49
CONTENTS AT A GLANCE Broadcast Radio Transmitter Operation
FM/AM Radio Receiver Operation Radio circuit operation
Tips for Making Your Audio Sound Better
Positioning your stereo speakers
FM Radio Antennas
Some Receiver Trouble Checks and Tips
Receiver Will Not Operate at All Intermittent Receiver Problems Some Receiver Service Don’ts
Loudspeaker Concepts and Precautions
How speakers are connected How tuned-port speaker systems
work
Cassette Players—Operation and Maintenance
General cassette care
Cassette tape circuit operation Tape head cleaning and
maintenance
Audiocassette problems, solutions, and corrections
Broadcast Radio Transmitter Operation
For you to become familiar with AM/FM radio reception, start by reviewing how the FM radio signal is developed and transmitted. FM stereo signals must be compatible with monophonic FM radios, but they must also simultaneously carry other information, such as SCA background music, paging, and much more.
The two basic components needed for any stereo radio system are the right (R) and left (L) audio channel information. Refer to the basic stereo FM transmitter block diagram in Fig. 2-1. These left and right audio signals are matrixed, resulting in sum information (LR) and difference information (LR). Matrix is something within which something else orig- inates or develops. To obtain sum information (L + R), +R was added to L; to obtain the difference information (LR), a negative –R of the same magnitude as the R (only 180 degrees out of phase) is added to L. Thus, LR, the difference signal, was created. The composite LR and LR information is now used as FM modulating components in this system. Normally, the LR information could immediately FM modulate the carrier.
However, to be certain that the LR information is in the same phase relationship to the LR information, as they were when they came from the matrix when the FM modulated the carrier, it is necessary to insert a delay network in the LR channel. The delay sys- tem is needed to shift the phase of the LR modulating component in such a manner that it will be in phase with the LR upper and lower 38-kHz sidebands when they also FM modulate the carrier.
In the FM stereo system of transmission, it is necessary that the LR information AM modulate a subcarrier. To create this subcarrier, a very stable crystal oscillator produces a 19-kHz signal. The 19-kHz signal is doubled to obtain a 38-kHz subcarrier that is then AM
L+R L+R
L-R
L-R L
R
Doubler 19kc Stereo
microphones Audio Matrix
Delay Network
FM Modulator Suppressed
subcarrier modulator 38kc
Subcarrier
19kc subcarrier
Pilot 19kc Oscillator Pilot Gen.
67kc Subcarrier
(SCA)
67kc SCA
Power amp.
stage
FIGURE 2-1 A block diagram of an FM stereo transmitter.
modulated by the LR information. The 19-kHz signal is also used as a pilot signal or syn- chronization signal and it also FM modulates the carrier. Because all of the necessary signal information in the subcarrier system is contained in the upper and lower LR 38-kHz side- bands of the AM-modulating envelope, the 38-kHz subcarrier need not FM modulate the carrier. Thus, the 38-kHz carrier is suppressed and only the remaining upper and lower LR 38-kHz sidebands are used to FM modulate the radio carrier.
The FM broadcast system now has three carrier-modulating components: LR audio information, two LR upper and lower 38-kHz sidebands, and the 19-kHz pilot signal. As stated previously, it is necessary that these FM radio systems be compatible with facsimile or SCA. So, another modulating component, a 67-kHz subcarrier for SCA, needs to be added.
FM/AM Radio Receiver Operation
The Bose Wave Radio, shown in Fig. 2-2, delivers sound quality for its small size that can’t be compared to conventional radios or to ordinary stereo systems.
Linking a special configuration of Bose’s unique waveguide technology and the Acoustic Wave Music System to a top-quality radio receiver, the Wave Radio generates sound far more spectacular than its compact size or the sum of its component parts would indicate. Despite its small size, the Wave Radio provides full, rich sound to fill most size home listening rooms. This remarkable audio breakthrough in sound quality comes from the 34-inch single-ended waveguide inside the unit. More on the Bose waveguide speakers later in this chapter.
All functions on the Bose Wave Radio can be regulated by a credit card-sized remote- control unit included with the radio. The Wave Radio features AM and FM stereo radio and a dual alarm clock modes. It offers 12 radio presets, mute, scan and automatic sleep features, as well as battery back up, in case of a power failure. You can set the Wave Radio
FM/AM RADIO RECEIVER OPERATION 51
FIGURE 2-2 The Bose AM/FM Wave Radio. Courtesy of Bose Corp.
so that you can fall asleep to one station and wake up to another. The volume will raise gradually to a volume level that you set.
