SERVICING THE LOW-VOLTAGE POWER SUPPLY (Guide to Troubleshooting Consumer Electronics Audio Circuits)

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Most electronic products found in today’s electronic entertainment field are powered by batteries or a low voltage power supply. The portable or shortwave radio might operate from several batteries wired in series or an AC adapter. A portable TV may operate from a battery pack, several batteries in series, or a self-contained power supply. The camcorder operates from a rechargeable battery while the large-screen TV contains several power sources, providing different voltages for the various circuits.

Often the batteries are switched out of the circuit when the electronic product is plugged into the AC power line receptacle. When the AC adapter is plugged into the portable radio or CD player, the batteries are switched out of the circuit ( FIG. 1). The AC/DC adapter might provide a 6V, 7.5V, 9V 10V 12V and 18 volt source. The universal AC/DC adapter might have an output of 3V, 4.5V, 6V, 7.5V, 9V and 12 volts with several different male or female plugs. The AD/DC adaptor might provide a dc voltage to the portable radio, CD player, cassette player, power tools and Nintendo games.

FIG. 3 The Sony portable CD player can be operated from the power line with an AC adaptor power supply.

The portable cassette player or radio might operate from batteries, external power, or the AC power line. When the AC cord is plugged into the player, SW1 switches the battery and external power socket out of the circuit ( FIG. 2,). Now, AC is applied to preamp winding of transformer (Ti) to a full-wave rectifier circuit. Likewise, when external power is plugged into P1, the power transformer and “C” cells are removed from the power circuits.

FIG. 2. SW1 switches the batteries or ac power supply circuits in and out of the portable cassette player.

The step-down transformer, (T1) supplies a low AC voltage to the anode terminals of rectifiers D1 and D2. D1 and D2 provide full-wave rectification that is switched by SW1 to a filter capacitor (C1). The 7.5 volt source is applied to the cassette motor and IC circuits of the cassette player.

HALFWAVE POWER CIRCUITS

There are two kinds of power rectifier circuits found in the AC power supplies that consists of a half wave and full-wave rectifier circuits. Half-wave rectification is used where small amount of power is required. In the halfwave circuit only one-half of the waveform is used, where the full-wave rectifier employs both halves ( FIG. 3). Although, the half-wave circuits were found in the early tube radios and TV chassis, you might still see a halfwave circuit found in today’s multiple power sources. You will find larger filter capacitors with higher capacitance in half wave rectifier circuits, to help smooth out direct current ripple effect. The halfwave rectifier might provide voltage to a tape motor, meter or indicator circuit.

FIG. 3. The half-wave rectifier contains one silicon diode while the fullwave circuit has two or four diodes.

The halfwave rectifier circuit might operate directly from the AC power line without a power transformer, from a tap off of a fullwave rectifier or from a separate winding of the power transformer. In the early AM/FM/MPX radios and cassette decks, the DC tape motor operated from a halfwave rectifier circuit ( FIG. 4) A separate winding provided AC voltage to D1 D2 and filter network (C102). The radios and amplifier circuits were powered from a bridge rectifier (fullwave) circuit.

FIG. 4. Notice the cassette motor operates from a halfwave diode while the amp and radio circuits have a fullwave bridge rectifier circuit.

THE FULLWAVE POWER CIRCUITS

The fullwave rectifier circuits might consist of two or four separate diodes. You will find only two silicon diodes in a fullwave rectifier circuit within the lower priced electronic products. Two silicon diodes might be found in the clock radio, cassette player, stereo recorder, and CD player. The fullwave or bridge rectifier circuits consist of four diodes in a bridge con figuration ( FIG. 5). You might find more than one bridge circuit in high-powered supply circuits. The bridge rectifier might have four diodes molded in one electronic component.

FIG. 5. The bridge rectifier circuit has four silicon diodes.

The fullwave bridge rectifier is found in today’s AM/FM/MPX receivers, cassette players, phonographs, amplifiers, CD players, and TV sets. The bridge circuit might have a single bridge component or four separate silicon diodes ( FIG. 6). When a bridge component is not available, four separate diodes can be wired into a bridge circuit.

FIG. 6. Here a Technics high-powered receiver has four diodes in a bridge rectifier circuit on a separate PCB.

The integrated stereo component system that includes an AM/FM/MPX tuner, tape deck, CD section, and high powered amplifier might have two or more bridge rectifier circuits. You might find more than one power transformer or two or more secondary windings feeding the bridge circuits. The bridge output circuits might have transistor, zener diode and IC voltage regulators ( FIG. 7).

FIG. 7. The regulator circuits might consist of transistor, zener diode and IC components or all three.

IC101 of the bridge rectifier provides a positive voltage, while IC102 has a negative volt age output to the high powered amplifier circuits. Q101 and ZD101 provides a voltage regulated source for the other different players. Notice the bridge symbol is provided with only one diode pointing in the positive direction. This diode symbol is found in many Japanese power supply circuits.

THE RECTIFIERS

After the tube rectifier, the germanium and selenium rectifiers were found in the early battery-tube circuits. The selenium rectifier consisted of large selenium plates stacked together to rectify the AC voltage which provided dc voltage to the tube filaments and B+ circuits. Very soon the selenium rectifier was replaced with the solid state silicon diode. The silicon diode is small in size, has high amperage characteristics, is very cheap to manufacture, and is very dependable.

