Repairing SMD Audio Circuits

Surface mounted devices (SMD) are found in many different consumer electronic products such as the TV, portable cassette player, compact disc, VCR, and camcorder. SMD parts are very miniature in size and are difficult to identify. Although, the SMD or SMT components are very tiny, they can be tested in the same manner as the standard or larger parts.

Because many surface-mounted devices have similar shapes and sizes, sometimes they are quite difficult to see upon the chassis. The SMD part might appear as a black, brown or silver part on the pc wiring. These surface-mounted components are so small as they might be mounted between a regular IC terminals upon the PC wiring side. SMD components are mounted directly on the pc wiring, usually on the bottom side, and standard parts are found on the top side of the PCB.

Commercial SMD resistors might appear as round, flat and leadless devices. The fixed resistor might appear black or silver with several numbers stamped on top, indicating the correct resistance. The round SMD resistor might have color-coded rings around the component for correct resistance.

FIG. 1. SMD components might be found between standard IC’s within the RV chassis.

The SMD ceramic capacitor might be flat with outside terminal connections and tinned ends. The SMD part terminals are found at each end, except transistors and IC chips; either end can be mounted between the pc wiring pads without any difficulty.

The commercial surface-mounted transistor might appear in a chip form with flat or gullwing contacts at one side, top and bottom, or on both sides. You might find more than one transistor inside one chip.

The same applies to fixed diodes and LED SMD components. Two or more diodes might be found in one component or chip. Each SMD part can be tested like standard components. They are easily checked and tested when wired or soldered to the pc wiring.

SMD CONSTRUCTION

The SMD parts are now available for electronic project construction. They are marked and mounted, somewhat like the commercial SMD component. These SMD parts are very small in size and must be handled with care. Since these parts are so tiny, they can easily be flipped out of sight. For surfaced-mounted resistors and capacitors, select the largest physically, highest wattage, and working voltage; they are much easier to work with.

Choose SMD electrolytic capacitors with at least a 16 volt rating for small 9 volt electronic projects. Select thick film resistors with a 1/8th watt size. Select a ceramic capacitor chip with a 50 volt working voltage. The ceramic electrolytic capacitor chip can be selected with a 5, 10, 16 or 25 volt rating. A radial or stand up electrolytic might have a higher voltage rating. These surface-mounted parts are soldered directly to the pc wiring. They are ideal for building small electronic projects.

The most common SMD parts available for electronic construction are capacitors, resistors, transistors, ICs, LEDs, diodes, and inductors. The SMD ceramic capacitor might be available in three or four different case sizes, 1210, 1206, 0805, and 0603. The 1210 and FIG. 2. Surface-mounted components are now available for project construction.

1206 case sizes are physically the largest chips to work with. For instance, the SMD 1210 case is 3.05 in. long, 2.54 in wide, and 1.27 mm thick, while the 1206 is the same length (3.05 mm), 1.5 mm wide and 1.27 mm thick. The smaller the capacity in picofarads (pF), the smaller the case size of the capacitor.

Most ceramic chip capacitors are available with a 50 volt working voltage. The capacity can range from 0.5 pF to 0.068 uF. These ceramic chip capacitors are available from some mail order firms in single (1), 10, 100, 500, or 1000 lot prices. It’s best to purchase SMD parts in a 10 lot price. The ceramic chip capacitors are used in bypass and coupling electronic circuits.

Remember the ceramic chip capacitor is a non-polarized capacitor. You can solder any end into the pc wiring without any problems. The ceramic capacitor might have a letter and number stamped on the top, indicating the actual capacitor value; while in other chips there are no markings or values with only end connections. Always keep these small SMD components inside the marked plastic envelopes, so they won’t get lost or mixed up ( FIG. 3)

FIG. 3. SMD parts are found on strips placed inside plastic envelopes.

The SMD aluminum electrolytic capacitors are polarized and should have at least a 16, 25, 35 or 50 volt working voltage. Don’t use an SMD electrolytic capacitor of 10 volts or less working voltage for a 9 volt circuit, as they have a tendency to break down.

