|Home | Audio Magazine | Stereo Review magazine | Good Sound | Troubleshooting|
(source: Electronics World, Dec. 1963)
By ROBERT W. WINFREE
Every FM-multiplex buff knows about that mysterious SCA service that may be a source of annoyance in FM J stereo reception. This Subsidiary Communications Authorization (SCA) provision for background music, or "store-cast," dates from 1955 and antedates by several years compatible FM-stereo broadcasting. Although of greater significance to broadcasters than to the public, this multiplexing was welcomed at that time by nearly two hundred stations as a means of supplementing their incomes through this private point-to-point service.
Prior to FM-stereo broadcasting, the FCC permitted SCA channels to be located anywhere between 25 and 75 kc. Popular frequencies for multiplexing were 37, 41, 42, 57, and 67 kc., with only the last being sufficiently separated in frequency from the present 23- to 53-kc. stereo passband to be interference free. Since stations are not required to move their SCA to 67 kc, until they convert to stereo, some of the other frequencies may be around for a long time. Any discussion of equipment here assumes operation on the 67-kc channel although minor changes in multiplexer design will make its use possible on other frequencies.
Although the SCA service is completely analogous to the stereo multiplex transmission, there are several important points of difference to be considered. While the stereo difference information ( L-R) is transmitted as amplitude modulated sidebands on a 38-kc. suppressed subcarrier at 80 to 90% modulation, the background music is frequency modulated on a 67-kc. subcarrier at a maximum of 10% of the total modulation. Besides this technical difference, the stereo program is "broadcast" while the background music is "transmitted," this difference being clearly distinguished by the FCC. While unauthorized reception and use for profit ( such as playing it in a place of business) is not allowed, reception of and listening to an SCA transmission in one's own home apparently does not constitute "reception and use for profit." While program quality may not meet hi-fi standards (most stations play tapes at 3.75 ips or slower), the program content of uninterrupted, easy-to-listen-to music is most attractive to the home listener.
Since most FM service areas have at least one station providing background music, the addition of a multiplexer will enable the owner of the average FM tuner, either with or without stereo multiplex facilities, to receive the SCA programs in the home.
The connection necessary between the FM tuner and the multiplex converter is to the output of the discriminator or ratio detector at a point ahead of the usual de-emphasis network. This is exactly where a stereo multiplexer would be connected and if the tuner has a "MPX Jack" not in use, this may do. Since the converter to be described has a relatively low impedance input circuit so as to be tolerant of connecting cable lengths, the addition of a simple one-transistor emitter-follower output circuit in the tuner will improve the operation for both stereo MPX and background-music use by isolating the tuner high-impedance detector circuitry from a multiplex converter.
Since the background music is transmitted on a 67-kc. Frequency-modulated subcarrier, all that is needed to listen to it is a suitable FM receiver operating on this frequency and plugged into the MPX jack on the tuner. Since this proves to be a scarce item on the market, a little construction work on your part will be necessary here. There are several approaches to this problem and each must meet the several requirements of such a unit. Selectivity or filtering must adequately attenuate stereo subcarrier sidebands and noise below 60 kc. and above 74 kc., and amplification and limiting circuits must pass a suitably tailored FM signal to the frequency-sensitive detector while rejecting AM noise and signal components in the passband. Of course, no tuning adjustments should be necessary since the operation is on a fixed frequency.
Of the several basic designs for such a unit, the one to be described has the important advantage of not requiring complex tuned circuits or filters operating at the subcarrier frequency. By using a crystal oscillator with conventional superheterodyne circuitry and a standard intermediate frequency, no special, hard-to-find, or home-made components are required in the equipment and no tuning adjustments are needed during use. In fact, aside from the 388.888-kc. crystal and the 455-kc. discriminator transformer, most parts can be found in the experimenter's junk box or are easily obtained at the parts house. The crystal is a surplus type available from, among others, Texas Crystals, Fort Myers, Fla., as the Type 241 channel 280 crystal for 50 cents plus 5 cents postage. The Type SSO-1 crystal socket is a good buy at 15 cents. The total cost of all parts if purchased new will be around $20.00. The output of the converter is a high-quality audio signal suitable for feeding the tuner or auxiliary inputs to a music-system power amplifier. An output level control allows setting to a level compatible with other program sources of about 2 to 3 volts and a separate low-impedance cathode-follower output at about one volt is provided to feed remote distribution amplifiers in the author's hone.
Having read this far, you are probably interested enough to take a look at the circuit diagram and parts list and follow a detailed circuit analysis. The composite input signal as provided by the FM tuner will consist of frequencies from a nominal 50 cps at the low end to at least 75 kc. at the upper range, unmodulated, amplitude modulated, and frequency modulated where the station provides all FM services. Where this composite signal is supplied from a high-impedance detector output you will want to add, in the tuner, the simple emitter-follower stage using one 2N508 transistor shown in Fig. 1. When adding this MPX output isolation amplifier, the diagram indicates where to tap off the signal for either this background music converter or a commercial stereo adapter.