The silicon diode is used today in half wave, fullwave and bridge rectifier circuits. Only one silicon diode is found in the half wave rectifier circuit, while two diodes are located in the fullwave, and four separate diodes work in the fullwave bridge circuit. Most low voltage power supply rectifiers are 1 and 3 amp silicon diodes. The electronic technician might replace all silicon diodes with a 2.5 amp type, except the power supply which requires the 3 amp diode. The defective diode can become leaky, shorted or appear open.

THE FILTER CIRCUITS

After the AC voltage has been stepped-down and rectified by the silicon diodes, the filter components must smooth out the remaining pulsating and dc ripple. The early radio and TV circuits included input-choke and capacitors. In fact, the early radio chassis used a magnetic field-coil as a choke to help eliminate the dc ripple in the low voltage power supply. The choke coil provides high impedance to AC current while practically no opposition to direct current.

The input-choke circuit provides better regulation than the capacitor input filter with a lower output voltage. A capacitor-input filtering network provides a higher output voltage and less filtering action ( FIG. 8). Although the choke-input filtering is no longer used in many of the present day electronic products, capacitor input filter action is still used with transistors, zener diodes, and IC’s as regulators.

CHOKE-INPUT FILTERING; CAPACITOR-INPUT FILTERING; DC OUTPUT; RIPPLE ACTION OF FILTER NETWORK; CHOKE

FIG. 8. The early rectifier circuits had choke or capacitor input filter circuits.

Very large and higher wattage resistors were found as voltage dividers and bleeders in the early power supplies. Some high-wattage resistors had sliding taps to provide a certain working voltage. Now, the voltage regulators, resistor and decoupling electrolytic capacitors form the different voltage circuits. The electrolytic capacitor-input circuits are found throughout the electronic field of entertainment products.

You can spot the electrolytic filter capacitors on the electronic chassis since they are very tall and large compared to other radial type capacitors (Fig 9). The defective filter capacitor might show signs of bulging sides, black and white substance oozing out of the bottom terminal area. A defective filter capacitor might become leaky, open, or shorted, damaging the silicon rectifiers and power transformer. The dried-up electrolytic can pro duce a very low voltage source. Most electronic technicians check the voltage across the large filter capacitors first to determine if the power supply is normal or provides improper voltages.

VOLTAGE REGULATOR CIRCUITS

The voltage regulator circuits might consist of either a transistor, zener diode, and IC or all three in the large power supplies. A zener diode might supply a positive or negative fixed voltage to the cassette motor, display tube, or operate in a transistor regulated circuit. The zener diode provides a simple voltage regulator whose output is constant and fixed. The zener diode regulator circuit consists of a zener diode and limiting resistor in series with the unregulated power source. The defective zener diode might appear open, leaky, shorted or overheated with burn marks.

The zener diode might be found in transistor regulator circuits. The diode is usually located in the base circuit of the transistor regulator. The transistor-diode regulator combi nation provides a low priced, dependable, and high current regulator circuit ( FIG. 10). An open transistor regulator indicates no output voltage while a leaky or shorted transistor regulator might produce a low or higher dc voltage source. Often the zener diode operates quite warm and might be leaky when the transistor regulator becomes leaky or shorted.

FIG. 9. The filter capacitors are the largest capacitors found in the power supply circuits.

The IC regulator might be found throughout the high-powered receiver, CD player, camcorder, amplifier, VCR’s and TV set. In the TV chassis, a large fixed high-voltage regulator is found after the bridge rectifier circuit. The bridge rectifiers and IC regulator operate directly from the AC power line. The TV-IC regulator might regulate the output voltage from +115 to +135 volts; this regulated line-voltage is fed to the horizontal output transistor.

You might find several different IC regulators within the AM/FM/MPX receiver, camcorder, VGA and CD player. The IC regulator might provide a positive and negative voltage source. Several motors within the camcorder or CD player might operate from an IC regulator. The IC regulator provides a fixed voltage source to critical circuits within the low voltage power supply ( FIG. 11).The IC voltage regulators might appear in low voltage power supplies that also contain a transistor-zener diode voltage source. The defective IC regulator might appear leaky or open, providing improper voltages.

FIG. 11. IC regulators provide a fixed voltage source in the power supply circuits.

SEVERAL DIFFERENT VOLTAGE SOURCES

Several different voltages sources are found in today’s electronic products. Although the camcorder operates from a NI-CAD battery, the dc to dc converter provides several different voltage sources to feed the different capstan, drum, and loading motor circuits. The compact disc player power supply sources feed different voltages to the various electronic circuits, loading, disc, turntable and up and down motors.

FIG. 10. The transistor and zener diode regulators are dependable and have higher current capabilities.

The TV chassis might have a line-operated regulated power supply that supplies voltage to the horizontal output circuits and in turn produces many different voltage sources from the flyback or horizontal output transformer circuits. In the RCA CTC1 57 TV chassis, the low voltage power supply provides 129 volts to the horizontal output circuits and the flyback supplies several voltages to operate the different circuits (FIG. 12). The 200 volt source feeds the color output transistor, CRT and boost circuits. The 44 volt source supplies voltage to the beam limiter transistor, vertical reset and vertical sawtooth circuits.

