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P O L Y T E C H . N U

ARC51 frequency selector

imageThis article is about a preset frequency selector panel. It's unknown where this frequency selector is used. The frequency selector is able to 'memorize' mechanically 20 frequencies. It's assumed that this panel is used for the ARC/51 airborn transceiver since the part number is '5300/CP ARC/51'.

The device I obtained had two missing screws that resulted in a freely rotating knob. I 'upgraded' the thread to M4 and new hexagon socket set screws are placed.

noteSome avionics devices (from before 1960's) are painted with (illuminating) radioactive paint.
This device is checked and no radiation is detected.

image The user operation is rather simple. At the front panel is only one knob available. This is an aluminium knob that can be rotated (in both directions) to select the desired frequency preset. Each preset is marked with a number between 1...20. The selected preset is marked with a white arrow. The pilot can select the desired radio frequency preset by just rotating this knob.

setting frequency presets
imageThe frequency is also rather simple. When the panel is removed from the airplane, the top side of the device is available. There's a spring loaded 'door' that can easily be lifted. Underneath the 'door' is a drum visible with movable studs. By strategically placing the studs, the desired frequency between 200,00 MHz and 399,95 can be selected. By rotating the knob at the front panel, the drum rotates. The corresponding channel number is also shown at the right. Remind that the channel number at the front is not the frequency that can be set at the same time due to the mechanical design. There's a channel number shift of five applicable. There's a built in tool connected to a spring loaded string to prevent losing the tool. With the tool it's possible to slide the studs to another position. The first stud is used to switch between 200,00 and 300,00 MHz marked with a '2' and '3'. The next three numbers are set by removing two studs that corresponds to the available truth table. There's a two out of five possibility that makes exactly ten possibilities where always two studs are used. The last number is to switch between 00 and 50 kHz. This makes that the channel steps are 50 kHz.

For reference, the frequencies programmed in my device were:
01: 317,500 MHz
02: 257,800 MHz
03: 385,400 MHz
04: 344,000 MHz
05: 368,300 MHz
06: 340,300 MHz
07: 364,200 MHz
08: 355,600 MHz
09: 312,900 MHz
10: 249,250 MHz
11: 278,350 MHz
12: 380,150 MHz
13: 373,900 MHz
14: 291,450 MHz
15: 243,600 MHz
16: 275,950 MHz
17: 242,600 MHz
18: 396,500 MHz
19: 267,350 MHz
20: 297,800 MHz

imageSince the device is rather simple, the connections are also simple. There's only one connector available. This is a circular connector shell size 16 with 26 pins of size #20.

inner working
The inner workings are rather simple. The rotatable drum has movable studs. Each stud lifts a mechanical contact that acts as a closed switch. So for each selected frequency are eight switches closed. The connected transceiver 'computes' the desired frequency and sets the desired transmit and receive frequency. By rotating the drum, the applicable contacts are lifted for each selected channel. The switch contacts are rather crude and are open switches, so failure is possible fore sure due to dirt on the contacts. This is not uncommon for the 'technology' back in de days. The good thing is that no sensitive electronics is used, so this rather primitive device will last very long!


reverse engineering/schematic
The reverse engineering was done within half an hour. Creating the digital drawing took almost more time... The device consists of 19 switches that have one common contact 'a'. Each other end of the switches are connected to an individual pin of the connector. There's also a chassis ground pin 'X' that is also used for the panel illumination power return. The panel illumination lamp is powered by applying power to pin 'Y'. I guess the schematic is rather self-explanatory.