Approved Electronic Instr. Corp. FM Tuner Schematic

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Product Information:
Model:FM Tuner
Manufacturer:Approved Electronic Instr. Corp.

Schematics Content:

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Tuned Lines; Brass Tubing, Silver Overlay.0005 Thick Shorting Contacts; Phosphor Bronze, Contact Surface
Silver Overlay.0005

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R. F. Section
The superheterodyne circuit employed in this tuner follows the Armstrong method of reception. However the R.F. section comprising the R.F. stage, mixer and oscillator, deviate from the common type variable capacitor / inductance combinations by the complete absence of a variable condenser. Instead a new and novel form of R.F. front end has been developed.
The R.F. section of the tuner consists of three Hi - Q actually shaped resonant lines terminating in small end inductances for a frequency coverage from 88-108 mc, with 180 rotation. Semifixed silver ceramics, temperature compensated, capacitors in circuit with the resonant lines and end inductances constitute the tuners total L / C.

This combination allows for an exceptional Hi - Q circuit design with excellent frequency stability and sensitivity. The mechanical construction of the tuners R.F. section is such as to give complete freedom from microphonics often encountered with variable capaci. tors when used at high frequencies. The lines are constructed from 5/32 Od. brass tubing with a heavy silver overlay.0005 thick. The shorting contacts are formed from.007 thick flat stock phosphor bronze with a con tact surface silver overlay of.0005 thickness. Silver overlay, although much more expensive than silver plating, was chosen for reasons of wearing qualities and contact surface. The shorting contacts are mounted up on lucite bars which are fastened to a common shaft to form a single control unit with the lucite bars centered between respective resonant lines and rotable over 180 . With the 4 " drive drum attached to the tuning shaft, a tuning ratio of 16: 1 is obtained with a pointer travel of 6 " resulting in a large full view easy to read dial calibration. The whole front end is of unit design and rubber floated. Full use of the new miniature tubes is made by using a 6AG5 as an R.F. amplifier with a 6J6 dual triode serving as combination mixer-oscillator. The oscillator operates at a frequency 10.7 mc. lower than signal frequency.

I. F. Section Following the mixer are two I.F. stages operating at a center frequency of 10.7 mc. into a single limiter stage which in turn feeds a standard discriminator. All I.F. transformers are constructed of high frequency ceramic throughout. Special iron cores are used that reach their peak Q " value at 10.7 mc. The fixed ceramic 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 I.F. - Discriminator transformers.

Carefully chosen L / C ratios with high " C " maintained during production of these transformers result in high adjacent channel attenuation.

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R. F. Frontend Alignment

To align the R.F. section of the Approved Tuner the following equipment is required. A signal generator with a frequency coverage of 88.108 mc. and preferably 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 108 mc. and the tuner dial indicator set to road likewise 108 me. The next step is to adjust the oscillators semifixed capacitor until the meter indicates maxi. mum 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 then 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 increase in voltage reading will be noticed. The next adjustment consists of tuning the antenna and mixer stages for maxi mum response. Like the oscillator, both stages are tuned by means of semifixed, silver ceramic capacitors.
The generator should now be set to 90 mc. and the dial indicator to the same frequency, and with nonconducting rod adjust the oscillator inductance until the meier again reads maximum voltage. A small adjustment of the oscillator inductance at 90 mc. may show up as a large frequency deviation at 108 mc, due to the inter-relationship of L to C. It may be necessary to re peat 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.

High Frequency Attenuator

High frequency pre-emphasis deliberately intro duced at the transmitter must be compensated for at the receiver i a linear audio response is to be expected. This de-emphasis network is connected across the dis criminator output and has a time constant of 100 u / sec. The voltage output of a discriminator does not depend upon the strength of a carrier but on the frequency swing caused by modulation and on the voltage / deviation characteristic of the discriminator transformer. Under average modulation conditions an R.M.S. vol. tage of approx. 2 volts can be expected. Any audio amplifier of sufficient gain may be connected to the Ap proved FM Tuner.

The input of the Approved Tuner has been designed to accommodate an FM antenna with a 300 ohm downlead 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 line. For construction information see Fig. 5.

Audio Amplifier

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 correspondingly good speaker.
No power connection need be made from the amplifier to the tuner. The only requirements are that a good ground connection be provided and a shielded lead from the tuner output to the amplifier input in order to avoid hum pickup. Hum level measured across the output of the tuner is 70 db. below average rms. output.

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I. F. Alignment The center frequency of the IF amplifier is 10.7 mc. Due to overcoupling of the IF transformers a band width of about 150 kc. can be expected and is of the doable humped variety. While it is possible to align the IF 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. A much more efficient and time saving procedure of I.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 D.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. con traction or expansion of the total sweep width and a simultaneously 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 IF 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 oscilloscope. 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 mica condenser and have sufficient R.F. voltage output to produce a small marker pip superimposed 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 protruding from the top of the IF 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.

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 repeated. 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 (6J6), where the oscillator volt age is injected. The 6J6 tube is a dual triode and half of the tube is used as a mixer with the other half employed as an oscillator. During the alignment of the first IF transformer, the oscillator should be made in operative by disconnecting the oscillators B + lead. The next step is to align the first IF 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.
Discriminator Alignment The alignment of the discriminator is comparatively easy. The output of the frequency modulated signal generator is applied to the grid of the limiter tube and the output of the AM generator is fed to the same point at 10.7 mc. The vertical input of the oscilloscope must be connected across the discriminator output 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. Symmetry must be obtained around the 10.7 mc. marker pip with linearity above and below the marker pip point.
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 D.C. vacuum tube voltmeter preferably one having a zero center scale and reading plus and minus 3 volte 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

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Secondary of the discriminator must now be adjusted until the meter reads zero Now change the generators frequency in equal steps above and below and note the voltage read on the Readings should increase linearly on either side of the center

CORRECT DISCRIMINATOR ALIGNMENT PATTERN WITH 10.7 MC MARKER PIP

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