FIG. 12. There are three different power supply circuits found in the RCA CTC157TV.

The audio and vertical output circuits are fed from the scan-derived 26 volt source. The clamp and pin cushion circuits are also fed from the 26 volt source. Besides these secondary flyback voltages, there are 33V, 12V, -12V, 9V, and 5 volt sources supplied from the scan-derived horizontal output transformer. A 33V, 1 29V and 150 volt source is powered directly from the power line voltage supply. A separate standby power transformer sup plies a 23.9V, 1 2V, and 5 volt source. You will find many different voltage sources in today’s electronic products that can break down and provide no or improper voltages.

FILTER CAPACITOR PROBLEMS

When a loud hum is heard from the speakers with the volume turned down, suspect a defective electrolytic capacitor in the power supply. A shorted or leaky filter capacitor can destroy silicon diodes and bridge rectifiers. A dried-up or open filter capacitor in the TV power supply can produce black bars on the picture tube. If the unit is left on too long with a leaky capacitor the power transformer can be damaged. Often the primary winding of the step-down transformer will open up with an overloaded power supply. This can occur at once within the electronic chassis that has no protection fuse in the primary winding.

The defective filter capacitor can dry up, appear open, leaky and shorted. Electrolytic capacitors, after several years old, might dry up the electrolytic paste between the capacitor elements and lose capacitance. The capacitor terminal leads might break inside the capacitor or come loose from the aluminum foil and appear open. An open or dried-up decoupling filter capacitor can cause a lower voltage source. The shorted or leaky electrolytic capacitor can destroy resistors, transistors and zener diodes in the voltage sources. Shunt the suspected filter capacitor with the same or greater capacity and working volt age, to determine if the hum disappears and the voltage source increases.

A quick voltage test across the main filter capacitor terminals and compared to the schematic can indicate if the power supply is normal. Suspect a leaky diode, burned isolation resistor or voltage regulator when the voltage is quite low across the capacitor terminals. Discharge the capacitor. Measure the resistance across the filter capacitor terminals. A low ohm measurement might indicate a leaky capacitor or a leaky component tied to the voltage source. Remove the positive lead from the electrolytic capacitor.

Check the suspected capacitor in and out of the circuit with a capacitor tester ( FIG. 13). A good electrolytic capacitor should have about the same capacity as marked on the capacitor or a higher measurement in microfarads. Replace the suspected capacitor with very little capacity or with internal leakage. A resistance measurement on the 20K ohm scale can indicate a good capacitor that charges and discharges with the meter hand. Reverse the test leads and the electrolytic capacitor will charge up again. Always observe correct polarity when replacing the defective electrolytic capacitor.

FIG. 13. Check the suspected or replacement electrolytic with a capacity tester.

DECOUPLING VOLTAGE CIRCUITS

The decoupling capacitor provides a low-impedance path to the ground to prevent common coupling of the various electronic circuits. A decoupling capacitor within the low voltage sources follows a voltage dropping resistor. Often, the decoupling capacitor has a low capacity value compared to the main filter capacitor. Suspect a leaky or open decoupling capacitor when one stage is weak or dead. A decoupling electrolytic capacitor might isolate two different voltage circuits.

in the car radio, the AM and FM circuits might be isolated from the amplifier circuits with a voltage dropping resistor and decoupling electrolytic capacitor. The AF stage within the auto radio amplifier circuits might be isolated from the same voltage that feeds the power output IC ( FIG. 14). R707 (680 ohm) resistor provides a lower voltage source and C705 (220 p is the decoupling capacitor. R703 (820 ohm) resistor is the collector load resistor of 0703. Suspect a leaky C705 when real low voltage is measured at the collector terminal of Q703. R707 might become burned and change resistance with a shorted C705.

FIG. 14. C705 and R707 provide a decoupling voltage circuit in the audio amplifier.

Check the low voltage source feeding the dead or weak amplifier stage. Improper voltage on the amplifier transistor or IC might result from a leaky decoupling capacitor. Suspect real low voltage applied to the amplifier stage when the decoupling capacitor dries up or opens. Low hum in the speaker might result from a dried-up decoupling capacitor. An increase in resistance of the voltage or isolation resistor results in a low voltage source.

POWER TRANSFORMER PROBLEMS

The power transformer might be located on the electronic chassis or off by itself on another PC board or mounted separately ( FIG. 15). The primary winding of the step- down power transformer is wound with smaller copper wire than the secondary winding. If a silicon diode electrolytic capacitor becomes leaky or shorted, the primary winding might open up in the electronic chassis that has no fuse protection. An overloaded circuit connected to the same voltage source can make the power transformer run very warm. When lightning hits the power line, the transformer can be damaged since there is no fuse protection or the on/off switch is located in the secondary winding. Check the resistance across the AC male plug to determine if the transformer winding is open.