Often the voltage and capacity are stamped on top of the electrolytic capacitor. The top black marked area indicates common ground terminal. Observe the correct polarity of electrolytic capacitors or if installed backwards, they can run warm, and blow up in your face. They are available from 0.15 to 1000 microfarads (uF). The aluminum SMD electrolytic capacitor stands up while the solid chip lays down on the pc wiring.

Tantalum electrolytic chip capacitors are found in a smaller capacity and can be purchased in the 16, 20, 25, and 35 working voltage types. The black or white polarity bar on the top side and at one end is the positive terminal. Most standard electrolytic capacitors have a black line that indicates a negative or ground terminal. These small SMD chip electrolytic capacitors have the reverse, a positive (+) polarity with a black or white bar at one end. Place the bar at the positive voltage connection with a SMD electrolytic chip capacitor.

The Tantalum electrolytics are available from 0.47 to 47 microfarads (uF). The SMD aluminum electrolytic capacitor is used in B+, decoupling and power supply circuits, while the lower capacity Tantalum capacitors might be found in coupling and bypass circuits.

The SMD fixed resistor might be identified by numbers stamped on the top side of the chip. These SMD resistors appear in thick film chips of 0805, 1206, 121 0 and 2512 case styles. Choose the 0805, 1206 and 1210 case styles for electronic construction. The 0805 resistor is 1/10th watt, 1206 is 1/8th watt, and 1210 is 1/4 watt. The 2512 case is a 1 watt SMD resistor. These fixed resistors appear in 0, 10 to 1.0 megohms. The zero (0) resistor might be used as a feed through or to tie two circuits together.

These SMD resistors can be purchased in 1, 10, or 100 lot prices. It’s best to choose SMD resistors in a 10 lot price of each value.

Remember, either end of the resistor can be soldered into the circuit with the resistance value at the top. For instance, the SMD resistor might have 122 stamped on top, where the first two numbers equal the amount with the last number rated in zeros ( FIG. 4). The number 1 and 2 equal 12 with two zeros at the end equals a 1200 ohm resistor.

FIG. 4. The SMD capacitor might have a letter and numbers, while the SMD resistors have numbers for the correct value on top of the component.

The surface-mounted transistor (SMT) might appear as a chip with flat contacts at one end, top and bottom, or both sides. You might find more than one transistor in one chip. The standard or conventional SMD transistor has a SOT-23 package outline, while the one watt power transistor has a SOT-89 outline with a heat sink. The SOT-89 and SOT-223 might consist of two transistors in one chip or in a Darlington arrangement.

The conventional transistor SOT-23 is a general purpose transistor that you will find in electronic projects; such as the Digi-Key FMMT39O4CT-ND (2N3904) NPN or the FMMT39O6CT-ND (2N3906) PNP type transistor. The Mouser listing for the same type of transistor has a part number of MMBT39O4 (2N3904) NPN or the MMBT39O6 (2N3906) PNP type. The conventional 2N2222 general switching transistor in the same SMT types is listed as MMST2222 at Mouser and FMMT2222ACT-ND at Digi-Key Corporation. The Digi-Key corporation and Mouser Electronics are electronic mail order firms.

The SOT-23 general purpose transistor has the collector terminal on one side (at the top) with the base to the left and emitter terminal to the right on the bottom side of the package outline. Some have flat or gullwing type terminals. These transistors can be tested with the transistor tester or on the diode-test of the digital-multimeter (DMM).

FIG. 5. The standard transistor with SMD size transistors below.

The SOT-23 general purpose FMMT39O4CT has a 1A stamped on top, the FMMT39O6CT can be identified with a 2A on top, and the CMPT2222A switching transistor has a C1 printed on top (body) of chip.

The SMD IC chip is constructed like the standard IC with gullwing terminals. These sensitive-static devices appear in dark static-sensitive bags.

Terminal 1 is identified by a “U” or indentation circle on top and is located on the bottom left hand corner of the chip. The linear LM386 low power amp IC is a SMT (LM386-1 -ND) and has an SO-B outline. The numbers 386 are stamped on top with an indented circle at terminal 1 ( FIG. 6).