RADIO CIRCUIT OPERATION
Now see how the various circuits of a radio receiver operate and the problems that can occur.
Refer to the block diagram in Fig. 2-3 as various sections are covered.
The RF tuner section The radio RF tuner selects the station you want to hear and also rejects any unwanted or undesirable radio signals or interference that is present at the antenna. The mixer stage is used to mix the RF station signal with the receiver’s oscillator to produce the IF frequency. An AGC voltage is applied to the RF stage to reduce the amplification of this stage when a strong radio signal is being received. This AGC control voltage is developed at the detector and is usually used to control the amplification of stages in the IF and RF circuits of the receiver.
Automatic frequency control (AFC) circuitry With any high-frequency oscillators, sta- bility is very important feature and these circuits require some type of AFC control to com- pensate for oscillator frequency shift. This is accomplished by taking a sample voltage from the ratio detector and feeding it via a varicap, a voltage-controlled variable capacitor, to the oscillator stage. The varicap is connected across the oscillator tuned circuit and acts as a
RF
AGC
2nd If 3rd If
AFC FM-RF
AGC
ant.
ant.
FM mixer
FM-AM 1st If
FM OSC
AM RF
AM mixer AM oscillator Power
supply
19kc amp
Stereo indicators witch Frequency
doubler
Stereo light 38kc amp Composite
amplifier Bi-plex
detector Ratio
detector
67kc trap
Left speaker Left audio amp.
Right audio amp.
Right speaker FIGURE 2-3 The block diagram of an AM/FM stereo receiver.
frequency-controlling device. If the oscillator should drift, a ratio detector unbalance occurs and a dc voltage is fed back to the varicap so that its changing capacitance will automatically adjust the oscillator frequency. Thus, it has an automatic oscillator frequency control that eliminates drift and simplifies station tuning. Analog tuners will usually have an on/off AFC switch. When tuning in a station, turn off the AFC switch to disable the AFC control to more accurately tune in the station. The newer receiver tuners are digitally logic-IC controlled and do not have an AFC switch.
Intermediate frequency (IF) amplifiers The FM IF frequency is usually 10.7 MHz and the IF frequency for the AM section is 455 kHz. The IFs in a receiver are used to amplify the RF signal and, with the addition of traps, make the receiver much more selective. The gain of the IF amplifiers is controlled by an AGC control voltage. The better receivers will usually have four stages of IF amplification. The processed signal is then fed to the FM ratio detector.
Ratio detector AND composite amplifier The 10.7-MHz amplified output signal from the last IF stage is fed to the ratio detector. The ratio detector is a standard FM circuit that consists of diodes or a special detector chip. Assuming that the FM station you are tuned to is transmitting in stereo and with an SCA program, the composite output signals from the ratio detector will be:
■ A 67-kHz SCA signal.
■ A 19-kHz pilot signal.
■ A L + R audio voltage signal.
■ Upper and lower 38-kHz sidebands.
The composite signal goes to the input of a 67-kHz trap. If the FM station you are lis- tening to is also sending out a 67-kHz SCA signal, it cannot be allowed to enter the detector or the audio will be very distorted.
Composite amplifier function With the 67-kHz SCA information trapped out, it is now necessary to amplify the remaining parts of the composite FM detected signal. The com- posite amplifier has a gain of nine or more times. The output of this composite amplifier is fed to two channels. The L + R audio voltage and the 38-kHz L–R upper and lower side- bands are fed directly into the biplex detector and are then recombined with the developed 38-kHz subcarrier, as well as simultaneous detection into L and R audio voltages. The 19-kHz signal is usually taken off of a transformer and fed to the 19-kHz pilot amplifier.
Other circuits in a stereo FM receiver consist of a 19-kHz pilot signal amplifier, 19-kHz doubler, 38-kHz amplifier, and a circuit to indicate when you are receiving a stereo radio broadcast. This is called the stereo indicator switch circuit.
Biplex detector operation Some receivers use a bilateral transistor in the biplex detec- tor circuit to accomplish stereo signal separation.
For biplex detector operation, the (L + R) audio signal appears at the “L” and “R” out- put circuits in equal amplitude of the same polarity. With only a few turns in the 38-kHz transformer secondary winding, there is only a low-resistance path for the (L + R) signal.
FM/AM RADIO RECEIVER OPERATION 53
The (L–R) 38-kHz sidebands are demodulated by the action of a transistor into two equal amplitudes, but with opposite polarity (L–R) regular audio signals in the same L and R output circuits. The biplex solid-state circuit thus acts to reinsert the 38-kHz contiguous wave (CW), which is a subcarrier into the (L–R) 38-kHz sidebands. At the same time, it demodulates this signal into the (L–R) audio signal and also provides the matrixing of the two sets of audio signals.