The damaged power transformer might blow the house fuse when the unit is turned on or appears real warm with no sound. The leaky transformer can keep blowing the main line fuse. Sometimes a loud hum and groaning noise from the transformer indicates an over loaded transformer. The electronic chassis might vibrate or buzz with a defective power transformer. Remove the secondary leads of the transformer from the fullwave rectifiers or bridge diodes. Make sure all secondary leads of the transformer are removed from the circuits. Replace the transformer if it has a loud hum and runs quite warm. No doubt the transformer has burned and shorted internal windings. Check the following components that can cause the transformer to run warm and overheat (Failure 3-16).

FIG. 15. The power supply might be mounted upon separate PCB in the portable radio or cassette player.

FIG. 16. Check the following components that might make T101 overheat.

Try to replace the power transformer with the original part number; it fits in the right spot and has the correct AC voltage. For instance, in a Sylvania R53-14/-18 AM/FM/MPX radio, phonograph and tape player the original transformer part number (55-14146-13) supplies an 1 8V, 1 3.6V, and 11.8 volt source. The center tap from the secondary winding provides 5.3 AC volts to several pilot lights. The secondary winding feeds 17.3 volts AC to a half wave rectifier with three resistors and three filter capacitors providing different volt age sources ( FIG. 17).

FIG. 17. The different voltage sources form a step-down power transformer in the Sylvania R53-141-18 radio, phono and tape player.

The 11.8 volt source feeds both tape preamp transistors (Q1 and Q2). A 13.6 volt source feeds both 2nd tape preamp transistors, while the 18 volt source supplies voltage to the 3rd tape preamp player. A separate power transformer and low voltage power supply provides several voltage sources to the volt amp, driver and push-pull output transistors. This is a special type of transformer and should be replaced with the original part number.

NOISY SOUND PROBLEMS

The most pronounced noises found within the audio amp circuits are filter hum, pickup hum, and frying or hissing noise. Hum heard in the speakers with the volume control turned down is caused by defective filter capacitors. A pickup hum might result from a poor microphone or tape head connection or open tape head. A loud rushing noise with no music can be caused by broken tape head wire. Sometimes an increase in resistance of a base resistor of a preamp stage can cause a low hum noise. A constant frying or hissing noise can be caused from a defective transistor or IC. The small ceramic capacitor connected to the collector or base terminals of a preamp or driver transistor can also cause a frying noise.

Shunt another known electrolytic across the suspected filter capacitor to see if the hum disappears in the speakers. Discharge both capacitors. Clip the new electrolytic capacitor across the suspected capacitor with wire clip leads. Don’t shunt the capacitor while the unit is operating. Now, notice if the hum is eliminated. Replace all electrolytic capacitors within one can if one is found to be defective; they will all go sooner than later.

Inspect the microphone cables, plugs and jacks for a poor solder connection. Replace all frayed cords and poor shielded connectors with gold plugs. Check all patch cables for poor connections. Pickup hum can result from poor input ground connections of the preamp circuits. Inspect all ground return wires and cables at the input microphone and phono cables.

Suspect a defective transistor or IC for a constant frying or hissing noise. Critical voltage, resistance and transistor tests won’t locate the noisy transistor. Determine if the frying noise is in the output or front-end circuits. Turn down the volume control and if the noise disappears, the defective component is in the front-end circuits. Several coats of coolant might spot a frying or hissing transistor or IC.

Shunt the base terminal of each transistor with a 100uF 50 volt electrolytic capacitor to ground. Discharge the capacitor each time so as not to damage the transistors. Another method is to short out the base terminal to the emitter terminals of each transistor. Make sure you have the correct base and emitter terminals or you can destroy the transistor if the base or emitter is shorted to the collector terminal. If the noise stops each time, the defective component is ahead. When the noise is present and the base terminal is shunted, you have located the noisy circuit. Replace the suspected transistor.

The frying noise might be caused by a defective ceramic or electrolytic capacitor in the base and collector circuits ( FIG. 18, Suspect a 0.1uF to 47uF coupling capacitor to cause a frying noise within the preamp or AF circuits. Shunt each coupling and bypass capacitor within the located noisy stage. You can signal trace the noisy component by going from base to collector of each preamp and AF transistor with an external amplifier. Likewise check the output and input terminals of the preamp and AF-IC components for a frying noise. Sometimes replacing the noisy component is the only answer.

SERVICING THE CASSETTE PLAYER POWER SUPPLY

The small cassette player might have a very simple fullwave or bridge rectifier circuit. Only two silicon diodes may be connected to a center-tapped power transformer. SW3 (AC! battery SW) switches the batteries out of the circuit when the AC cord is inserted. SW2 is a leaf type switch that turns on the cassette motor and power to IC1. The dc ripple is filtered out with C22, C21, and C20 ( FIG. 19, 1. IC1 is powered from C22 and the erase head voltage is taken from C20 and R20 (470 ohms).

FIG. 19. The power supply and motor circuits within a typical cassette player.

FIG. 18. Check the following components that can cause a frying noise in the amplifier circuits.

Suspect the dc power supply when the tape player operates on batteries and not from the AC receptacle. Measure the dc voltage across C22 (1000 pF). Check the dc voltage at either cathode of silicon diodes D3 and D4. If a dc voltage is found here and not at the filter capacitor (C-22), suspect dirty switch contacts at SW3 and SW2. Clean up the switch contacts with a piece of cardboard or fingernail file. Spray cleaning fluid into the switch area; work the switch back and forth.

