Approved Electronic Instr. Corp. A-710
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|Manufacturer:||Approved Electronic Instr. Corp.|
Model 710 tuner chassis described here
is a high quality AM - FM tuner, designed for connection to any good amplifier, radio or television receiver.
Its small and compact size lends itself to custom installations, remote and numerous other applications.
Because of the fact that in some cabinets the tuner must be mounted in an inverted or upright position, a special dial scale is made available, with dial calibrations from top to bottom. An attractive bronze finished escutcheon is supplied with each tuner.
The tuner does not carry its own power supply and an external source is required. A 3 - foot power cable emerging from the rear of the chassis is color coded for easy identification.
DATA ON THE APPROVED FM-AM TUNER
The ever-increasing number of FM stations on the air, and the spiraling demand for high-quality FM reception are a challenge to the ingenuity of circuit engineers who must steer a course between very cheap designs of such poor performance that dealers are coming to recognize them as spurious models, and FM receivers which are too high in price for the mass market at this time.
It was with the middle course in mind that our engineers at Approved Electronic instrument Corporation designed the unit shown in the circuit diagram.
First of all, it employs a tuned RF stage with two limiters and a discriminator, giving high sensitivity and effective static reduction. These assure reception that does full justice to FM broadcasting. Then, a simplified electrical design was developed which assures high stability and freedom from drift.
This tuner contains all the controls required for operation, and one stage of audio amplification common to both FM and. AM section. Thus it can be connected to a special amplifier and speaker, or to the audio system of an existing television set capable of supplying the necessary filament and 8+ power requirements of the tuner
FM - I.F. SECTION
Following the mixer are two 1.F. stages operating at a center frequency of 10.7 mc. into two limiter stages which in turn feed a standard discriminator. All TF. transformers are constructed for high frequency operations throughout. Special iron cores are used that reach their peak " Q " value at 10.7 mc. The fixed capacitors are of the compensatory type. Wave shape tests
Have shown these transformers to possess excellent symmetry and stability. The discriminator transformer has been designed to provide extreme uniformity of wave shape with equal positive and negative peaks resulting in high voltage output with very good discrimination, A band width of 200 kc. is the nominal value of all 1.F. Discriminator transformers. Fig. 3 and Fig. 4.
The center frequency of the If amplifier is 10.7 mc. Due to over coupling of the IF transformers a bandwidth of about 150 kc. can
be expected and is of the double humped variety. While it is possible to align the 1F amplifier with an ordinary AM signal generator and meter, for maximum response, it does not follow that this method produces the correct alignment for proper bandpass characteristic. much more efficient and time saving procedure of the L.F. amplifier alignment is the visual method requiring a frequency modulated signal generator, an oscilloscope and for double check purposes a deviation meter to be connected across the discriminator output. The meter is a 0.C. V.T.V.M. zero center and calibrated -3. - 0 +3 volts. The frequency modulated signal generator must be capable of sweeping through a range of about 10.5 to 10.9 mc. in sawtooth fashion with a possible adjustment for contraction or expansion of the total sweep width and a simultaneous Ty generated sweep voltage is necessary for horizontal deflection of the Oscilloscope. A good AM signal generator with a wide spread around 10.7 mc. completes the total test instruments necessary for proper lf amplifier alignment. Using the visual method of IF alignment, the sweep voltage output of the frequency modulated signal generator must be connected to the horizontal deflection input of the oscil Toscope. The controls of the scope should be adjusted that the trace covers almost the full width of the screen. Connect the vertical deflection input of the oscilloscope across the grid return resistor of the limiter stage and with the output of the frequency modulated signal generator applied to the grid of the second IF stage, adjust the generator to sweep from about 10.5 to 10.9 mc. Due to grid rectification action of the limiter stage, a signal corresponding to the amplitude response of the preceding circuits is then available, and by careful adjustment of the oscilloscope controls a picture of the response curve will be visible on the screen. Never apply more generator voltage than required to produce a good image on the screen.