FIG. 6. A standard LM386 IC compared with SMD gull wing audio output IC’s.

These IC devices are made of ceramic or plastic moldings. IC chip devices are not heat- proof or shockproof; they should not be subject to a hot soldering iron or magnetic components. Don’t apply unnecessary stress to the chip. Handle SMD semiconductors with extreme care. Keep the SMD IC chips inside the plastic container until ready to be mounted. Install the chip on the printed circuit board. Use a low wattage or battery iron to solder the small terminals.

The leadless, fixed, Schottky barrier, zener, and variable capacitance diodes appear in SOT-23 packages. These diodes might look like a three-legged transistor. The SMD diode can be identified by two alphabet letters or numbers. You might find one or two fixed diodes in one chip ( FIG. 7). Only two of the terminals are used with one fixed diode and all three terminals are used for two diodes in one SOT-23 chip. These diodes can be purchased in one (1), 10, and 100 lot pricing. SMD components can be obtained for construction projects from several mail order firms.

FIG. 7. The SMD transistor and diode connections inside the SOT- 23 package.

The SMD diode might appear in several different packages. The SMD signal diode SD91 4 or MMB914 might appear in a SOT-23 package, while a SMD silicon signal diode appears in a round LL-34 chip. The SMD zener diode might be found in a flat, round chip, or SOT- 23 package. The case and power rating of a 6.1 volt zener diode rated at 200 mW, 300 mW, and 400 mW is found in a SOT-23 package. The LL-34 case zener diode is rated at 500 mW in a round package. The SMD on (1) watt zener diode appears in a round (SM-i) or PSM flat chip. You can test the SMD diode chip with the diode Test of the DMM.

WHICH SIDE IS UP

Mount the SMD resistor with the numbers on top and black side upward. A bypass or coupling chip capacitor might not have any markings on the body and should be mounted with the contact points downward on the end pads. Mount the small chip electrolytic capacitors with the capacity and voltage listed on top with contacts at the bottom. Make sure the white line on the top side of the SMD ceramic capacitor connects to the positive voltage. The top black edge of the aluminum electrolytic capacitor is connected to ground.

The transistor is mounted with the number and letter (example 1A) upward with the terminals over the three PC pads. Place the indented dot of the small IC at terminal one on the PC wiring. Make sure all terminals line up with the pc pads and connections.

There are only a few more tools needed than those found on the service bench. Select a hand-held magnifying glass to locate and identify each SMD component. A pair of eye brow tweezers are required to hold the SMD part in position. Select a small screwdriver to help lineup the SMD component and a low wattage soldering iron. Select a 30 or 40 watt, or less, soldering iron, a battery operated soldering iron is ideal for miniature connections. The bench-type temperature control soldering iron does a good job in soldering sensitive-static ICs and microprocessors.

IDENTIFY SMD PARTS

Within the latest TV chassis, surface-mounted parts are soldered directly to the pc wiring, while standard components are mounted on the top side of the board. The SMD part found on the electronic chassis might look like tiny brown, black and gray specks. The fixed resistor might be marked with white numbers on the black chip.

The SMD fixed Component might be shiny and round with color marked rings to show the value of the part. A ceramic capacitor chip might have a letter with a number along side to identify the value. Some bypass and coupling SMD chips are not marked at all. The electrolytic ceramic chip can be identified with a white line at one end, indicating the positive terminal. The electrolytic capacitor might have three numbers marked on top with a value (222) that equals 2200 uF An aluminum electrolytic capacitor has a black line on top, indicating the negative or ground terminal. Besides the polarity marking, the SMD chip electrolytic capacitor might have the capacity value and working voltage stamped on top of the plastic body. A number on top such as 3R2 equals 3.3 uF, where R is considered a decimal point.

FIG. 8. Soldering up a round SMD resistor pin on the PC board with battery soldering iron.

FIG. 9. You can remove and replace SMD components with a small screwdriver, tweezers, battery iron, and magnifying glass.

