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FS-5000 X-Unit
Transmitter and DSU test device

The X-Unit was a small test device that was supplied as part of the Toolkit of the FS-5000 Field Station. It measures only 9 x 5.5 x 2 cm and has connectors at three sides. One side has a DB25 female socket which is marked DSU. At the opposite side is a simular (male) connector marked TRANSMITTER. At the short side are three banana-type sockets: black, red and yellow.
The test device is shown in the image on the right, with the DSU connector at the rear. At the top right is a green LED that is used as a power indicator. The test device contains two fully independent circuits and offers three functions:

(1) Test the output power of the transmitter. This can be useful if you suspect the transmitter might be broken. (2) Allow the transmitter to be connected directly to the antenna. This can be used as an emergency measure when the ATU is broken. (3) Test the interface of the DSU. This requires stand-alone (battery) use of the DSU.
X-unit (test box)

Please note that the X-Unit can not be used to connect the DSU directly to the transmitter. The box contains two fully independent circuits and there is absolutely no connection between them. The functions of the X-Unit are further described below.
Contents of the toolkit X-unit (test box) X-unit (test box) Side of the X-unit X-unit, top view LED on X-unit Banana sockets on the X-unit Antenna connected to the X-Unit

It was foreseen by the designers that situations might arise in which the user is uncertain about correct operation of the transmitter. As the combination of Transmitter and Automatic Antenna Tuner (ATU) is a rather complex one, there are various places where things can go wrong.
In such situations, the ATU should be removed from the transmitter by loosening 4 recessed screws on top of the ATU. Once the ATU is removed, a socket on top of the transmitter becomes visible. This connector carries control and return signals, but also the direct output of the transmitter, which can be 30W or more.

The X-Unit can now be used as an indicator of the output power, by placing it on top of the transmitter's socket. This is shown in the image on the right. When a transmission is now started, the green LED on top of the X-Unit should be lit.
Installing the X-Unit on the transmitter

If the LED lights up, the output stage of the transmitter (PA) is working fine and the transmitter produces sufficient power for a transmission. In this case it is possible that the ATU is broken. As an emergency measure the transmitter can now be connected directly to the antenna.
As the transmitter produces an output power of 30W or even higher, the energy reflected by a badly matched antenna could easily damage the transmitter's output stage (power amplifier).

This problem is overcome by connecting the X-Unit in between the transmitter and the antenna. The X-Unit is fitted to the transmitter's socket as described before. The antenna and a suitable counterpoise (ground) is now connected to the red (antenna) and black (ground) banana-type sockets on the side of the X-Unit. This situation is shown in the image on the right.
Antenna connected to the X-Unit

The X-Unit contains a so-called dummy load of 59 Ohm which is used here as an attenuator. The maximum output power with a load of 50 Ohm is now 10W, which can no longer damage the PA amplifier of the transmitter when being fully reflected. The function of the yellow TEST-socket is currently unknown.
Transmitter (without ATU) seen from the rear Transmitter output connector seen from the rear Installing the X-Unit on the transmitter X-Unit connected to transmitter, seen from the rear. X-Unit placed on the output socket of the transmitter Antenna connected to X-Unit (seen from the rear) Antenna connected to the X-Unit Antenna connected to the X-Unit

Testing the DSU
The Digital Storage Unit (DSU) is normally used as part of the complete FS-5000 field station. It is connected to the DB25 socket on the side of the receiver. When powered down, the messages and settings in the internal memory of the DSU are retained by the internal 7V backup batteries.
If the FS-5000 does not work as expected, the DB25 interface of the DSU can be tested. For this test the DSU is used in stand-alone mode and is powered by the internal backup batteries.

The image on the right shows how the X-Unit is connected to the DB25 socket at the bottom of the DSU. Once connected, the DSU is powered-up by pressing the two blue buttons together.

After the normal Power-On-Self-Test (POTS) sequence (RAM Test, Crypto Test, etc.), the text Interface Test appears on the screen.
Connecting the test device to the DSU

The DSU will now test each line of the DB25 interface and will show the progress on the LCD screen. Each test should produce an 'OK', e.g. Frequency Signals OK, Receiver OK, etc. When all tests are performed as expected, the message 'Interface OK' appears. The test is now complete.

Please note that this test can only be performed if the DSU contains healthy backup batteries. Although the original batteries will probably show 3.5V each when measured with a volt-meter, this voltage drops when the DSU is turned on. More information on the DSU-page.
Complete FS-5000 radio station DSU with test device (X) Connecting the test device to the DSU DSU with test device (X) Test device connected to the DB25 socket of the DSU Performing an Interface Test Interface OK Replacing the back-up batteries

The test unit is housed in a die-cast aluminium case with a removable lid at the top. The image on the rights shows the interior of the X-Unit after the lid has been removed. It is shown here with the DSU connector towards the front.

A vertical PCB is mounted directly to the DSU connector. It contains circuits for testing the interface of the DSU. The bottom PCB contains the dummy-load/attenuator, a small filter circuit and the power indicator. The connector for the transmitter is at the rear. Most of the pins are unused; only the antenna signal is used.
X-Unit interior

X-Unit interior X-Unit interior X-Unit interior X-Unit interior, top view Dummy load and indicator The DSU test-board at the rear Detail of the DSU test-board Detail of the DSU-board

  1. Feldstation FS-5000M, Technisches Handbuch, Teil 1, Beschreibung
    Field Station FS-5000, Technical Manual, Part 1, Description (German).
    March 1994.

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