The demodulation efficiency of the multiplex “average-type” detectors is about 30 per- cent. The demodulation efficiency of the biplex detector circuit is near 60 percent. Fur- thermore, the L and R channel separation is improved to better than 6 dB at the higher audio frequencies between 8 kHz and 15 kHz. The biplex circuit is designed to provide about 25 dB of separation between the L and R channel signals at 1000 Hz.
One of the most desirable features of the biplex detector is that when tuning across the dial, both stereo and non-stereo (monophonic) stations are received at approximately the same volume level. During monophonic FM program transmissions, the 19-kHz pilot sig- nal is not transmitted. If the 38-kHz switching signal is not applied to a switching transis- tor, it will remain turned off. In this case, the L + R audio signal will be divided between the two channels and fed to both the left and right audio amplifier channels.
The two stereo audio amplifier stages boost the signal level high enough to drive loud- speakers. They can be two or more speakers for each channel. The stereo amplifier stages will also have tone, loudness (volume), and balance adjustment circuits and controls for you to adjust to various room arrangements and to your listening preference.
The Dolby recording technique First, see how an ordinary standard audio recording is produced.
Making a standard audio recording Figure 2-4 illustrates how music consists of dif- ferent loudnesses, separated by intervals of silence.
Loud and soft sounds are shown here as long and short lines. The music represented by this drawing starts loud and gradually becomes very soft and quiet.
Figure 2-5 represents noise. Any recording tape, even of the highest quality, makes a constant hissing noise when played. At very slow speeds and narrow track widths (used in cassette players), tape noise is much more noticeable than with a professional tape record- ing and CDs (although some noise is on these recordings, also).
Figure 2-6 depicts both noise and music on a tape recording. When a tape recording is played, the noise of the tape conceals the quietest musical sounds and fills the silence when
FIGURE 2-4 The music, represented by this drawing, starts loud and gradually becomes very quiet.
no sound should be heard. Only when the music is loud will the noise be masked and usu- ally not heard.
However, tape noise is so much different from musical sounds that it sometimes can be heard even at these times.
How a Dolby recording is produced Let’s now see how the Dolby recording is made and what happens during tape playback.
The Dolby system “first” listens Before the tape recording is made, as shown in Fig. 2-7, the Dolby system “listens” to the music to find the places where a listener might later be able to hear the noise of the tape surface. This happens mainly where the quietest parts of the music are recorded. When it finds such a place, the Dolby system automatically increases the volume being recorded so that the music is recorded louder than it would be normally.
Figure 2-8 gives you an indication of what the Dolby system is doing during recordings.
In a Dolby system, recording the parts of the music that have been made louder, stand out clearly from the noise.
FM/AM RECEIVER OPERATION 55
FIGURE 2-5 A blank tape will make a hissing noise
FIGURE 2-6 Tape background hiss can even be
FIGURE 2-7 The Dolby system “listens” to the when played back.
heard on some quiet music selections.
music first and adjusts the music level accordingly.
As a result, the Dolby system recordings sound brilliant and usually clearer—even when played back without the special Dolby system circuit.
What the Dolby system does during playback is illustrated in Fig. 2-9.
When the tapes are played on a high-fidelity (hi-fi) tape recorder equipped with the Dolby system circuitry, the loudness is automatically reduced in all of the places at which it was increased before recording. This restores the music to its original loudness once again.
At the same time, the noise that has been mixed with the music is reduced in loudness by the same amount, which is usually enough to make it inaudible.
Tips for Making Your Audio Sound Better
Of course, the placement of your stereo speakers is a very personal matter, depending mainly on the arrangement and layout of your listening room, speaker positions, and the way you listen to music. Where you place your speakers does make a difference in how your system will sound. Before settling on a final arrangement, try several arrangements.
Bass response is very dependent on speaker location. For maximum bass, place the speakers in the corners of your room. Placing the speakers directly on the floor will pro- duce an even stronger bass response. If the bass sounds boomy and exaggerated, move the speakers away from the corners slightly, pull them out from the wall, or slightly raise them up off the floor.
FIGURE 2-8 When Dolby is used for recording, it
FIGURE 2-9 When a Dolby recorded tape is played makes the louder music stand out with brilliant sound.
back on a Dolby machine, the loudness is automatically reduced in all places that it was increased before.