Check each silicon diode with the diode test of the DMM when AC voltage is measured on the anode terminals. Suspect a poor AC cord or power transformer with no AC voltage at either diode. Remove the AC plug from the wall and take a resistance check of the primary winding, Across the AC plug prongs. A continuity measurement indicates the cord and primary winding are good. Often the primary winding of the transformer will be open when the silicon diodes (D2 and 03) become leaky or shorted.

Test each battery for correct voltage in a battery tester, when the cassette player won’t operate in battery mode. Make sure the AC cord is not plugged into the cassette player. Clean up all battery contacts. Suspect a dirty switch contact of SW2 or SW3 after installing a new set of batteries ( FIG. 20). Double check each battery polarity as one might be turned around, when they were replaced.

FIG. 20. A dirty leaf switch might cause a dead or intermittent cassette player operation.

TROUBLESHOOTING THE BOOM-BOX POWER CIRCUITS

The high-speed dubbing cassette deck in a boom-box player might have a transistor and IC regulator circuits in the AC power supply. SW6 applies AC voltage to the bridge diodes (D501 -D504) from the power transformer (T501). C502 (2200 uF) provides filtering action before the 18 volts are fed to transistor voltage regulators Q501 and IC501. The 12 volt source out of regulator IC501 provides power to the audio AF and output circuits, and to the motor drive circuits. 0501 and D503 provide transistor-diode regulation to R353 and electrolytic filter capacitors C505 and C344. The +13.5 voltage source is fed to the Dolby, tape head and preamp circuits ( FIG. 21).

FIG. 21. A typical regulated power supply circuit in boom-box cassette player.

Notice, T501 is on all the time when the AC plug is plugged into the wall receptacle. No fuse protection is found in this boom-box player. Check the dc voltage (18V) across the largest filter capacitor C502 when the boom-box player is entirely dead. Suspect defective silicon diodes (D501 through D504), SW6 and T501. Measure the AC voltage across the secondary power transformer winding. Check for a bad cord or open primary winding with no secondary AC voltage.

When the tape drive motors won’t rotate and there is no sound at either speaker, suspect regulator IC501. Measure for 12 volts at the “out” terminal of IC501. Sometimes the silicon diodes and IC regulators are damaged when the boom-box is hit by lightning or a power line outage condition. An open IC501 might have no or very little output voltage. A leaky IC501 may have a lower than normal output voltage. Remove the 12 volt lead from C503 and out terminal of IC501 to see if the voltage increases, indicating an overloaded circuit connected to the regulated IC501.

Suspect a defective Q501 regulator circuit when the motor rotates and no voltage is applied to the front-end circuits. Rotating the volume control up and down can indicate if voltage is present in the amplifier circuits with a rushing type noise in the speaker. No voltage can be measured at the emitter terminal of Q501 if the transistor is open. Replace the intermittent (open) Q501, if the player is on for a few minutes and shuts down with the motor operating. When Q501 appears leaky or shorted, the output voltage might be lower with an overheated D503. Replace both D503 and Q501 when the regulator transistor becomes leaky. Critical voltage measurements within the low voltage circuits can quickly locate defective diodes and regulators.

REPAIRING THE DELUXE AMP POWER SUPPLY CIRCUITS

The integrated stereo component system might have a main and a sub power transformer with several different voltage sources. The main power transformer might supply +60 volts and -60 volts to a high powered amp output IC of 100 watts on each stereo channel (FIG 22). The push on-push off power switch applies the 120V AC power line voltage to the primary winding of Ti. Fl provides fuse protection in the primary winding.

A blown fuse (F1) might indicate a shorted bridge rectifier (D510), leaky 0512 or C514, and a leaky power IC501. Suspect a defective component within the power supply when no sound is heard in the speakers. Locate the largest electrolytic capacitors (C512 & C514) on the main power board and check for correct voltage across each filter capacitor. No voltage might indicate a blown fuse, shorted or leaky bridge rectifier (D510), and C512 or C514 capacitors. Low or improper voltage might result in a leaky power amp IC or defective filter capacitors. You might find a higher supply voltage (33 to 75 volts) feeds the high-power amp IC or power output transistors.

Check each silicon diode with the diode test of DMM when the fuse keeps blowing ( FIG. 23). Take a resistance test across each filter capacitor (C512 and C514) after discharging each capacitor. A low resistance measurement to common ground might indicate a leaky filter capacitor or IC501. Remove the positive or negative lead from each capacitor and take another test. Suspect a leaky or shorted power amp output IC with normal capacitor tests. Check terminals 20 and 21 to common ground for a low leakage test, indicating a leaky power output IC501.

SERVICING HIGH AND LOW TV POWER SOURCES

Most TV chassis today have a low voltage power supply that operates directly from the AC power line without a power transformer. The only transformer that might be found in the TV chassis is a small stepdown power transformer for the standby remote circuits. The 120 VAC power line voltage feeds into a protection fuse, line filter, degaussing coil, on/off switch and bridge rectifier circuits.

