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A good turntable with a quality phonograph cartridge provides good-quality listening music. The turntable with a high signal-to-noise ratio and phono cartridge with a wide frequency response probably provides the best sound. The signal-to-noise ratio indicates how quiet the turntable rotates so the cartridge will not pick up vibrations or rumbling sounds. The high-quality phono cartridge ( FIG. 1) with wide frequency response reproduces all the sounds recorded on the record.
Types of turntables
A fully automatic turntable operates with only one button, which might include repeat and pitch control, auto antiskate, complete sit-down, and shutoff features. The semiautomatic changer requires you to start the tone arm manually, although it will pick up the arm and return it at the end of the record. The semiautomatic turntable might have auto return and shutoff, reject control, auto antiskate, and belt drive with synchronous ac motor.
The new linear-tracking tone arm moves laterally across the record with no tracking-angle errors. The motor-driven tone arm moves across the record with automatic shutoff features. Some of the linear-tracking tone arms might select portions of the record using automatic set-down features. Actually, the linear-tracking tone arm just plays the record as it was originally cut.
The upright or vertically mounted turntable might provide dual-side playing with automatic turn-on, shutoff, and single-play operations. Variable speed-control and front-loading features are usually included on the upright turntable.
The turntable might be driven by an idler wheel, belt, or direct-drive mechanism. In the older manual and automatic turntables, the turntable was rotated by a friction motor pulley that rotated an idler wheel that then rotated the turntable. The large platter turntable was often rotated by a large belt around the motor pulley. With a heavy, machined platter, room or motor vibrations are not transmitted in the sound. In the direct-drive turntables, the motor shaft is actually the record spindle assembly. The direct-drive turntables have accurate speed regulation but are prone to motor vibration sounds.
The automatic changer
The automatic turntable might play 78, 45, or 33-1/3 rpm (revolutions per minute) records. Some of the latest turntables might have only two speeds, 33-1/3, and 45 rpm ( FIG. 2). The correct speed and record size is usually selected using controls on the top side of the changer. A cue stick raises the arm so certain music selections can be played. The tone-arm adjustment applies more or less tone-arm weight on the stylus and record. All records are loaded on top of the center spindle and held in place with a keeper arm.
Dead, no rotation Listen for the motor rotating under the turntable. Suspect a defective ac switch, cord, or motor if there is no motor rotation. The motor switch is engaged when the reject or start lever is pressed. Check the ac switch contacts with the low-ohm scale of the ohmmeter if the lever is moving the switch assembly ( FIG. 3).
Take a continuity measurement across the motor terminals. Sometimes the motor terminals are found right on the coil assembly. If not, trace the motor wires to the switch and turntable plug. Inspect the motor coil assembly for brown or burned marks, indicating the motor is running too warm.
Note if the motor is making a humming noise. The motor bearings might be frozen, preventing rotation of the armature. Place the tip of a screwdriver blade against the metal motor assembly and see if it vibrates. A vibration noise indicates the motor is receiving ac voltage but has a frozen shaft bearing ( FIG. 4). Remove the motor mounting bolts and drop the motor down. Disassemble the motor and clean out each bearing with cleaning fluid. Wash out the shaft bearing areas before assembling the motor.
The motor might be dead if the arm lever does not engage the On/Off switch. Check for a missing C washer. Suspect a defective switch or binding-arm lever if the motor runs all the time. Check the price of a replacement motor, because a new motor assembly might cost half as much as the whole turntable.
Speed problems--A worn idler wheel or gummed motor bearing might cause slow speeds. Remove the idler wheel and inspect the rubber drive area. Look for small pieces of rubber around the motor pulley and idler wheel, indicating the idler wheel should be replaced. Apply a dab of phono lube upon the idler wheel shaft before installing a new wheel ( FIG. 5).
Check the turntable bearings for dried grease or dirt inside the bearing area. Clean off the bearings and turntable hub with alcohol and cloth. Lubricate bearings and hub with phono lube grease. Only a dab is needed.
Excess oil on idler wheel or motor pulley might produce slow speeds. Clean with alcohol and cleaning stick. An overheated or shorted motor might cause slow speeds. The turntable might accidentally be left on all day or night and overheat, producing frozen motor bearings.
Improper or erratic speeds--Check the turntable bearings for hair or foreign material that could produce incorrect speeds. Very jerky music might result if the idler wheel operates part way on the 45 rpm part of the spindle in the 33-1/3 rpm mode. Readjust the idler wheel adjustment screw so the idler wheel runs on the center area of the spindle in each speed setting ( FIG. 6). If the adjustment is way off, you might find only one speed on the turntable.