In order to insure correct center frequency setting, it is now necessary to apply a marker frequency, conveniently obtained from the standard AM signal generator, unmodulated and applied in parallel with the sweep frequency generator. The output of the AM generator should be isolated by means of a small rica condenser and have sufficient R.F. voltage out put to produce a small marker pip super imposed upon the response curve trace, with the AM generator set to exactly 10.7 mc, observe the position of the marker pip and if the pip falls in the center of the response curve, the alignment to follow consist of equalizing the peaks on either side of the marker pip by means of the iron core adjustment screws from the top and bottom of the F transformers. If the AM generator possesses a good frequency spread around 10.7 mc., the marker pip can be used to measure actual band width by slowly moving the AM generators frequency to either side of center frequency, noting where the pip begins to slide off the center of either hump, and adding both frequency differences from center frequency. This equals the total bandwidth. A correct alignment pattern is shown in Fig. 7A. Greater amplitude of patterns indicate higher gain and therefor all adjustments made must be based not only upon symmetry but gain as well.
The generators, both AM and FM are now shifted to the grid of the preceding stage and the whole procedure as outlined re peated, It will be necessary to reduce the output of the generators due to the gain of the added stage. When this stage has been properly aligned, the signal generators are then shifted to the grid of the mixer tube 16C4 where the oscillator voltage is injected. During the alignment of the first le transformer, the oscillator should be inoperative by removing the 6J6 tube. The next step is to align the first lf transformers prim. and sec. The pattern appearing on the screen is then a picture of the overall response of the complete IF amplifier and should be symmetrical with the highest possible amplitude for maximum gain.
The alignment of the discriminator is comparatively easy. The output
of the frequency modulated signal generator is applied to the grid of the 1st limiter tube and the output of the AM generator is fed to the same point at 10.7 mc. ilie vertical input of the oscilloscope must be connected across the discriminator out put with the ground side of the scope to the grounded side of the discriminator. The controls of the scope should be adjusted for the best image possible with a minimum of signal generator voltage applied to the grid of the limiter.
Sym metry must be obtained around the 10.7 mc. marker pip with linearity above and below the marker pip point. A correct discriminator pattern is shown in Fig. 8.
The adjustment of the primary of the discriminator transformer controls the linearity of the discriminator curve, If
Meter alignment is preferred, or no oscilloscope available, a simple 0.c. vacuum tube voltmeter preferably one having a zero center scale and reading plus and minus 3 volts is connected across the discriminator output. A frequency of 10.7 mc. from an AM signal generator is fed to the grid of the limiter stage. The meter will probably read off center. The secondary of the discriminator must now be adjusted until the meter reads zero volts. NOW change the generators frequency in equal steps above and below 10.7 mc, and note the voltage read on the meter. Readings should increase linearly on either side of the 10.7 mc. Center frequency. Checks and rechecks with simultaneous adjustment of the discriminators primary may be necessary before a curve is obtained that resembles the pattern in Fig. 8.
R.F. Frontend Alignment
To align the RE section of the Approved Tuner the following equipment is required. A signal generator with a frequency coverage of 88-108 mc. and prefer ably on fundamentals, a D.C. vacuum tube voltmeter with a low scale reading of about 3 volts or a D.C. meter having at least 20,000 ohms per volt impedance. The meter should be connected across the grid return resistor of the limiter stage. The output of the generator is then applied to the input of the tuner with the frequency set to 106 mc. and the tuner dial indicator set to read likewise 106 mc.
The next step is to adjust the oscillator trimmer until the meter indicates maximum voltage. If the meter tends to read off scale, reduce the R.F. input voltage and hold the meter reading to about 2 volts average. The oscillator has been designed to operate at 10.7 mc. lower than signal frequency and proper setting of the oscillator frequency can be readily checked with a small absorption type wavemeter. At resonance, a large dip or i
increase in voltage reading will be noticed. The text adjustment consists of tuning the antenna and mixer stages for maximum response, Like the oscillator, both stages are tuned by means of silver ceramic capacitors,
The generator should now be set to 90 mc. and the dial indicator to the same frequency, and the oscillator, Rs and antenna inductance slugs adjusted until the meter, again reads maximum voltage. A small adjustment of the oscillator inductance at 90 mc. may show up as a Target frequency deviation at 106 m. due to the inter-relationship of L to C. It may be necessary to repeat the alignment procedure several times before good tracking is finally obtained, with a perfectly aligned tuner, tracking error should never be more than 3 db.