FIG. 22. A +60 and -60 voltage source feeds the high- powered output amplifier.

FIG. 24. A power line- operated power supply found in the Emerson model MS198ORTV.

In an Emerson MS1 980R TV power supply, a large filter capacitor C506 (1000 uF) helps filter out the dc ripple current and is fed to an SCR (D508) regulator circuit. The 125 volt source is fed to the horizontal driver and output transistors. The 125 volt source is also fed to the UHF and VHF tuner circuits through a 30 volt regulator (IC502) providing a voltage source to the tuners. The zener diode (D41 0) 6.6 volt source provides a dc voltage to the horizontal and deflection circuits. A +155 volt source feeds to the stereo sound driver and output transistors.

The primary winding of the standby stepdown power transformer connects to the 120 VAC line after the 4 amp fuse. This power transformer is on all the time. The secondary AC voltage feeds into a bridge rectifier circuit and is filtered by C104 (470 uF) and IC104, a 6 volt regulator IC. The standby voltages are a 13 volt, two 5 volt, 5.3V, and a 3.9 volt source. The 5 volt source provides voltage to IC101, IC102, IC103, OS101 Recocon receiver, AFT balance, screen reset, power relay pre-driver Q104, LED drive and reset transistor Q1 13 ( FIG. 25).

FIG. 25. The different standby voltage sources provide remote control operation in the TV.

A 13 volt source provides voltage to operate power relay drive transistor (Q1 03) and relay (Ry 101). The other 5 volt source supplies power to the screen generator IC1 02. The 5.3 volt source feeds voltage to the F. Syn Micon (IC1 01) and reset transistor Q102. A 3.9 volt source goes to hold terminal (34) of IC1O1 Micon.

Before the scan-derived secondary voltage sources of the flyback (FB4O1) can operate, the low voltage power supply, horizontal, and flyback circuits must perform. The high voltage winding of FB4O1 provides a 27.5KV to 30KV voltage to the anode button on CRT, a focus voltage from 4.0KV to 8.0KV, and a screen voltage around 411 volts for the picture tube.

Besides these voltages three different scan-derived windings provide 8 different voltage sources (FIG. 26). The 183 volt source furnishes voltage to the color amps and output transistors tied to the cathode elements of the CAT. A 12.8 volt source feeds the tone control IC371 and IC103. The 8.6 volt source feeds Q201 IF preamp transistor. A 9 volt (A) source provides voltage to the IC201 VIF/SIF/chroma, AGC, 1st transistor buffer Q203 and 2nd transistor buffer (0604), the sound SIF of IC2O1 and sound buffer transistor (0301), the chroma section of IC201 and the brightness (y) buffer transistor (0602).

FIG. 26. Besides the HV, screen and focus voltage, the flyback circuits provide many different voltage sources.

The 9 volt (B) source feeds voltage to the MTS IC951, SAP amp transistor 0951, SAP amp 0952, SAP amp Q953 and 9 volts to noise reduction lC952. A 9 volt (C) source powers the SAP LED indicator, SAP SET LED, switching transistors 0111 and 0110, and also lights up the stereo LED connected to IC951. The 8.9 volt source feeds the kinne bias of IC901 and a peak ACL transistor (Q903).

in this Emerson model the TV sound stages are powered by a high +155 volts and the 9 volt A, B, and C voltage sources. You may find several different voltage sources feed the stereo sound circuits. Most mono TV sound stages are fed from one high voltage source. Always check the low voltage sources feeding the audio circuits when a dead, weak, distorted or intermittent sound symptom is found. Try to secure a wiring diagram when repairing the stereo TV audio circuits. Remember, when the voltage source feeding the sound circuits in the TV chassis are protected by the flyback, both power supply, horizontal and HV circuits must operate.

Most TV sound problems occur in the audio output stages. Locate the output transistor and IC circuits. Take a quick voltage measurement of the collector (body) terminal of a power transistor or voltage supply pin (VCC) of the audio output IC. If low or no voltage, go directly to the TV low voltage power supply sources.

If the TV symptom is that the whole set is dead, the TV chassis must be repaired before any sound stages can be tested. When the TV symptom is no sound, distorted audio or weak sound, suspect the audio output circuits. Rapidly, rotate the volume control up and down to hear a rushing noise. Go directly to the audio output circuits without a rushing noise symptom. Proceed to the audio input stage if a noise or hum can be heard in the speakers.

Check all voltage sources that feed the sound circuits from the main or flyback power supply circuits. When one stage has low or improper voltage, see what voltage source feeds this audio circuit. Look for open transistor regulators and burned zener diodes. The suspected IC regulator might cause no or improper voltage at the sound circuits. Critical voltage measurements within the low voltage source can quickly determine if the power supply circuits are defective. Don’t overlook an open resistor in series with a leaky silicon diode within the scan-derived voltage sources.

SERVICING THE CD POWER SOURCES

The portable CD player has a very simple power supply compared to the CD player found in the compact entertainment system. A small portable CD player might have a power line adapter to supply a voltage source. The portable boom-box CD player with AM/FM/MPX radio and cassette player might have transistors, IC’s and zener diode regulators.