The SMD transistor has three terminals with two on one side and one on the top side. The SMD diode might have the same form as a three-terminal transistor. The terminals might be marked 1, 2 and 3. Check for the correct part number printed in white on the TV PCB.

The SMD transistor or IC might have the part marked on top or no markings at all.

The ceramic IC chip has many terminals on each side, while some microprocessors have gullwing-type terminals. You will find many ICs on the camcorder and CD player PCB. The IC chip has a indented terminal 1 or a white dot. You might find a white dot also upon the pc board, indicating terminal 1. The small IC might have 8, 16, or 18 terminals while the microprocessor has up to 80 soldered terminals. Of course, you must have a magnifying glass under a strong light, to identify the small numbers and letters stamped on the tiny SMD component.

FIG. 10. Notice the indented zero indicating pin terminal 1 on most SMD IC’s and microprocessors.

There are many different surface-mounted devices found in the CD player. The D/A converter and audio amp ICs might be SMD components. These chip devices have many gullwing terminals. The chips are usually made of ceramic or plastic molding, and they should not be subject to a direct shock. Always wear a grounded wrist strap, when working with sensitive devices. Most SMD replacements are obtained from the manufacturer because they fit in the required space and replace the original part number.

REMOVING SMD COMPONENTS

After locating the defective component or damaging the new SMD part with too much heat, the component must be removed from the PCB. A cheap and easy method is to apply the iron at one end and slip a small screwdriver blade under the part as heat is applied at the other end. Solder-wick or a desoldering iron with sucking bulb can remove the excess solder. A desoldering station is ideal, but they are rather expensive. Throw the removed component away. Don’t try to use it again.

Heat each individual terminal of the transistor and pry up each terminal with a pocket knife or small screwdriver. Likewise, remove each gullwing terminal from the defective IC until all terminals are removed. Make sure all terminals are loose on the SMD IC before prying them out of the mounting pads. Be careful, it’s very easy to damage the PC wiring. Touch up the soldering pads with solder-wick or mesh-braid and soldering iron. Lift the excess solder up from the pc pads and wiring.

REPLACING THE SMD COMPONENT

Before replacing a defective SMD part, test out the fixed resistor or capacitor with a resistance test. Likewise, test all transistors, inductors and connections with the low ohm scale of a DMM. Place the DMM fine probe tip at each end of the resistor and check for correct resistance. The fixed ceramic capacitor can be checked with the 20K ohm scale by touching the two ends with ohmmeter test probes.

A low capacitance capacitor might show a resistance test for only a few seconds. The electrolytic capacitor will charge up and down according to the capacity measured. For instance, a 2200 uF electrolytic will charge up the numbers for several seconds. Then reverse the test probes and the capacitor will charge up again. Charging action without any leakage on the meter indicates the capacitor is normal and probably okay. Check the capacitor with a capacity tester if one is handy.

Check each SMD transistor or diode with the DMM diode-test as standard solid-state devices are tested. Be very careful as these miniature parts can fly or flip out of site while being tested. Place the SMD part on a white sheet of paper for replacement and tests. Double check the polarity of diodes and electrolytic capacitors before and after installation.

Take one component out of the package or off of a replacement card at a time ( FIG. 11) Remove the part from the strip (usually capacitors and resistors) by placing a razor or knife blade under the piece of cellophane. Place the SMD device on a white sheet of paper, where you can easily see it.

Seal the remaining parts on the cut strip with a piece of cellophane tape. Return all other parts back into the original packet or bag. Seal up with tape or staple the plastic packet so the parts won’t spill out. These parts can easily be mixed up as they all look somewhat alike.

FIG. 11. Replace the strip of SMD components back into the plastic or sensitive-proof bags.

Grasp the tiny part with a pair of small tweezers and hold the ends to be soldered over the correct set of pads. Make sure the wiring pads are tinned with solder. Choose the smallest diameter of rosin core solder available for those tiny connections. Tack one end to the pad with the soldering iron. Only a dab will do. Then go to the opposite end and apply enough solder for a clean joint. Now you are soldering the component ends of the SMD part to the PC wiring pads. Go back and resolder or touch up the tacked-end side and make a good soldered joint. Notice that a good soldered bond upon each end of an SMD component will have a bright and clean connection.

