View Of Transistorized Ssb Exciter

The audio and r-f networks are mounted on a phenolic sheet attached to the bottom of the chassis. The I2BYTA linear amplifier and shield are mounted on a small aluminum bracket at the left of the assembly. Grid coil L3 is beside the tube, and plate coil Lt is behind the tube. Coil Li is to the right of Ls with sideband switch St behind it. The two r-f balancing controls Ri and Rt are at the extreme right of the chassis panel.

to reduce the 12-6 volt primary supply to 9 volts for the 2N170 stage, and to 4 volts and 2V2 volts for the preamplifier stage. The exciter is designed for use with a primary supply having the positive terminal of the battery grounded.

Exciter The exciter is built within

Construction an aluminum case measuring iyA" x 4" x 21/2" in size. Miniaturization of the audio phase shift network was accomplished by measuring a quantity of resistors and capacitors on a bridge. Final selection was made from those having the closest tolerances. These hand picked components are mounted on a phenolic terminal strip. This same sheet is used to support the small r-f chokes as well as the balanced modulator components.

It is a good idea when building equipment to fit in a limited space to construct everything in the form of sub-assemblies. This will permit the components to be mounted in the open, facilitating wiring. In this particular unit, the audio components and the r-f modulator are mounted on a phenolic board on one side of the box and the r-f components are attached to the opposite wall of the box. The oscillator coil Li is placed within an old i-f transformer can as is the linear amplifier grid coil La. The 12BY7A tube is mounted on a small bracket at one end of the box, and has a tube shield placed over it. The plate coil L< of the amplifier stage is directly behind the tube (figure 11). ' Various voltage dropping resistors are mounted on a small terminal board at the center of the chassis, and the audio transistors are placed in the area between the lower terminal board and the rear wall of the case.

The r-f circuits are wired after the component boards are mounted in place. Wiring is straightforward and simple. The two r-f transistors are mounted by their leads to three terminal phenolic tie-point strips. Be sure to protect both the transistors and the modulator diodes from excess soldering heat, as described in section 28-1. The reader is referred to this section concerning the adjustment and test procedure for the home-made phase shift network.

Testing The The transistorized SSB exciter Exciter may be bench tested by running the transistors from a group of IV2 volt batteries. It is wise to conduct preliminary tests at an operating potential of about 9 volts. The voltage may be boosted to the operating value after tuning and alignment adjustments have been completed- Volt age should be first applied to the transistor oscillator. R-f output may be measured at the terminals of the bifilar coil with the aid of a vacuum tube voltmeter. The slug of coil Li is adjusted for maximum reading. Under diode load, about 4 volts r.m.s. may be measured from either side of the winding to ground. The bifilar winding is relatively critical. Too many turns will load the oscillator to a point of instability, and too few turns deliver insufficient drive to the modulator. Two turns, loosely coupled to the primary winding, seem to be about correct. When the bridge is unbalanced, approximately 0.2 volt r.m.s. may be measured from either arm of output potentiometer Ra to ground. This voltage is stepped up by means of a pi-network circuit so that maximum excitation voltage is applied to the base of the 2N114 r-f transistor.

Preliminary adjustments, in general, follow those described for the vacuum tube exciter described in section 28-1. Potentiometers Ri, Rs, and R.i are adjusted for minimum carrier output in the collector circuit of the 2N114. It may be necessary to add a 10 /¿/ufd variable ceramic capacitor from one side of the modulator bridge to ground to achieve maximum carrier suppression. The final adjustment is to apply an audio signal and touch up the audio phasing potentiometer R« for maximum rejection of the unwanted sideband.

When these preliminary adjustments have been made, the 12BY7A tube may be inserted in the socket, and the shield slipped over the tube. Grid inductor La is resonated for maximum grid excitation with the aid of grid dip oscillator, and the plate inductor Li can be tuned in the same manner. A dummy load consisting of two 6.3 volt, 150 ma. flashlamp bulbs (brown bead) connected in parallel is attached to output jack Ji. Plate voltage and excitation are now applied to the linear amplifier stage and L3 and L4 adjusted for maximum output. When audio excitation is removed from the exciter the r-f amplifier will probably break into oscillation and the antenna bulbs will continue to glow. Neutralizing capacitor G should now be adjusted with a fibre screwdriver until oscillation stops. After the exciter is operating properly, full battery voltage may be applied to the transistor stages with a consequent increase in grid drive and power output of the linear amplifier. Approximately 2 volts r.m.s. grid drive may be measured with the vacuum tube voltmeter at the grid pin of the 12BY7A linear amplifier stage under conditions of maximum excitation.

+1 0

Post a comment