Palstar VHF-1200 Owner's Manual - page 13
13
Tuned Input Circuits
The tuned input circuits utilize an L-C-L or "T" impedance matching circuit with a high Q
design. These circuits employ RF phase compensating inductors to reduce intermodulation
products. The use of mica trimmer type capacitors allows adjustments to precisely match the
transceiver to the amplifier. The rear panel provides easy access to these trimmer capacitors.
Tuned Input Adjustments
Your Commander amplifier has a mica trimmer capacitor which is easily accessed through
the rear panel. The tuned input circuits are factory tuned and should not require any readjust-
ment. You can easily make adjustments for any change in your preference for operating fre-
quency range. Also, slight adjustments may be necessary because of slight variances in imped-
ance between your transceiver and the tuned input circuitry of the amplifier.
1. Install a SWR meter between the transceiver and the amplifier.
2. Make sure the Operate/Standby switch is in the operate position and Your amplifier
should also be properly tuned and loaded.
3. Apply drive and observe the SWR, and adjust the trimmer capacitor for minimum SWR.
Be careful not to overdrive the amplifier.
Output Filter Circuit
The π variable network filter transforms the plate load impedance from approximately 1600
ohms down to 200 ohms, Two air variable capacitors and an associated inductor accomplish
this transformation. A design Q of 14 allows for good harmonic attenuation all across the band.
The utilization of a special reactance tuned powdered iron core 4 to 1 transmission line trans-
former steps the nominal 50 ohm antenna impedance up to the 200 ohm output of the π circuit.
This also achieves further harmonic attenuation of the output in the same manor as an "L" coil
in a π - L network. Additional harmonic attenuation is accomplished with a bandpass filter on
the output.
High Voltage Supply
The high voltage supply operates from a 200/234 VAC 20 ampere line. The primary of the
high voltage transformer is switched on/off with solid state relays that during power up only
conduct when the phase angle of the AC power line is at the 0 to +30 degree crossover point.
This minimizes line surge and inrush current while the high voltage filter capacitors charge. The
front panel on/off switch activates the solid state relays with 2 mA AC to turn the unit on. Ap-
proximately 900 VAC is fed to a full wave voltage doubler rectifier circuit. This supplies
approx. 2600 VDC to the 3CPX800A7's anodes. Metering of the tubes anode and grid current is
accomplished by shunt resistors located in the negative return of the 3CPX800A7's cathodes.
Plate voltage metering is accomplished by a resistor multiplier network in the B+ line of the
high voltage circuit.