In fact both may be used at the same time if desired. Even if your tuner does not have wide-band i.f. and discriminator circuitry so desirable for FM stereo, this background music converter will give good results. Remember, we are working with a frequency-modulated subcarrier here instead of the amplitude-modulated subcarrier for stereo and a falling off of the signal level at the higher frequencies due to tuner shortcomings which is so disastrous in stereo, is easily made up by amplification in the unit.
Referring to Fig. 2, the input tuned circuit, T1, a slug tuned TV horizontal width coil, tunes to 67 kc. with the step-up provided by autotransformer action giving a signal level of about 100 to 150 mv. at the mixer input grid. Grids 1 and 2 function in the oscillator circuit of the 388.888-kc.
crystal oscillator with feedback controlled by the 82- and 100-pf. capacitors. The mixer plate circuit contains the desired 455.888-kc. i.f. signal as well as a strong component of the oscillator frequency. This undesired component is blocked by T5, a miniature 455-kc. i.f. transformer which, by the addition of a parallel 20-pf. capacitor, tunes sharply to the undesired 388.888 kc. signal. Elimination of this signal allows better limiting in the i.f. stages. The interstage i.f. transformer, T2, couples the converter stage to the 6BJ6 first i.f., which provides most of the gain and some limiting due to grid current. Most of the limiting action takes place in the 6BH6 second i.f. stage operating at very low plate voltage.
Grid rectification in this stage provides a d.c. voltage on the order of 25 volts at the grid which is brought out through a 100,000-ohm resistor as a test point, designated "T.P," for alignment purposes.
The remainder of the circuit is unusual and worthy of mention. The detector uses two germanium diodes in a balanced bridge discriminator which, at first glance, may appear to be a type of ratio detector. This circuit, when properly balanced, is capable of the high AM and noise rejection required here, as its electrical balance produces no output from these sources. This circuit is similar to that used in the "Dynatuner" and various industrial equipments. Loading of the discriminator is optimized by the high input impedance of the cathode-follower triode, ½-12AX7A. A low–impedance output is tapped off here for remote audio lines. Then follows the de-emphasis network and volume level control feeding the ½-12AX7A output stage operating with plate-to-grid feedback. The maximum audio level available from this stage is in the order of 3 volts.
The power supply is entirely conventional and need not be built if suitable facilities are available in the tuner or elsewhere. The transformer 6.3-volt winding supplies all tube heaters while the 125-volt supply is half-wave rectified by a silicon diode followed by adequate filtering and decoupling circuits. The high resistance of the transformer winding obviates the use of the usual surge resistor in the rectifier circuit. The total d.c. current drain is under 20 ma. with no signal. No a.c. switch is included since the unit normally plugs into a controlled outlet on the author's tuner. Obviously you may include a switch if you wish.
The emitter follower transistor multiplex output stage added to a typical FM tuner is diagrammed in Fig. 1. It is most important to locate the proper point in the detector circuit to pick off the multiplex signals. This must be before the de-emphasis filter, usually a series 68,000-ohm resistor and shunt .001-pf. capacitor. The usual 10.7-mc. filter elements of 50to 100-pf. capacitors will help locate the right place and corresponding locations for both a typical discriminator and a ratio detector are shown. The transistor socket and other components may be hung by their leads under the tuner chassis in a convenient location, providing short connections, while the 150,000-ohm dropping resistor is adequate for all tuner d.c. voltages from 100 to 250 volts. Be sure to observe polarity when connecting the electrolytic capacitors.
In addition, for longest transistor life do not plug in or remove the 2N508 from its socket while the power is on.
It is most convenient to feed the new MPX output to an RCA-type jack as usually provided for such connections. The full mono program will be present at the jack as well as all MPX subcarriers. Circuit values permit use of any type of multiplex adapter that the builder may desire to employ.
Construction & Alignment
A look at the photographs will show the logical layout of components on the 4" x 6" x 2" aluminum chassis. At the front edge are three jacks for MPX input, low audio output, and high audio output, with the oscillator crystal socket just to the rear of the input jack and the audio level control in back of the high output jack. The three 455-kc. i.f. transformers are mounted with the clips supplied and the filter can by its twist tabs. The four shield base tube sockets are fastened with 4-36 screws and shields are used on all tubes.
Orient the sockets to insure the best lead arrangement under the chassis. Be sure to mount everything that fastens to the chassis before beginning any wiring. A few suitably placed lug strips will provide terminals for those resistors and capacitors not soldered directly to the sockets and coil lugs.
