MODEL FMF - 3 Tuner

TECHNICAL AND ALIGNMENT DATA FOR FMF - 3 TUNER The tuning slugs used in our tuners have a nominal operating range from 87.5 to 108.5 megacycles. They are held within 1 % tolerance in permeability. The oscillator operates 10.7 megacycles higher in frequency than the signal This means that the effective permeability of the tuning slug is greater in the oscillator coil than in the signal coils. Some method, therefore, is required to reduce the actual frequency coverage of the oscillator. This is done by means of a coil in shunt with the oscillator coil which reduces the latters frequency coverage to agree with the signal coils. The inductance of this coil is made adjustable by means of an iron core. Frequency stability of the oscillator through careful design and the use of capacitors of the proper temperature co-efficient is equalled only by the use of crystal controlled circuits. The mixer circuit is of purely conventional design. Oscillator voltage is injected directly into the grid of the mixer by a.68 mmf. capacitor from the oscillator plate,
The RF stage consists of a cathode follower-grounded grid amplifier. At VHF this type of circuit offers a considerable advantage over the more ventional pentode stage. The loading effect of the tube on the coil is many times less than a pentode at these high frequencies. Consequently the " Q " of the circuit is improved which increases both the sensitivity and the image ratio.

ALIGNMENT INSTRUCTIONS
It is necessary that an accurately calibrated signal generator be employed that covers the range of 86 to 109 mc. It should be frequency modulated but not necessarily so. It will be assumed that the alignment work is being done in a completed FM radio with some kind of tuning indicator on it. The fol owing table lists the operations to be performed:

1. Set oscillator tuning slug to 1/32 " from end of winding with tuning slugs all the way out

2. Connect the signal generator through a Imfd. condenser to the grid of the mixer tube. Remove RF tube.

3. Set the signal generator to 87.5 mc. and run the tuning slugs all the way in the coils.

4. With an insulated screwdriver adjust the oscillator trimmer until a signal is heard. Make sure that the oscillator is on the high side of the signal by swinging the signal generator to 108.9 mc. The image should be heard practically as loud as the signal was. If the image is not heard the oscillator trimmer should be re-adjusted by reducing the capacity of the trimmer until another signal is heard. The proper setting will be with the screwdriver slot at approximate right angles with the front of the tuner.

5.Run the tuning slugs all of the way out of the coils and check the frequency. If the coverage is too great unscrew the shunt coil core two or three turns and repeat steps three and four. If too narrow screw the shunt core in two or three turns and repeat steps three and four. This may have to be repeated several times until proper coverage is obtained. The oscillator trimmer will have to be adjusted each time this is done. The tuning range now should be 88.5 to 108.5 megacycles.

6. Remove signal generator lead from mixer grid and connect it to the antenna. Insert RF tube.

7. Set mixer tuning core to 1/32 " from end of winding with the tuning slugs all the way out. Adjust mixer trimmer for maximum output.

8. Set RF tuning core flush with end of winding with slugs all the way out. Adjust RF trimmer for maximum output.

9. Set signal generator to 106 megacycles, tune in the signal, and re-adjust the RF and mixer trimmers for maximum output.

10. Set signal generator to 90 megacycles, tune in the signal, and check alignment by adjusting the RF and mixer trimmers to see if they are at optimum alignment. If circuits do not track do the following: a. If the RF trimmer has to be screwed in at 90 megacycles to obtain tracking, screw the RF tuning slug in about 1 turn, and repeat steps 9 & 10 un til tracking is obtained. b. If the RF trimmer has to be screwed out at 90 megacycles to obtain tracking, the RF tuning slug out about 1 turn & repeat steps 9 & 10 until

MODEL FMF - 3 Tuner

Alignment steps equally apply if the mixer stage require's tracking. Tracking within 3DB may be obtained if the Alignment steps are carefully followed.