e d t h e l i n e a r i t yproblems every t ime. The tests theyperformed with the Z Meter turned upnothing.

Measuring one of these capacitors on the ZMeter showed a value of 24.5 µF and lessthan the maximum allowable leakage. Itsure appeared to be a good capacitor,However, by pressing the value button asecond time after measuring the leakage,t h e c u s t o m e r f o u n d t h a t t h e v a l u emeasured 13% lower than it did before hemeasured leakage. As he continued to holdthe value button, the reading steadi lyincreased to its first value. The Z Meterwas showing the capacitor as bad, havinga 13% dielectric absorption value change.

Any capacitor that has more than 5% D/Ashould be questioned, especial ly in acritical waveshaping circuit like the verticalstage of a TV.

Dielectric absorption is the result of acapacitor remembering a charge that wasapplied to i t . The results of this D/A“memory” vary, depending on the circuitthe capacitor is in. In the previous examplethe capacitor was not allowing the verticalramp to be properly shaped. As the voltageon the capacitor changed, the memorye f fec t o f D /A opposed the change,resulting in a non-linear picture.

Some circuits (like this vertical stage) willbe severely affected by D/A. In othercases, a small amount of D/A tells you thatthe capacitor has aged to the point where itshould be replaced, even if it isn’t causinga circuit problem yet because it is near theend of its life.

Capacitor Leakage Upsets BiasVoltages

Capacitors are supposed to block DC while

Fig. 3: This schematic shows the vertical output stages of an RCA CTC97 chassis. Evenafter the defective transistors and bias resistors were replaced, the 180 volt power waslow. The problem was traced to a leaky filter cap, C3071.

passing an AC signal. When a capacitordevelops leakage, it allows DC current topass through in large amounts. Thiscauses bias voltages to become incorrectand power supplies to load down. That isw h a t h a p p e n e d w i t h o n e p a r t i c u l a rchassis.

The set came in for repair having only asingle horizontal line across the screen.Several quick checks isolated the problemto the vertical output stage. A little closeranalysis of the output stage showed thatmost of the transistors were biased intocutoff. When the technician checked thetransistors, he found that most of themwere bad.

Further checking showed that the biasresistors (See Fig. 3), R3116 and R3117,had greatly increased in value. When theywere replaced, a full raster returned, yet acontinuous f l i cker ing ind ica ted tha tsomething was still wrong. Measuring thevoltage at Ml showed that the 180 voltsupply was low, even after all thetransistors and related bias componentswere confirmed good.

One possible culprit remained - C3071.The capacitor’s value was OK, but theZ Meter leakage test showed high leakage.The capacitor had so much leakage that itwas acting like a resistor and loading downthe 180 volt power supply.

While a leaky capacitor acts like a resistor,you cannot f ind a leaky cap with anohmmeter or even a voltmeter. Leakage in acapacitor is not linear; it depends on thevoltage applied to it. The only way todetermine if a capacitor has leakage is tomeasure the current through it with itsrated voltage applied (Figure 4).

Maximum Allowable Leakage(in microamps) for

Standard Aluminum Electrolytic Capacitors

Fig. 4: The acceptable leakage for acapacitor depends on its value and voltagerating. In order to be sure that a capacitoris good, you must measure leakage withthe cap’s rated voltage applied.

Fig. 5: Schematic of a speaker crossover network. In this circuit, Cl’s ESR had limited thesignal delivered to the Tweeter.

ESR Is More Of A Problem InToday’s High Frequency Circuits.

ESR is the Effective Series Resistance of acapacitor. It is a standard characteristic,expressed in ohms, that represents all ofthe energy losses in the capacitor, despitethe source. Capacitor ESR is a combinationof lead resistance, foil/capacitor plateresistance, resistance of the lead welds,and the energy lost in the dielectric itself.

ESR robs power from the circuit. Chargingc u r r e n t f l o w i n g i n t o a n d o u t o f t h ecapacitor must f low through the ESR.

Energy is lost in the form of heat. In powersupplies operating at 60 Hz, heat loss isfair ly low. However, in many moderncircuits, such as switching mode powersupplies, the operating frequency is in thekilohertz range. This causes the current toflow at a higher rate, and the heating effectincreases.

ESR decreases the ability of a capacitor tofilter correctly. In effect, you are placing aresistor in series with the capacitor. ESRalso affects the time constant in criticaltiming circuits. Let’s look at an example ofthe failures caused by high ESR.

Example: In one defective speaker system,t h e r e w a s n o o u t p u t f r o m t h e h i g hfrequency tweeter. It was suspected thatth i s speaker had been damaged byexcessive power. The tweeter was checkedand, as suspected, found to be defective.When the new tweeter was installed, theo u t p u t w a s l o w a n d d i s t o r t e d . N e wsuspect? The crossover network. It wasthe only other component that could causea loss of high frequencies in this unit.Figure 5 shows the schematic of thecrossover network. Notice that the onlycomponents that could affect the output tothe tweeter are L1, L2, Cl, and R1. Thehigh frequency level control, R1, waschecked; it tested good.