Take a quick voltage measurement across the large filter capacitor (C201) when the CD-boom box player is weak or dead. Check the bridge rectifier or silicon diodes for possible leakage. A shorted diode might open up the primary winding of the power transformer (T201) ( FIG. 27).

FIG. 27. A typical boom-box CD player power supply voltage circuit.

Suspect a blown fuse (F1), open 0201 and damaged D201 when no or real low voltage is found at the emitter terminal of Q201. A weak sound problem might be caused by low-voltage in the power supply sources. Suspect dried up filter capacitors, leaky transistors and IC’s for low voltage sources. Shunt C203 and C205 for a low +10V and +5V source.

Remove one end of a silicon or zener diode when they don’t measure up in the circuit. Check each diode with the diode test on a DMM. Test each transistor regulator in the circuit for an open or leakage measurement. Monitor the output voltage of each emitter of Q201 and Q203, when the sound is intermittent. The regulator transistors have a tendency to operate and then break down under load or after they have been operating for several hours.

REPAIRING THE AUTO RADIO-CASSETTE POWER SUPPLY

The typical early auto radio-cassette chassis operated directly from the car battery. A 3 to 5 amp fuse is found in the “A” lead harness with the stereo speakers and ground connections. L901, C905 and C906 provide a choke input network to eliminate hash-noise from entering the battery line (A 3-28). The power switch (S1) turns power on to the sound circuits. C906 filters out any hum that appears in the power hookup cable. Suspect an open or dried-up electrolytic capacitor (C906), when hum is heard in the audio output stages.

AMP IC TO POWER

FIG. 28. Switch S1 provides voltage to the output IC while S2 switches in either the tape player or radio operation in the auto player.

Besides a blown fuse, poor switch contacts and a defective filter capacitor (C906) provide most of the service problems within the power source of the auto radio. Check the 3 amp fuse and poor switch contacts of S1 for a dead auto receiver. Clean up switch contacts of S2 for intermittent radio and tape operation. Suspect R901 for open conditions when the tape motor won’t operate. Take a quick voltage test across motor terminals to determine if R901 or S2 is defective. Shunt C906 when a loud hum is heard in the speakers with the volume control turned down.

The audio circuits within the auto CD player are fed from a +5V, 10V or 12 volt source. The DIA converter and line amplifiers might have a positive and negative voltage feeding the audio IC component. Many of the auto CD players contain a DC to DC converter unit connected to the 14 volt battery source. You might find several IC, transistors and zener diode regulator circuits in the compact disc player. A lot of the headphone amp circuits are fed from a +5 volt source.

The DC to DC converter is fed from the 14.4 volt battery source and the transistor or IC oscillators provide a pulsating voltage to transformer (T101). The secondary winding might have a half wave silicon diode or a bridge rectifier circuit ( FIG. 29). You might find transistors, zener diodes and IC components as regulators. Several electrolytic capacitors provide filtering action in the dc output circuits.

FIG. 29. The block diagram of an auto CD player-DC-converter power supply.

Check the voltage supply source feeding the audio and headphone amplifiers within the CD player. Suspect a defective transistor or IC regulator with no voltage output. Measure the voltage into the regulator. Check the dc voltage at the silicon diodes in the secondary winding of the power transformer. Accurate voltage measurements and scope waveforms of the IC or transistor operated dc to dc converters solve most auto CD power supply sources.

CHECKING THE MOSFET POWER SOURCES

To provide high output power in a 100 to 200 watt auto stereo amplifier, higher dc voltage must be greater than the battery (14.4) voltage source. In a 170 watt auto stereo amplifier, the positive and negative (34V) source was acquired with an IC oscillator and several dc to dc converter MOSFET transistors. The 14.4 volts DC battery voltage was fused by a 30 amp fuse and fed into the primary winding of transformer T101. The positive and negative 34 volt source might feed 10 to 14 transistors of only one stereo channel in the high-wattage amplifier.

FIG. 30. A block diagram of the auto DC power supply circuits.

The PWM signal is fed from U3 to Q401 and Q405, with MOSFET transistors Q409 - 0411 connected in parallel with power transformer T401. Likewise MOSFET transistors 0406 - Q408 are connected in parallel with the otherT40l winding. The dc-dc converter transistors, U3 and T401 provide a positive and negative (34) volt source. D405 and D406 rectify the output voltage of the secondary winding of T401. A 2200 uF electrolytic capacitor filters the dc voltage source.

FIG. 31. The MOSFET DC-DC power source to power high wattage amp circuits.

Suspect a blown fuse when the pilot lamp is out and no sound from the high-wattage amplifier. If the fuse keeps blowing, check for a shorted or leaky MOSFET transistor (D406-D411) and a leaky D405 and D406. Don’t overlook an overloaded power output transistors in the high-powered amplifier channels. Inspect transformer (T401) and Li 01 for poor soldered connections. Check and test all MOSFET transistors when a 14.4 volt source is found on the primary winding and no output voltage.

Remove one end of each diode when a leaky measurement is found across the terminals. Inspect the silicon diode terminals D405 or 0406 with single 20 amp silicon diodes when the original parts are not available. The six power MOSFET transistors can be replaced with aYTF541 universal type. Shunt electrolytic capacitors C402, C403, C405 and C406 when a low hum is heard in speakers. Replace C402 and C403 with a 35 volt working voltage and C405, C406 at 50 volts.

