126. Frankfurt 42 Mine Detector

a. General. The Frankfurt 42 mine detector was the standard German Army metallic-mine detector and was designed to detect mines of low metal content. It was superseded late in World War II by a modified version of the Wien 41 (par. 127) because of the develop-ment of the frequency-induction fuze.

b. Description. The Frankfurt 42 mine detector (fig. 155) is an induction bridge type. The search head is cylindrical in shape, with a flat area along the top and bottom. It is made od molded plastic in two trough-shaped halves joined and held together by screws. The extension handle consists of four extension pieces made of aluminum tubing. The lowest piece is permanently attached by a bolt to the search head at the hinge bracket. This piece will collapse against the search head to facilitate packing. A canvas case is provi-ded to store the extension piece and search head when the detector is dismantled (fig. 156). The detector unit and power supply are housed in a black plastic pack case. The pack case is the standard German pack case used with most of the mine detector sets. The case measures 14 inches by 13 inches by 4 3/4 inches. It is provided with a canvas carrying strap, and metal hooks for attachment of the pack harness. Access to the in-terior is provided by removing the end covers which are held in place by toggle clamps. The cover of the left-hand end bears the operator's printed instructions on its inner side. Removing this cover exposes the control panel for the detector set. At the bottom of the panel is the off-on switch. Above the switch is the nameplate, then the voltage tester and two 2-terminal sockets for head-sets and one 3-terminal socket for a microphone. The voltage tester consists of a double window and a test button. When the detector set is switched on, and the test button depressed, both parts of the window should glow. If the voltage is low, only the lower part of the window will glow. A control box for senitivity adjustment is also clipped into a socket on the control panel. The control-box cable plugs into this socket when the control box is detached and carried on the opera-tor's belt. The search-head cable plugs into a socket on the right-hand end of the pack case. Two standard headsets are included in the detector set. The entire detector set is stored in a wooden shipping case when not in use. The interior of the shipping case is divided by wooden partitions into compartments for the pack case, the search head and extension pieces in their canvas case, the pack harness, a spare battery, and spare electron tubes. The shipping case is provided with a folding metal handle to allow it to be carried sultcase fashion. The total weight of the detector set and shipping case is 54 pounds.

Figure 155. Frankfurt 42 mine detector.

c. Functioning. The Frankfurt 42 detector functions on the induction bridge principle. A push-pull oscillator has as its inductance an iron-core coil in the search head. Another iron core in the search head is connected to the input of a two-stage audio amplifier. The output of this amplifier is fed to the headset. Normally, the voltage induced in the coils is very small. On approaching a metallic object, the condition in the coils is upset and allows a stronger signal to emerge from the amplifier.

d. Operation.

(1)

Assembling.

(a)

Open the shipping case (fig. 157), keeping is nearly vertical, and remove the pack case. Keep the pack case standing upright. The battery tends to lose it electrolyte if it is not kept upright.

Figure 156. Components of the Frankfurt 42 mine detector.

(b)

Lay the shipping case down and remove the extension pieces and search head from their canvas case.

(c)

Thread the search-head cable through the extension pieces, starting with the sleeve end, and threading through the section with rim last.

(d)

Assemble the extension pieces and adjust the search head so it is horizontal in the sweeping position.

(e)

Plug the search-head cable into the socket inside the pack case on the side opposite the control panel and close the hinged cover.

(f)

Plug the headset into either socket labeled Fernhörer.

(g)

Remove the control box from the control panel and attach the male end of the control cable to its socket on the control panel.

(h)

Plug the female end of the control cable into the control box.

(i)

Attach the pack harness to the hooks on the pack case and place the pack case on your back.

(j)

Put on the headset.

(2)

Operating.

(a)

Turn the off-on switch clockwise.

(b)

Turn the control knob on the control box until the yellow dot appears. A loud signal should be heard. The yellow dot indicates high sensitivity; the blue dot indicates low sensitivity; and the red dot intermediate sensitivity.

(c)

Hold the search head well clear of metal and the ground.

(d)

Remove the adjusting tool from the lowest extension piece and rotate the re-cessed heads on the sides of the search head, first one, then the other, until the signal is reduced to a barely perceptible level. Sensitivity can also be redu-ced by turning the control knob.

(e)

Test the senitivity by bringing the wooden end of the test stick in contact with the bottom of the search head. A noticable increase in the signal level should be heard.

(f)

Commence sweeping, holding the search head 3 inches from the ground.

Figure 157. Shipping case for the Frankfurt 42 mine detector.

f. Sensitivity. The Frankfurt 42 mine detector, when properly adjusted, is one of the most sensitive German mine detectors. It is very sensitive to small iron objects.