POSITIONING YOUR STEREO SPEAKERS
Stereo speakers should be placed from 6 to 8 feet apart. Putting them too close together reduces the stereo effect, but placing them too far apart reduces bass response and creates a “hole effect” in the middle of your room. Generally, most speakers have a tweeter dis- persion angle of close to 60 degrees. For this reason, your listening position should be in the overlap zone, so you want to angle the speakers toward you for better stereo sound.
FM Radio Antennas
Usually, the built-in antennas in most receivers are adequate for good reception. However, if you are having reception problems, try the following hints.
For better FM reception, you can build the folded dipole shown in Fig. 2-10. Just splice together 300-ohm TV twin-lead, as shown. Apply a small amount of solder and heat to the
FM RADIO ANTENNAS 57
Solder Solder
Solder
Solder
Antenna terminals FM 300 ohms
4 ft 8 in (142cm)
FIGURE 2-10 An FM dipole antenna that you can build up.
twisted ends until the solder flows over each wire strand. Attach the lead-in to the 300-ohm terminals on back of the receiver. The antenna can be stapled or tied to the back of the receiver or placed on a wall. Turn or move the antenna around for the best reception.
You can use a set of TV rabbit ears, or buy an FM antenna. Some deluxe antennas fea- ture electronic “tuning” for a more directional station reception.
An outside VHF/UHF/TV antenna will also work well for your receiver. A “splitter”
will let you connect a TV and FM receiver to the same antenna. If you live in the country side, a specially designed FM antenna can receive an FM station from greater than 100 miles away.
Some Receiver Trouble Checks and Tips
Now go over some radio receiver problems.
RECEIVER WILL NOT OPERATE AT ALL
If your receiver is completely dead, check the power supply with a dc voltmeter for a pres- ence of B+ voltage. If there is no B+ voltage, check for an open fuse (Fig. 2-11). If your radio has a built-in cassette tape deck and/or CD player and they are working ok, then the
FIGURE 2-11 Check for a blown fuse if the receiver and audio amplifiers are dead.
power supply should be working and the problem is in the radio RF tuner, IF stages, or detector/multiplex circuit stages. To repair these stages, you need a voltmeter (VTVM), transistor/diode checker, signal tracer, and oscilloscope. These repairs require a profes- sional electronics shop or technician.
If the receiver or cassette/CD player has no audio, then the problem would be in the audio power amplifiers or speakers if the power supply checks out OK. Amplifier prob- lems could be caused by poor solder connections, cable plug-in sockets, defective ICs or transistors, open coupling capacitors, or burned (open) resistors and coils. An audio signal tracer can be used to isolate a loss of audio in these amplifiers. When you are signal tracing in either the RF, IF, or audio stages, the dead stage will become apparent when a signal is found at the input of a stage, but not at the output of that stage.
Another quick check is to place the tracer probe at the speaker-output coupling capaci- tors. The same signal should appear at both ends of the capacitor if it is good (or shorted), but an open capacitor will have an input signal, but no signal at its output con- nector. Open electrolytic coupling capacitors between the output stages and the speaker are fairly common. So, if the left or right audio channels are dead, you should check this out first.
If no signal is found at either end of the speaker coupling capacitor of the dead channel, move the probe to the driver stage output and then to the input. A signal at the output but not at the input proves that the stage is defective.
INTERMITTENT RECEIVER PROBLEMS
Signal tracing is effective if the intermittent condition can be induced. To speed up the break down, you can use a heat gun (hair dryer) or some cooling spray to make the inter- mittent condition start or stop. After you find that a thermal condition triggers the fault, the heating and cooling should be applied to a small circuit area until the trouble can be pin- pointed to one component.
Capacitors are a common cause of intermittents. They can become intermittently leaky, shorted, or open. A leaky capacitor can change the bias on a transistor or IC and cause it (and other components) to fail. Some intermittent problems will change the B+ voltage, so closely check this to determine if voltage change might be the cause or the effect.
Other receiver intermittents are caused by various controls and switches that need cleaning.
Check the front-panel controls and switches. Notice the push-button switches in Fig. 2-12 and, while operating them, listen for any intermittents. These controls and switches can be cleaned with a special spray contact cleaner. If spraying with a cleaner does not correct the intermittent problem, the control or switches will have to be replaced.
If the station tuning dial will not move the pointer, the cord is probably broken. If it is broken, you can replace the cord by restringing it. The tuning cord is shown in Fig. 2-13.
If the cord is slipping, you can apply some anti-slip liquid or stick rosin compound.
SOME RECEIVER SERVICE DON’TS
When working on solid-state (transistors and ICs) receivers, key voltage and resistance checks can usually be used to find the fault. However, before you start probing around,
SOME RECEIVER TROUBLE CHECKS AND TIPS 59