When the 33-1/3 rpm speed is too fast, suspect the idler wheel is running on the 45 rpm part of the spindle. A screw out of the speed lever arm or a gummed-up plastic selector assembly will not let the turntable change speeds. Check for old and gummed grease around the spring-lift area.
Will not shut off--Inspect the record arm keeper for dry or gummed parts that will not let the keeper fall down. If the keeper does not fall, it will not shut off the motor after the last record is played. Note if the rubber washer at the end of the arm keeper is out of position and keeps the arm up so the motor will not shut off. If the arm keeper is loose and will not stay in place, check for a loose set screw. A frozen or dry shutoff assembly might prevent the player from shutting off. Check to see if a bent arm shaft at the bottom side of the tone-arm assembly is not striking or applying enough pressure against the trip lever of the cam assembly.
Tone arm will not set down--Improper adjustment of the height screw might prevent the arm from setting down properly on the record. Check to see if the cue stick is up instead of down. In some changers, a plunger-type assembly is found under the tone arm at the pivot point. A dry or frozen plunger assembly might prevent the arm from setting down. Clean the plunger assembly and apply a coat of grease to the plunger assembly.
Record do not fall--Make sure the spindle assembly is seated properly. Note if the trip lever at the top of spindle move up high enough to move the record off of the spindle. If it does not move high enough, see if the trip assembly at the bottom of the spindle assembly is bent out of line under the turntable. A bent arm keeper might not apply enough pressure against records to move them down the center spindle post. Check for a bent spindle assembly, and replace it if operation is sluggish or erratic in operation.
The semiautomatic turntable
Often with automatic shutoff, the tone arm is lifted upon the turntable to start the motor rotating at the end of the record in a semiautomatic turntable ( FIG. 7). A heavy or machined platter is belt-driven or directly driven in these turntables. The tone arm might have a removable headshell cartridge assembly. Usually there is a counterweight at the end of the tone arm for proper stylus pressure.
Vertical tracking force (VTF) is the amount of pressure applied to the record by the stylus and tone-arm assembly. If the VTF is set too low, high-frequency signals are distorted and attenuated. If it is set too high, low-frequency signals are distorted. The vertical tracking adjustment is made by rotating the counterweight at the end of the tone arm. Rotate the counterweight until the correct weight (in grams) is applied as specified by the manufacturer.
The automatic or semiautomatic turntable might have additional features such as antiskate control, cueing lever, speed select, and stop button. The antiskate bias control compensates for the inward lateral pressure imposed on the tone arm by the rotation of the record grooves. The antiskate control should be set to the same gram setting as the cartridge stylus pressure.
In some turntables, you might find an electronic speed control with 60 Hz strobe markings. The markings are found at the outside edge of the heavy platter ( FIG. 8). A pitch control is rotated until the strobe lines appear to stand still in either 33-1/3 or 45rpm speeds. If the platter is rotating too slowly, the dots will appear to move counterclock wise. When rotating too fast, the dots will appear to be moving clockwise.
The cartridge is usually a magnetic type with easy replacement. The new p-mount arm allows the cartridge to be plugged in without changing pickup wires. These moving-magnet or coil-type cartridges have a wide frequency response range from 10 to 35,000 Hz. Of course, many of the p-mount type cartridges are rather expensive to replace.
Dead, no movement--Often the large belt is off the turntable when there is no platter movement. Check to see if the motor is running. Remove the large turntable and check for a loose belt. In some models, the belt can be replaced by lifting the rubber mat and reaching through the slots in the heavy turntable ( FIG. 9). Sometimes when the speed is changed, the belt is lifted or lowered to a different spot on the motor pulley and the belt will fly off. Check the belt for oily areas. Clean off with alcohol and cloth. Hold the belt with your left thumb through the cutout provided on the platter. Place the belt on the rib of the platter and motor pulley.
Measure the belt for correct length if the belt is too loose or broken. Hold the broken pieces together to measure the length. Select a belt 1/4 inch shorter than the measured length ( FIG. 10). These drive belts can be purchased at most electronics music stores.
Suspect a defective motor, switch, or broken accord if the motor will not rotate ( FIG. 11). Check the switch and motor for continuity with the low-ohm scale of the DMM. Rotate the motor pulley to see if the motor bearings are frozen. Check for 120 Vac at the motor terminals with the switch in the on position ( FIG. 12).