For full enjoyment of high quality reception possible, an amplifier having a flat response of 50-15,000 c.p.s. within 2 db. should be used with a corresponding ly good speaker.
Power connection external or from the amplifier to the tuner must be made and a shielded lead from the tuner output must be used to the amplifier input in order to avoid hum pickup.
The input of the Approved Tuner has been designed to accommodate an FM antenna with a 300 ohm down lead impedance. it must be remembered that the higher the antenna above ground the greater its effectiveness. A simple folded di-pole antenna may be constructed from the new type 300 ohm fine. For construction information see Fig. 5.
AM 1. F. Section Following the converter are two IF transformers operating at 456 kc. Spe cial i roncore tuned IF transformers of high quality, high " O " construction con tribute to the excellent stability.
AM -1. F.
The following instruments are required for the complete alignment of the AM receiver section.
1. A signal generator with Audio Modulation covering the 456 kc. and 540-1800 Kc, band,
2. Voltmeter (AC) preferably vacuum tube (Voltmeter). The alignment of the AM section should be as follows:
Connect the AC output meter to the audio terminal from the 6AT6 tube, set meter to a suitable scale, volume control half on, selector switch in AM position.
Connect the signal generator to the converter signal grid and ground and feed in a 456 kc. signal. Align all. IF transformers for maximum meter reading. Use as little generator input as possible. amplifier is connected to the tuner use the.output meter across the voice coil, modulate signal generator and align for maximum sound output.
R. F. Alignment
Connect the output meter as above and feed a 1450 kc. signal into the antenna terminais. Check dial pointer to coincide with dial calibrations. Adjust all AM trimmer condensers for maximum output. Use as little generator input as possible.
Set dial pointer to 600 kc. (generator likewise to 600 kc.) and adjust ironcore slug of the AM oscillator coil for maxi mum output. Rock variable condenser slightly back and forth, note output and adjust tuning slug until maximum output is obtained near 600 Kc. Return dia setting to 1450 kc., generator setting likewise and repeat trimmer adjustment. This procedure may be repeated until no further increase in output is obtained at 1 450 kc, and 600 kc. Due to the great sensitivity of this tuner, only a small antenna is required for average reception.
To the left of the tuning capacitor is the FM section with the antenna coil indicated directly between the 6AG5 tube and variable capacitor. Directly behind the antenna coil follows the mixer coil and tube and this is followed by the oscillator tube and coil. All three coils are slug tuned and these slugs are used for inductance adjustment around 90 Mc. only. Temperature compensated silver ceramic trimmer condensers are shunted across the main tuning sections and are used for high frequency adjustment around 106 Mc. only. AIT FM alignments are critical and must be made with great care in order to obtain the maximum performance of which the tuner is capable. Model 710 has been aligned at the factory by the sweep generator and scope method for maximum efficiency and no adjustments should be disturbed unless the service man has the necessary tools to perform alignment service. Approved Electronic Instrument Corp. Sweep generator Model A 400 is a Sweep generator ideally suited for FM and television alignments. Page 7 shows a tube layout.
The top view of the tuner shows clearly all important parts and are identified as to their function. To the right of the tuning condenser and located near the dial scale is the AM antenna coil. Directly in line, but under the chassis is the AM RF coil. These coils are fixed and have no other adjustment but the trimmers on the variable capacitor. Again in line and under chassis is the AM oscillator coil. This coil has an ironcore slug and its adjustment is rather critical and should only be used and adjusted around 600 Kc, To the right of the chassis are located the IF tubes and 456 kc. If transformers.
in good signal areas where fairly strong FM signals are available, an indoor antenna as described on page 5 will work satisfactorily; however, for maxim performance an outdoor dipole with 300 ohm line is strongly recommended. For those who intend to use the tuner in conjunction with a television receiver, we re commend a simple 2 pole knife switch arrangement to transfer the television antenna to the tuner, using the same antenna for either television or FM reception.
Fig. 1. CIRCUIT DIAGRAM OF THE APPROVED FM - AM TUNER LESS POWER SUPPLY