User's Manual

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3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions

335B, Rev. 0 4-31

Front Panel Metering

The front panel meter (A9) uses the front

panel Selector switch S1 to monitor the

AGC Voltage, % Forward Power, %

Reflected Power, the power supply

voltage, and the current. The meter in

the AGC position will read between 1 and

2 volts. The power supply voltage

reading is calibrated using R86 on the

AGC control board. The % Output Power

is calibrated using R44 and the %

Reflected Power is calibrated using R53

on the AGC control board. With S1 in the

Current position, S2 can be switched to

read the idling currents, no RF drive

applied, of the low-band amplifier pallets.

Typical readings are an idling current of

1.8 amps in the I

, I

, and I

, positions

and 3 amps in the I

position.

Reflected Power Sample

The reflected power sample from the 3-

way combiner board is fed back to the

AGC control board at J5. On the board,

the reflected sample is connected

through a detector circuit, CR13 and

associated components, to the VSWR

cutback circuit, consisting of U5B and

associated components. If the reflected

power increases above 20%, the output

power of the tray, as set by R59, the

VSWR cutback adjust, will be cut back to

maintain a 20% reflected output level.

The red VSWR Cutback LED DS4 on the

front panel will remain lit until the

reflected level drops below 20%.

Thermal Protection Devices

There are three thermal switches in the

tray for overtemperature protection.

Two of the thermal switches (A4-A6 and

A4-A5) are mounted to the (A4) heatsink

for the three low-band amplifier pallets

and the third thermal switch (A5-A2) is

mounted on the (A5) heatsink for (A5-

A1) 3-way combiner board. The thermal

switches close when the heatsink on

which they are mounted reaches a

temperature of 175° F. The closed

thermal switch causes the AGC control

board to remove the enable to the

switching power supply. This eliminates

the +30 VDC and lights the red

Overtemperature LED DS5 on the front

panel and the AGC control board will

extinguish the green Module Status LED

DS3.

4.3 (A8) 3 Way Combiner Assembly

(1065241; Appendix D)

The outputs of the three VHF amplifier

trays (750 Watts each) connect to the 3

way combiner assembly. They are

combined to give approximately 2200

Watts at J5 of the assembly. A thermal

switch is mounted to the heatsink of the

combiner for overtemperature protection

in case of amplifier failure or a problem in

the output filtering network. The thermal

switch closes when the heatsink on which

it is mounted reaches a temperature of

175° F. The closed switch connects an

overtemperature command through FL1

& FL2 on the combiner to J11-1 & J11-35

on the VHF Exciter. The

overtemperature command will cause an

overtemperature fault, that will remove

the enables to the VHF amplifier trays

and place the transmitter in Standby.

4.4 (A13) Harmonic Filter, (A14)

Bandpass Filter and (A16) Coupler

Assembly

The RF output of the 3 way combiner at

(A8-J5) is fed through 1/2" superflex to a

7/8” to 1-5/8” adapter that connects to

J1 on the (A13) harmonic filter. The

harmonic and bandpass filters screen out

the -3.58-MHz, -4.5-MHz, +8.08-MHz,

+9.00-MHz intermodulation products, as

well as all of the visual and aural

harmonic frequencies. The output of the

(A14) bandpass filter is fed through 1-

5/8” hardline to (A16) the output

coupler. The coupler provides a reflected

and a forward power sample to the VHF

exciter for VSWR and metering purposes.

The forward sample connects to J8 and

the reflected sample connects to J9 on

the VHF exciter tray. The output of the

coupler, 3300 watts, at the 1-5/8”