Improper speeds--Inspect the belt and motor pulley for oil spots. Clean off the belt and pulley with alcohol and cloth. Dirt on the motor pulley or belt might produce wow, flutter, and rumble. Determine if the belt is too loose and replace the belt. A dry or gummed turntable bearing might cause slow speeds.
Erratic shutoff--Remove the platter and note if the small trip lever is bent out of line. Bring the tone arm across to see if the trip lever will strike the hub of the turntable. Inspect the trip lever for a gummed or frozen bearing on the cam assembly ( FIG. 13). When the record has finished playing, the arm lever places pressure against the trip lever. The trip level strikes the platter hub, causing the tone arm to return and shut off the motor.
Correct arm adjustment--Most arm-screw return adjustments are found at the bottom of the tone-arm assembly. Securing or fine-adjustment screws are turned to make arm-return adjustments. Adjust the screw so the auto-return mechanism triggers within a range of 57 to 65 millimeters or according to the manufacturer’s specifications.
Motor speed controls--The 33-1/3 and 45rpm speed can be selected by switching resistance into the leg of the motor assembly ( FIG. 14). The dc turntable motor operates from a dc bridge-rectifier circuit. Check for dc voltage (under 16 V) at the large filter capacitor. Inspect the speed select switch (SW1) for dirty contacts if the motor is erratic or does not change speeds. A worn, variable-resistor pitch control (R102) might cause erratic or no 45 rpm speed.
A speed-control circuit with a neon lamp strobe and pitch control resistor had no 33-1/3 speed in a JC Penney Model 6500 belt-driven turntable ( FIG. 15). The 45 rpm speed was normal and could be controlled with VR101. If the changeover switch was pulled to one side, the 33-1/3 rpm speed would start to work. The rotary switch was sprayed with cleaning fluid and restored the 33-1/3 rpm speed. Several coats of spray were needed before the switch contacts were clean enough to operate properly.
Directly driven turntables--In a directly driven turntable, the record spindle is actually part of the motor shaft. Most direct-drive motors are either a dc servo or quartz-locked motor that keeps perfectly accurate speed. The motors are dc types that are controlled from a dc power supply. The speed-control circuits might consist of a bridge-rectifier circuit, speed compensation, driver and speed detection, or strobe-type circuits. In FIG. 16, a Hall IC determines the correct speed of the speed-detection unit. The signal is amplified and sent to the rectifier circuit, which applies voltage to the separate motor windings.
Just about every turntable manufacturer has a different servo-motor speed circuit. You might find IC components in the latest directly driven, motor-speed circuits. The low-voltage, bridge-rectifier circuit supplies voltage to regulator transistors Q1 and Q2 ( FIG. 17). A 21.6 and 22.8 V supply provides voltage to IC101.
IC 101 delivers a BFG method (back-electromotive-force frequency generator) that is generated in the drive-coil winding of the motor to determine the speed of the turntable (terminals 8, 9, and 10). In addition to using a trapezoidal wave-generating circuit, a pulse-generating circuit, and a sampling-integration circuit, the BFG output frequency is converted to a voltage that maintains the rotation speed of the turntable. The operation control circuit functions as a control output voltage control, keeping the rotation speed of the platter constant (pin 22).
The operational amplifier in the active filter provides ideal filter operation (pin 20). As a result, the signal-to-noise ratio is high but produces very low wow and flutter.
Prompt starting of the motor is accomplished by a large-capacity power transistor in the integrated circuit, which has a high starting-torque circuit. Smooth motor rotation results from three differential switching circuits by means of the signal from the position signal coil and start circuit of the power transistor on pins 12, 13, and 14. There are several different motor windings found in this particular direct-drive circuitry.
Resistors R4 and R3 vary the 45 and 33-1/3 rpm speeds with the help of a neon-lamp strobe and turntable. SW2 switches in the 33-1/3 or 45rpm speeds or turns the turntable off. Accurate voltage measurements on IC 101 might indicate a leaky or open IC when the motor is erratic. Improper supply voltage at Q1 and Q2 might indicate a leaky or open regulated power transistor. Check the bridge rectifiers for leaky conditions when T1 becomes warm or if there is improper dc output voltage. Some manufacturers specify that the whole PC board be replaced with the motor if the servo motor or components need replacement.