Don’t leave the iron on the joint too long to damage the SMD part or lift the pc pad and wiring. Double check the soldered connection with the magnifying glass. After installation, check for correct resistance across the fixed resistor and capacitors for leakage.

The semiconductors are the most difficult SMD components to solder into the circuits. They have such tiny connections and so many of them. Try to center the three transistor terminals over the right soldered pads or tabs with a pair of small tweezers. Tack in one terminal to hold the part in position. Then carefully solder all three terminals with the fine point of the soldering iron. Be careful not to apply too much heat from the iron to destroy the transistor.

Make sure terminal one (1) of an IC or microprocessor is at the right pad. Check for the indentation circle or dot on top of the IC that indicates terminal 1. A white dot on the PCB can indicate terminal 1. Double check to determine if all terminals are over each pc wiring tab. Like other SMD parts, tack in one terminal on each side of the IC so it will stay in place. Now solder all IC terminal connections to the pc wiring. Inspect each connection with the magnifying glass.

Take a resistance test between each IC element or terminal with the 200 ohm range of the DMM for leakage. Sometimes too much solder will lap over and cause leakage between two or more terminals. Make an in-circuit diode-transistor test of each diode and transistor. Make sure the transistor or IC are not damaged and has good soldered connections.

These tests can be made upon the PC wiring where the semiconductor terminals are connected. A sharp-pointed test probe does the trick.

The resistance and diode tests of resistors, capacitors, transistors, inductors, and ICs insure that no parts are damaged, the correct part is in the right place, and good soldered connections are made. Besides, the electronic chassis will perform after all parts are mounted. There are no greater rewards, when the electronic project or product chassis is fired up after repairs, and it begins to play.

LOCATING THE DEFECTIVE SMD IC

Signal trace the audio signal in and out of the preamp or output IC with the scope or external audio amplifier. If the signal is found at the input of the IC and no output, suspect a defective IC or surrounding components. Take critical voltage measurements of each IC terminal and compare them with those shown upon the schematic. Also compare the same voltages with the other stereo channel. When one or more terminal voltages are improper or low, check for a leaky component tied to that terminal or a leaky IC. A leaky output IC might have lower then normal voltage on the voltage supply pin (Vcc). Take critical resistance measurements from each terminal to common ground.

LOCATING AND REPAIRING DEFECTIVE POWER ICS

Locate the defective power output IC with signal in and out tests of the scope or external audio amp. Notice if the power IC is running extremely warm on the heat sink. A leaky IC might have a very low supply voltage. The shorted IC can contain overheat marks with burned PC wiring. Often, the leaky or shorted power output IC can blow the main power or speaker fuse in the receiver or amplifier.

FIG. 12. Check the signal in and out, and take a voltage measurement upon the supply pin to locate a defective output IC.

After locating and removing the defective power output IC, obtain the manufacturers correct replacement. If the correct replacement is not available, check the semiconductor replacement manual for a universal replacement. At the present time, some universal replacements are very difficult to locate for critical output ICs and processors.

Before mounting the new replacement, clean up all PC terminal pads. Make sure all pads are clean and bright. Smear the back side of the power IC with silicone grease before mounting. Snug up both metal screws to the heat sink. Now solder all terminal contacts to the power IC. Some smaller ICs and microprocessors have glue between the component and PCB. It’s not necessary to apply glue to the new replacement, before mounting and soldering the component to the PC wiring.

INTERMITTENT PC BOARDS

Most intermittent problems found in the SMD audio output stages are related to intermit tent components or poor PCB connections. The pc board might tend to warp and bend, breaking contacts between the tiny SMD parts and pc wiring. Most SMD symptoms are caused by poor board Connections.