A three-point strip, center ground, fastened on one of the power transformer screws will mount the 1N2070 rectifier and the limiter test point resistor and capacitor.
A strip with three insulated lugs, fastened in the center of the chassis between the discriminator i.f. transformer and the input i.f. transformer, provides distribution points for the two d.c. voltages used in the i.f. amplifier and for the common terminal of the discriminator 68,000-ohm bridge resistors and output capacitor. Another three-lug strip, center ground, between the discriminator can and the 12AX7A socket, is used to tie the detector diodes to the bridge resistors and bypass capacitors.
Follow the usual practice of putting in the heater and power supply wiring first, then a bare wire bus connecting the grounded points, and finally add the small components, building from the chassis out. The small 24-gauge solid hookup wire is a good size to use. The power transformer primary leads are brought out through a rubber grommet and terminated in an a.c. plug: you may want to use a regular line cord here. The resistors and capacitors are all supported by their terminal leads in a point-to-point wiring arrangement, as is the midget i.f. transformer 388-kc. trap coil.
The tune-up requires at least a d.c. voltmeter, preferably a 20,000-ohms-per-volt v.o.m., and a station on the air with an SCA signal.
A visual check of the wiring and parts values after the wiring is completed may save considerable grief later. If no trouble is found, plug the unit into the a.c. line before putting in the tubes, then check the "B +" d.c. voltages. They should be around 200 volts with no load. Plug in all tubes and check the voltages when they settle down, comparing with the diagram values. Then with the v.o.m. on the 10-volt range, connect to the test point and chassis ground to measure the 6BH6 limiter negative grid voltage. If this is about ½-volt, plug in the channel 280 crystal. The leak-through signal from the oscillator on 388.888 kc. will increase the negative grid voltage to 5 to 10 volts. Now carefully adjust the 388-kc. trap coil 'I'5 slug-tuning to dip this voltage to a minimum of less than 1 volt.
Cable the unit up to the tuner NIPX output and adjust the input coil tuning slug until it is out about 1/16" beyond the end of the form. Now tune to a station providing SCA service or tune over the band while watching the limiter grid voltage with the meter on the 50-volt range. Adjust the input coil slug and top and bottom slugs of the two i.f. transformers to peak the meter reading at maximum. Be sure the F\l tuner has been exactly centered on the channel with its a.f.c. turned off while doing this. If the tuner is off resonance, a spurious indication may appear on a station without SCA. The voltage should be 20 volts or more when everything is peaked.
It is advisable to unplug the tuner and check the trap coil adjustment again for best oscillator rejection. Now clip the v.o.m. between the junction of the two 68.000-ohm discriminator resistors and ground, using the 10-volt range on the meter, and carefully screw in the discriminator slug (top of cam) to produce a maximum voltage, positive above ground, of about 3 to 5 volts. After noting the voltage reading, screw the tuning slug oust, watching the voltage decrease through zero and increase with the opposite polarity. You will swap the meter leads here and continue to screw the slug out until a peak is reached. If this voltage is within 10 percent of that previously noted, the primary tuning is satisfactory as is. If not, adjust this tuning (bottom screw) by running it in about one turn.
Now repeat the top tuning procedure, checking the balance of the peaks. If the positive and negative voltage readings are more nearly the same, the primary adjustment just made was in the proper direction. If there is a greater discrepancy between the readings, the adjustment was in the wrong direction and it should be reversed for the next attempt. Continue this juggling until the voltage maximums, positive and negative, produced by the secondary tuning are about equal. This leaves the primary tuning on frequency.
Now adjust the secondary tuning to the zero voltage point halfway between the peaks. A small correction may be made later but this will leave the secondary very nearly correct.
All this can be done in less time than it takes to describe it.
For signal generator users, the discriminator peaks are about 30-kc. apart and the characteristic is quite linear over the +- 7-kc. modulation swings.
Now connect the audio output jack to a power amplifier and speaker and check the audio signal for music quality and background hiss. During the silent period in the music, carefully adjust the discriminator secondary to minimize any noise and modulation from the main program. Objectionable hiss here will be due to weak signal level and poor signal-to-noise ratio on the main carrier. As with stereo multiplex, antenna improvements to boost signal levels will be helpful.
For average signals, the noise may be barely noticeable during pauses.
Stability of the oscillator and other components is very good and no trouble will be experienced with the unit drifting off the center of the channel. Very accurate frequency measurements of subcarrier signals reveal that not all SCA subcarriers are exactly on 67 kc. A slight variation here, within the range encountered in commercial operations, will not cause tuning troubles since the discriminator will accept signals as much as one kc. off center with no strain. If you find that you prefer a particular station's programming, however, all adjustments may be optimized to it. Finally, while enjoying the easy–to-listen-to music in your home, remember that you are really intercepting point-to-point transmissions intended for someone else.
Prev. | Next