The linear high-tech tracking tone arm plays records as flawlessly as if they were recorded with virtually no tracking-angle error. The tone arm operates with a worm-gear system to move the arm across the radius of the record. The tone arm will automatically set down at the beginning of the record or at any point you choose. After the record is finished playing, the arm comes over the start/rest position and shuts off the turntable. Of course, only one record can be loaded at a time on this type of turntable. ( FIG. 18).
In some linear turntables, an optical sensor locates each track, and lets you play any one track or all the programs in any sequence. Just pushing a button automatically advances to the next song or music. You can select the repeat mode to play a side up to eight times in a row.
In addition to automatic shutoff and auto repeat, many linear-tracking turntables are belt driven or direct driven with electronic-strobe speed adjustments. Automatic muting, silicone damping, and cueing controls are added features. Almost all linear-tracking turntables have die-cast aluminum platters. Accurate vertical tracking and antiskate adjustments are also located on the base of the turntables.
Radio Shack model LAB-2100
Linear-tracking automatic turntables might have a separate PC board with the various IC and transistor components that control turntable operation. A separate power supply, signal-processing circuit, submotor operating circuit, and main-motor circuits are found on a separate board ( FIG. 19). The tone-arm sensor, strobe indicator, tracking sensor, solenoid, and up and down signal are controlled from the various circuits.
A critical microprocessor is the MP-2001, which is the heart of the operation control circuits ( FIG. 20). The various switches are tied directly to the processor IC. Take critical voltage measurements on the IC when one or more functions do not operate. Check the various switches for good contacts. A dirty speed switch might cause improper rotation. Check the MP-2001 and the submotor control for poor or erratic tracking.
You might find several different power-supply sources within the linear-tracking turntable ( FIG. 21). Here the positive (+) and negative (-) 15 V source supplies voltage to IC 102, the submotor drive and the solenoid. The positive and negative 5 V source connects to the logic circuits, and the 22 V source supplies power for the main motor.
A low or improper 5 or 15 V source might be caused by regulator transistors X101 and X102. Check zener diodes ZD101 and ZD102 for leakage. When a regulator transistor becomes leaky, often the corresponding zener diodes are damaged. If there is no 5 or 15 V source, check for a shorted or leaky bridge rectifier, (BD101). Check BD102 for improper voltage (22 V) at the main motor terminals.
Tone arm does not move--Inspect the sensor assembly with no tone-arm movement. Measure the voltage applied to the sensor PC board assembly. A defective submotor might prevent the arm from moving. Measure the voltage at the motor terminals ( FIG. 22). Check the submotor driver transistors for open or leaky conditions. Inspect the play/cut switch for defective or dirty contacts. Note if the linear-tracking carrier mechanism is stiff or will not move. Rotate the drum pulley and note if the guide bar assembly is deformed, dented, or nicked, or otherwise preventing movement of the tone arm.
PCB schematic of Radio Shack model LAB-2100 linear-tracking turntable.
Tone arm does not descend to disc--Check the contacts of the up leaf switch. Often the leaf-switch contacts are closed when the power switch is off and the tone arm is up. Measure the voltage across the leaf switch. A defective solenoid might cause the tone arm to remain in the up position. Sometimes improper adjustment of the solenoid lift mechanism prevents the tone arm from descending.
Start and stop problems--Improper starting might result from a jammed worm gear or a defective electronic servo-control circuit. If the arm does not start to move, make sure voltage is applied to the motor tracking circuits. Inspect the belt and motor-drive assembly in the belt-driven models. Replace with a new belt ( FIG. 23).
When << button is pressed:
1. Submotor begins to rotate clockwise.
2. Pulley rotates clockwise.
3. Worm assembly rotates clockwise.
4. Drum pulley and worm wheel rotate clockwise.
5. Wire rope rotates clockwise.
6. The tone arm moves to the left.
When >> button is pressed:
1. Submotor begins to rotate counter clockwise.
2. Pulley rotates counter-clockwise.
3. Worm assembly rotates counter clockwise.
4. Wire rope moves counterclockwise.
5. The tone arm moves to the right.
Suspect foreign material in the worm-gear assembly or a defective speed-control circuit if the arm stops in the middle of a section ( FIG. 24). Check the sequence in repeat mode. See if the arm will start up or reject to the shutoff position. Note if the small motor belt or drive mechanism is off the tracking motor. Take quick voltage and resistance measurements on IC and transistor components in the variable motor speed control circuits.