Try pressing up and down with an insulated tool or a long pencil with the eraser pressed against the board connections ( FIG. 13). Sometimes by moving the board up and down around the various audio components, the intermittent connection can act up. Spraying the suspected SMD component with coolant can make some parts become intermittent. Solder all SMD part connections after locating a section of the board that appears intermit tent, to try and locate the defective connection. A cracked board in a portable cassette player or hand-held TV might result from the unit being accidentally dropped on a hard surface.

FIG. 13. Locate the intermittent SMD component with insulated tool or pencil.

Besides poor SMD part connections, the intermittent sound might result from poor socket connections. Move the connections around as the unit is playing to locate a poor socket connection after several years of operation. Pressed socket wires have a tendency to make poor connections after a few years. While flexing the wire cable, take critical low ohm resistance measurements between connecting wires and socket. A bright light and magnifying glass can help to spot poor board connections.

REPAIRING SMD CIRCUITS IN THE CASSETTE PLAYER

The SMD components are usually found in the pocket, mini or micro cassette player and recorders. A few standard parts might be found on the top side of the pc board and tiny SMD components mounted directly on the pc wiring ( FIG. 14). A trace of the pc wiring and part numbers might be found upon the top side of board. Most problems related to the various cassette players are defective components, dirty switches, worn parts and dam aged boards.

FIG. 14. The SMD components found on the PC wiring of a personal cassette player.

The stereo cassette mini-amplifier might consist of a dual-preamp IC and a dual-power output IC. Some small cassette players might have all of the audio amplifier circuits in one IC. The stereo tape head circuits are switched into the preamp circuits. A noisy volume control can be cleaned up with cleaning fluid. Replace the control if excessively worn. One channel might be noisy, intermittent, or distorted, pointing to a defective output IC.

Although the small cassette audio circuits are very simple to follow, the biggest problem is locating the defective SMD part on the PCB, with many parts cramped together. Locate the defective or distorted circuit by inserting a test cassette and signal tracing with the scope or external audio amplifier.

Suspect a small coupling capacitor between volume control and input terminal of the out put IC for a weak and frying noise. Suspect a defective power output IC with a low rushing noise in the left channel with the volume turned down. A weak audio signal might result from a leaky preamp transistor or IC. The dead or distorted speaker might be caused by a leaky output IC.

For a very weak recording or playback, check for weak batteries. A noisy right channel might be caused by a dirty headphone jack. Clean the male plug and female jack with cleaning fluid. Spray the function switch with cleaning fluid when a bad or noisy recording is noted. A jumbled recording might result from a ground wire coming off at the erase head. Replace a defective function switch with intermittent recording and playback. First solder all function switch terminals for intermittent reception.

SERVICING SMD AUDIO CIRCUITS IN THE PORTABLE CD PLAYER

The portable CD player might contain a SMD D/A converter processor with SMD audio preamp ICs, mute line and earphone SMD transistors. SMD resistors and capacitors might be found in these circuits with a few standard components. The left and right audio channels are taken from pins 4 and 7 of the D/A converter ( IC7). The weak audio signal is amplified by a dual-SMD IC8-1 and IC8-2. Q101 and Q102 provide muting of the left and right output line jacks.

Signal trace the left and right audio pin terminals 4 and 7 from the D/A converter with the scope or external audio amp. Check the audio input signal at pins 3 and 6 of IC8. Likewise check the amplified audio signal at pins 1 and 7 of the IC amplifier. The CD player audio signal can be traced to each audio line output jacks or stereo headphone jack.

A no left audio channel can be caused by a defective Low Pass Filter network (LPF1 Q1) or D/A converter ( IC7). Replace IC7 for static or frying noise in each channel. Check IC8-1 or IC8-2 for distortion in both channels. Replace IC7 for a background noise. Resolder all terminals of ICB for intermittent sound in the right channel. Suspect a leaky 10 uF electrolytic capacitor for no sound while the disc is playing. Replace the D/A converter (IC7) for distorted audio. Check Q101 for a leaky SMD transistor with a muted left channel.