Capacitor Cl was removed from circuit andchecked. The value read well within thetolerance range for this capacitor. Leakageand dielectr ic absorption were withinacceptable l imits. ESR was tested, i tmeasured 176 ohms. The Z Meter pullchart showed that this capacitor wasallowed 33 ohms of ESR (maximum). Sincethis capacitor was in series with the levelcontrol, it dropped most of the signal anddecreased the speaker output.

Check Replacement Caps BeforeYou Put Them In Circuit

Don’t count on replacement capacitorsbeing good; capacitors change value,become leaky, increase in ESR, anddevelop dielectric absorption just settingon the shelf (Figure 6). You should always

Fig. 6: In this Sylvania set, C32 shorted causing the set to stay locked onto one channel. The replacement capacitor was also defective,resulting in the set randomly tuning channels.

check a replacement capacitor beforeinstalling it.

Value Change Is Common InElectrolytics

Electrolyt ic capacitors have a moist,“paste-like” dielectric. The capacitor’sbody is sealed to prevent air from dryingout the d ie lec t r i c . Never the less , thedielectric eventually dries out and thecapacitor’s va lue drops dras t i ca l l y .Ceramic capacitors are also subject todrastic ' ‘on shelf’ ' value changes. Insteadof the dielectric drying out, the plates inceramic capacitors become cracked frommechanical stress. An example of thisfailure happened to a technician who wasworking on a Sylvania set (Figure 6).

The set came into the shop several daysafter an electrical storm hit the area. Thecustomer complained that the set wasstuck on a part icular channel and hecouldn’t choose any other. Being familiarwith this symptom, the service technicianquickly found the problem: C32 on thememory module was shorted.

He found a replacement capacitor in hisparts bin and replaced the defect ivecapacitor. It didn’t take too long for theshop to get a call from the customer. No,the se t wasn ’ t s tuck on a par t i cu la rchanne l ; the se t changed channe lsrandomly.

After studying the output l ines of theencoder and the keyboard, the techniciandecided that the encoder was causing theset to switch to a different channel - notunlikely since the memory/encoder chipwas probably damaged by the electricalstorm. Of course, he didn’t have the IC, sohe had to order one. After a week it finallyarrived. The technician carefully installedthe new IC, only to find that the set stillrandomly switched channels. He knew itwasn ’ t the ch ip , so maybe it wassomething on the chip’s input. One by onehe removed and checked the capacitors onthe input lines. They all checked OK,except C32, it was shorted.

Watch Out For “On-Shelf” Leakage

As electrolytic capacitors age on the shelf,they often develop leakage. Placing a leakycapacitor into a circuit can have disastrousresults, as this technician discovered:

The technician worked for a company thatmanufactured photographic equipment. Heexplained that they were using a 1240 µF,

360 VDC capacitor in the power supply of acamera strobe unit. At the time ofproduction, the company ordered an extra3,000 capacitors for their service centersto use as replacements. Most of thereplacement capacitors sat on thewarehouse shelves for 3 years before theywere needed. Then, one by one they wereused in service units.

shows the importance of doing all fourcapacitor checks, especially on off-the-shelf replacements:

A TV came into the shop with a verticalhe igh t p rob lem. The p ic tu re wasn ’ tcompletely collapsed, but it had very poorlinearity. Suspecting bad capacitors, thetechnician checked every capacitor in the

Fig. 7: The capacitors inside a power supply for a flash strobe must quickly charge torelatively high voltages. If one of these capacitors has excessive leakage, it quickly buildsup internal pressure from overheating and may explode.

The technician was in for a surprise whenhe turned on the power to one of theserv iced un i ts . Severa l rep lacementcapac i to rs had deve loped excess iveleakage, while sitting on the shelf. Whenpower was applied, heat and pressurecaused the capacitors to explode, spewingoxide paste throughout the power supply.

You can prevent this from happening inyour shop by checking every capacitor youtake off the shelf. If the leakage is below(or quickly fal ls below) i ts acceptableamount, you can put the capacitor intocircuit without worry. If the leakage isabove the maximum allowed, try reformingthe capacitor’s dielectric. The dielectriccan reform when a voltage, with a limitedcurrent, is applied to the capacitor ’sterminals for a period of time. Your Z Meterprovides the necessary leakage voltageand current limiting to allow you to reformelectrolytic capacitors.

Capacitors Develop O/A On TheShelf, Too

Dielectric absorption can also developwhile capacitors sit on the shelf. Here’s anexperience a servicer told us about that

vert ical feedback loop with his “valueonly” tester. When each checked good, hedecided to recheck them with his Z Meter.Each capacitor showed good value, goodleakage, and good ESR. One of the 50 µFcapacitors had 50% dielectric absorption.

Qu i te cer ta in tha t he had found theproblem, t h e s e r v i c e r g r a b b e d areplacement 50 µF capacitor out of theparts bin. Just to be sure, he checked it onhis Z Meter. It read the correct value, hadlow leakage, and low ESR. It too measuredabout 50% dielectric absorption! Another50 µF capacitor in the parts bin checkedgood on all four tests. When this capacitorwas put into the circuit, the set workedfine.

Z-METER is a trademark of Sencore, Inc. Form 4008 Printed in U. S. A.