Vertical mount/upright turntables
There are only a few upright or vertically mounted turntables on the market ( FIG. 25). These record players can play either side of the record and is automatic. The player has automatic turn-on and shutoff with front loading features ( FIG. 26). Like the semiautomatic turntable, a variable-speed control is found on most models.
Check the switch and motor for a no-start symptom. A jammed or binding mechanism might prevent the turntable from starting up. Because the turntable weight is on the center hub area, check for adequate lubrication of the turntable bearing. Aside from the above considerations, the upright turntable can be serviced in the same manner as any record player.
After repairs are made on the defective turntable assembly, inspect all moving components for correct lubrication. All cams and sliding levers should have a touch of light grease applied to the working areas. Inspect the turntable bearings for gummed bearings or hardened grease. Remove the bearing ring of the automatic turntable, wash out the old grease, and apply a coat of light phono lube.
Check the motor for dry bearings. A squeaking motor might need only a squirt of light oil. In older open-type motors, remove the motor assembly and the armature. Wash out the bearings with alcohol or silicone shield lubricant. Do not overlook gummed motor shafts. Lubricate bearings with a light oil. Do not over oil or over grease any moving bearing. The oil or grease might drip or land on the motor belt or rotating turntable, causing slow-speed problems. Wipe off excess oil with alcohol and cleaning stick.
All turntables seem to collect dust, especially if there is no dust cover for the turntable. Wash off all dust and dirt with a window-cleaning spray. Spray the cleaning fluid on the tone arm, turntable, and mounted surfaces. Get into the small areas with a brush. Be careful not to hit or damage the stylus or cartridge. Removing dirt and grease from the top of the turntable assembly makes it not only perform better, but makes it shine like new.
Cartridge and stylus replacement
A dirty stylus or needle might cause distortion or fuzzy music. Excessive dirt between stylus and cartridge might produce tinny and weak sounds. First clean out around the stylus with a small brush. Look at the stylus under a magnifying glass to determine if the point is worn or chipped. Always replace the stylus with one having a diamond needle. A chipped diamond stylus might cause scratchy music and gouge out the recording.
To remove the defective stylus, drop the cartridge down, straighten the stylus clip upward, and then pull outward ( FIG. 27). Some needles are removed by pulling straight out. The stylus may slip under a keeper metal piece to hold it in position. When removing the stylus be careful that the small saddle attached to the crystal or moving metal vane is not broken.
A defective cartridge might cause excessive distortion and mushy music. Very low or no volume might be produced by a defective or broken cartridge. It sometimes happens that the music volume can be fairly normal but without clear vocal recordings. This can also be caused by a defective cartridge. Excessive motorboating sounds also might be caused by a defective cartridge. Replace if in doubt.
Check the continuity of all magnetic cartridges with the low ohmmeter scale ( FIG. 28). Both left and right channels should be equal in resistance. The open or shorted winding should be replaced with a new cartridge. Often the cartridge terminal clip leads come off the right channel, and red (+) and green (—) with white (+) and blue (—) or black off the left channel.
Intermittent or erratic phono reception might be caused by broken wires or loose clips on the cartridge. Check each connection at the cassette terminals. Hum might be present if there is a broken negative or black ground wire. Measure the continuity of each channel with the ohmmeter where the female phono plugs are found on the turntable. This test will detect an open magnetic cartridge or broken wire lead. Crystal cartridges should have infinite resistance.
Remove the head-shell mounted cartridge by loosening the small mounting screws. In models with the cartridge incorporated with the head shell, it is recommended that the two be replaced together. The head shell attached to the tone arm should not be inclined to the right or left. If necessary, loosen the screws attached to the tone arm or unscrew the cartridge head from tone arm. Make sure the stylus is perpendicular to the record surface. Some cartridges are held in position with a front metal clip. Pull down on the clip and the cartridge will release.
The p-mounted cartridges -- NOT RECOMMENDED -- plug right into the cartridge mount and are easy to replace. The stylus can be bonded, nude, micro, or linear with a bi-radial or elliptical shape. Prices range from $19.95 to $250.00.
Try to replace cartridge and stylus with exact replacements. Select a cartridge with the same voltage and frequency response when replacing with an unusual component. Take the old cartridge and stylus to a record or music store or to where you purchased the player, along with the model number of the turntable for correct replacements. Remember, there are hundreds of different stylus cartridge designs on the market.
Table 1. lists some additional problems along with their possible causes and suggested troubleshooting procedures.
Table 1. Troubleshooting chart for automobile and semi-automatic turntables.
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