REPAIRING SMD COMPONENTS IN THE STEREO AMPLIFIER

SMD audio components are found in the portable cassette and compact disc players, portable and auto receivers, and power amplifiers. The portable compact disc might have SMD parts throughout the audio circuits. A portable cassette player can have both input SMD audio components and standard power output parts. The preamp audio circuits might consist of some SMD parts with standard components in the output circuits. A dual-preamp IC and dual-output ICs are found in the low wattage amplifier circuits. A separate SMD audio board might be located in the cassette and CD players.

FIG. 15. The bottom view of a portable CD player with SMD IC’s and components.

Signal trace the SMD audio circuits like any standard audio circuit. Start at the DIA analog output audio circuits within the portable CD player. Check the audio signal from the tape head terminals through each stereo speaker in the cassette player.

Begin at the signal diode detector within the radio circuits to the volume control and small PM speakers. Compare the audio signal at the volume control to determine if the preamp or audio output circuits are defective. Then compare the audio signal from base to collector of each audio transistor, from input to output of the dual-preamp or dual-output IC, or at any given point in the circuit.

TROUBLESHOOTING SMD TV AUDIO CIRCUITS

The portable hand-held TV and small screen receivers might have SMD parts in the audio circuits. The SMD parts and component layouts are found in some service manuals. You cannot tell by looking at the schematic which parts are SMD components. By checking the schematic, notice what ICs or processors have gullwing terminal connections, the part list will tell you this, as well.

All semiconductors such as diodes, transistors, ICs and microprocessors will be listed as all chip types. The SMD resistors might be listed as metal glass chips. A star next to a capacitor in the part list indicates a chip type capacitor.

You might find both SMD components and standard parts within the audio output circuits. The standard parts are mounted on top and the SMD parts are found underneath the PC wiring. The low wattage resistors (1/10 watt) can be SMD components. The ceramic low voltage (under 50v) capacitors are used in coupling and bypass circuits. Often, low voltage Tantalum capacitors under 16 volts are found in SMD bypass, coupling and decoupling circuits. Most high-capacity electrolytic capacitors are standard components in the audio circuits.

FIG. 16. Check the normal signal in and distortion out of IC8-1 and 2 of the analog amplifier CD section.

Although, SMD components are found in very low voltage circuits, they do fail and break down. Besides semiconductors, coupling capacitors and SMD resistors produce most of the SMD TV sound problems. Check for cracked pc wiring around heavy heat sinks and intermittent terminals on IC components. Suspect cracked SMD resistors for distorted audio. High ohm SMD resistors have a tendency to increase in resistance, causing weak and distorted audio. In a Sharp 19 inch portable TV, a feedback resistor R3206 (100K) produced distortion in the speaker with increased resistance ( FIG. 17).

FIG. 17. Feedback SMD resistor (R3206) had increased in resistance producing distortion in a Sharp portable TV.

SERVICING SMD COMPONENTS IN HIGH-POWERED AUDIO CIRCUITS

You might find SMO components in the front-end audio circuits where low voltage is fed to preamp transistors or ICs. They are often located under large standard parts upon the pc wiring side.

The preamp stereo circuits might contain two op amps in one SMD component. The op amp part might be found in the input treble and bass tone circuits. Most of the audio circuits within the auto high-powered amplifier have Mylar, polypropylene, and ceramic capacitors with carbon film or metal oxide standard resistors. Standard high-wattage components might be located on a double-sided PCB in the high-powered auto amplifier.

The high-powered auto amplifier can be checked with signal in and out tests. Check the chassis for overheated components and PC wiring. Connect a 4 or 8 ohm- 100 watt load resistor across the left and right output speaker terminals; test speakers can be connected if no voltage is found at the output terminals. Keep the gain or level controls low as possible, if found at one-end of the high-powered amplifier.

FIG. 18. Look the chassis over for possible burned SMD parts and PC wiring.

Inject a 1 kHz signal from a function or audio signal generator to both input terminals. Connect a bench power supply (13.6v) to the amplifier. Scope each circuit and compare the audio signal stage to stage in the high-powered amplifier. When the audio drops out or becomes distorted, you have located the defective stage. When possible, replace all critical audio components with original part numbers.

. ===