TROUBLESHOOTING THE TUBE RECTIFIER CIRCUITS

in early audio amplifiers, a fullwave rectifier tube such as a 5R4GY, 5Y3GT, 5U4GB, 5Z4, 6X4, 5AR4, and GZ34 provided a high B+ voltage to the amplifier tubes. The twin diode tube was replaced with the selenium rectifier and then silicon diodes. Since vacuum tubes require higher voltages to operate, the power transformer provides a much higher AC voltage to the rectifier tube ( FIG. 32). In some tube chassis you might find a voltage doubler circuit to boost the dc voltage.

FIG. 32. A typical tube or PA amplifier’s power supply with higher voltage sources.

Besides providing a high AC voltage to the rectifier tube, a separate winding for the tube filaments (6.3V AC) and another winding for the pilot lights and bias circuits might be found in the PA. amplifier. The power supply circuits were protected with a 1 or 2 amp fuse and in larger amplifiers with a 5 amp fuse. A 5 volt AC winding provided heater voltage for the rectifier (5U4) tube. Most tube rectifiers have a choke or capacitor input filter network; others contained a resistor-capacitor filter circuit. The filter capacitor working voltage averaged 450 volts, since the output voltage to the amp were quite high compared to today’s amplifiers.

Most problems found in the tube rectifier circuits were defective or weak tubes, dried-up filter capacitors and blown fuses. A quick voltage test across the filter capacitor will indicate if the power supply is normal. Suspect a leaky rectifier tube or filter capacitor with a blown fuse. A shorted or leaky output tube can blow the line fuse. Check the tube heaters or filament for open conditions with the ohmmeter.

A loud or low hum in the speakers indicates an open or dried-up filter capacitor. Shunt each capacitor until the defective one is located. Choose an electrolytic capacitor with the same working voltage and capacity or one with a higher rating. Replace the entire container of electrolytic capacitors when only one is found to be defective. After replacing or shunting the electrolytic capacitor and a low hum is still heard, suspect a burned choke winding or voltage dropping resistors. Usually the windings are burned, providing poor voltage regulation. A leaky electrolytic capacitor can damage the power transformer.

Remove the rectifier tube and replace the open line fuse. If the transformer runs hot or smokes, suspect shorted windings in the power transformer. Remove all transformer secondary wires from the power supply circuits. Now if the fuse blows or the transformer appears warm or hot in a few minutes, replace it.

LAST BUT NOT LEAST

Shorted or leaky silicon diodes in the power supply can cause the fuse to keep blowing after replacement. A poorly soldered connection at the base or collector terminal of the voltage regulator transistor can cause no sound or relay-click in the speaker of a receiver. The audio might cut out, become intermittent, and the speaker relay clicks off when one of the diodes in the bridge rectifier becomes defective in the receiver. No audio in the receiver might be caused by a shorted or leaky decoupling capacitor lowering the dc voltage. A defective regulator transistor or IC might cause audio distortion after several hours of receiver operation.

The radio receiver volume might go all the way up or down with one or two defective zener diodes. A defective voltage regulator transistor might cause audio distortion after the receiver has warmed up. Weak or no audio, and/or relay click, might result from a defective voltage regulator transistor and diode in the receiver power supply circuits of a deluxe receiver. A protection relay might not turn on the speakers with a leaky zener diode in the B+ line of a receiver.

No sound within the TV chassis might be caused by shorted filter capacitors in the low voltage power supply. Check for burned or open isolation resistors in the TV power supply for no audio symptom and a dead chassis. No or weak audio might result from defective zener diodes, transistor or IC regulator circuits. The no sound symptom might result from open or dried up 1 uF to 10 uF decoupling capacitors in the TV supply sources. Suspect open resistors with shorted diodes in the flyback voltage sources feeding the sound circuits for a no audio symptom. No audio might result from burned resistors and poorly soldered joints on capacitors in the power supply.

Check for dried-up or open electrolytic capacitors in the voltage sources for weak audio in the TV chassis. The weak sound symptom in the TV chassis might be caused by a dried- up 4.7 uF 50 volt electrolytic capacitor. A garbled sound in the TV speaker might result from a defective regulator transistor. Check for an improper voltage source when distorted sound is found in the TV chassis. Low hum in the audio can be caused by defective electrolytic filter capacitors in the decoupling circuits. Garbled audio might be heard in the TV speaker from a defective regulator transistor. Check the large filter capacitors with no audio and a loud hum in the speakers. Suspect a defective zener diode in the low voltage sources with weak audio and poor sound adjustment.

The low hum symptom with no audio might be caused by an open low-ohm resistor and regulator transistor. Check for a defective zener diode by listening for a buzz or hum in the speaker. Intermittent sound with no audio might result with poorly soldered connection of capacitors in a voltage source. Don’t overlook a defective scan-derived voltage source when the sound quits with vertical fold-over in the TV. Remember, the dc voltage source must equal that found on the schematic and feeding the sound circuits, for normal sound conditions.

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Updated: Wednesday, 2014-12-24 23:43 PST