The WA2FNQ 4-400 Transmitter
The WA2FNQ 4-400 transmitter was designed to interface with the linear AM transmitter in an auxillary/main configuration.
Keying voltages, VFO and modulation monitoring are provided by the linear AM transmitter. Transmitter selection and
additional antenna switching are provided by the 4-400 transmitter. This transmitter was desinged to operate on 75/80 meters
The left picture is the front of the 4-400 amplifier. The controls are pretty simple. They are plate tuning, coase and fine
loading, and grid tuning. Grid tuning is a screwdriver adjustment and may need to be touched up when a major frquency change
is made. The meter panel has meters for monitoring final grid current, final plate current, final/modulator plate voltage and
modulator plate current. The switches on the right side of the meter panel are local transmit. transmitter select and power.
The right picture is the front of the modulator, audio driver and RF driver. The modulator is super simple. It's a
pair of zero bias 572B's. The grids go to the audio driver transformer and the center tap is to grounded. The plates go to
modulation transformer and he center tap goes to the high voltage supply. The modulation transformer was made by RCA and is
from some old kilowatt broadcast rig somewhere. The audio driver was inspired by the push-pull, class A, 6AQ5 modulator found
in an old ARRL Handbook. It was re designed to improve it's audio performance and a tone control stage was added. Again, the
audio driver transformer is a big RCA unit removed from some old broadcast rig.
The RF driver is an Eico model 720 transmitter. It was modified for lower output. In it's original configuration it would over
drive the 4-400.
This is the audio driver, modulator and RF driver slid forward for servicing.
Left is a close up view of the audio driver. The RCA audio driver transformer can be seen to the right of the audio
Right is a picture of the two 572B modulator tubes.
This is a shot of the back of the transmitter showing all the connections to the audio driver chassis. One of the modulator tubes can be seen to the
right. The choke on the floor in the background is part of the linear AM transmitters modulation choke.
This is a look through the screen in the top of the 4-400 amplifier. You can see the 4-400 on the left. Next to it in the picture
is the final RF choke and DC blocking capacitor. Both were removed from the RF driver stage of a Gates BC5P2 which is...
a 4-400. The chassis is pressurized by a blower which blows cooling air throgh the bottom of the 4-400. Cooling air is directed
at the plate cap by a tube extending up from the chassis. Yes, the end is off the filler tube of a toilet tank assembly. There is
also another fan on the left side of the cabinet which blows cooling air across the 4-400. On the right is the final tank coil. The
tuning and loading capacitors are out of view. The capacitors seen at the top of the cabinet are for the modulation monitor RF sample.
The 4-400 transmitter is built in to a 2 1/2 foot, open frame rack. So where's all the "big iron"? How about on the floor behind
In the picture on the left the large transformer is one of two plate transformers used for the high voltage supply. This
transformer was installed after the transmitter was moved to this location in 1989. It produces a slightly lower voltage than
the original plate transformer allowing power reduction to 350 watts. Also seen in this picture are the filament transformer
and the components for other low votage supplies. The minibox is a coupling network for the modulation monitor. At the bottom
center is part of the blower that pressurizes the chassis.
In the picture on the right the big transformer is the modulation transformer. The 4-400 screen supply and bias supply along
with the transmitters high voltage supply are under the basement stairs. The original plate transformer with step start relays
mounted on it can be seen all the way in the back. The screen choke is out of view beyond the modulation transformer.
Here's a closer look at the modulation transformer (picture on the left) and the plate transformer (picture on the right). The wires in the foreground
have nothing to do with the 4-400 transmitter. Two pieces of equipment were removed from the rack next to the 4-400 transmitter so these pictures could
be taken. The wires are from the equipment above.
This is the RF switching panel. Attached to the Drake low pass filter is the relay that switches between the 4-400 and the
linear AM transmitter. From this panel transmitters can be switched to either of two Matchboxes, the HF dipole antenna, tri band vertical
or 10 meter vertical dipole.
These are the two Johnson Matchboxes used for antenna tuning. In the old days these Matchboxes would not be able to handle the
power output of the 4-400 transmitter. Back then the 4-400 transmitter used a homebrew tuner designed specifically for 75 meters.
Today, with the relaxed power level of the 4-400, these Matchboxes are at their limit but are just fine. Why two Matchboxes? One
can be left on one frequency/band and the other on another frequency/band eliminating the need for a lot of re tuning.
The homebrew tuner mounted on the back of the wall of the old K2AAW Ham shack.