Abstract:
By providing dummy on projections  61   a ′ to  61   d ′ to cause dummy on status of the switches  62   a  to  62   d  at particular positions on the movale contact member pressure projections  61   a  and  61   d  in the rotor section  57  which turns on and off a plurality of switches  62   a  to  62   d  of the piano-touch type switch corresponding to the operation mode of the VTR, an operation mode detection device  56  which can clean the switch contact surfaces  62   a ″ to  62   d ″ by dummy on operation of the switches  62   a  to  62   d  using the dummy on projections  61   a ′ to  61   d ′ when the VTR proceeds from the standby status or the PLAY/REC mode to a next operation mode can be achieved. Thus, even when the piano-touch type switch is used for operation mode detection of the VTR, the switch contact members in the switches  62   a  to  62   d  are protected from reaction with substances in the air during the VTR is in standby status. This solves the problem of generating errors in operation mode detection and control resulting from noises generated during ON/OFF operation of the switches  62   a  to  62   d  due to sulfurization, oxidation or dust contamination caused by such reaction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an operation mode detection device to detect the operation mode position in the driving mechanism which drives and controls the tape cassette loading and the tape running statuses, such as recording, playing, fast forwarding, rewinding and stop of the magnetic tape in the magnetic recording and reproducing apparatus, and particularly, to a switch structure which enables cleaning of the contact members in the piano-touch type switch used as the operation mode detection device. 
     2. Description of the Related Art 
     A magnetic recording and reproducing apparatus (hereinafter referred to as “VTR (Video Tape Recorder)”) carries a tape cassette into the VTR, guides the magnetic tape in the tape cassette to a certain tape running mechanism and the circumference of a cylinder incorporating a rotary head and, according to various operations input from the input means of the VTR, runs the magnetic tape corresponding to the recording, playing, fast forwarding, rewinding or stop mode. 
     The VTR mechanism which arranges the tape cassette carrying and the magnetic tape running mode is described below with referring to the plan view of FIG. 4 showing a magnetic tape driving mechanism of the VTR. FIG. 4 shows the magnetic tape driving mechanism laid out on the back of the VTR chassis only. The driving mechanism comprises a plurality of mechanism members  31  to  55 . 
     In the figure, on the right side of a chassis  31  in which the magnetic tape driving mechanism of the VTR is placed, an electric motor  32  serving as the driving source is provided. The electric motor  32  is controlled by the operation mode setting means (not shown) for its revolution speed, number of revolutions and revolution direction. Mounted and fixed to the rotational axis of the electric motor  32  is a worm gear  33 , which makes rotations coaxially with the rotational axis of the electric motor  32 . A first gear  34  is engaged with the worm gear  33 . The first gear  34  consists of a large diameter gear  34   a  engaged with the worm gear  33  above and a small diameter gear  34   b  coaxial with the large diameter gear  34   a . Engaged with the small diameter gear  34   b  of the first gear  34  is a second gear  35 . A bevel gear (not shown) is formed on the flat surface of the second gear  35 . Engaged with the bevel gear on the second gear  35  is a large diameter gear  36   a  of a third gear  36 . The third gear  36  comprises the large diameter gear  36   a  and a small diameter gear  36   b  coaxial with it. Engaged with the small diameter gear  36   b  of the third gear  36  is a large diameter gear  37   a  of a detection gear  37 . The detection gear  37  comprises the large diameter gear  37   a  and a small diameter gear  37   b  coaxial with it. The small diameter gear  37   b  of the detection gear  37  is configured as a pinion and is engaged with a first rack  39  linearly formed at an end of a slider  38 . 
     In other words, the rotational driving of the electric motor  32  is reduced by the worm gear  33 , the first gear  34 , the second gear  35 , the third gear  36  and the detection gear  37  and transmitted to the first rack  39  and slides the slider  38  in the horizontal direction shown with an arrow in the figure. 
     Formed on the slider  38  are a sliding guide groove  40  to guide the linear sliding in the horizontal direction, a first guide groove  41 , a second guide groove  42 , and a third guide groove  43 . With the guide grooves  40  to  43 , guide posts  40 ′ to  43 ′ are mated allowing sliding. The guide post  40 ′ is fixed to the chassis  31 . Though details are not shown for the remaining guide posts  41 ′ to  43 ′, the guide post  41 ′, for example is provided with a pinch roller driving lever. When the slider  38  makes sliding operation, the pinch roller driving lever is moved corresponding to the shape of the first guide groove  41  so that the magnetic tape is held between the rotational axis (capstan) of a capstan motor  52  to be described later and the pinch roller so that it can run. Further, the guide post  42 ′ of the second guide groove  42 , for example, drives a reel base brake (not shown) and the guide post  43 ′ of the third guide groove  43  is, for example, a driving post to drive the tension lever for tape tension adjustment. The guide grooves  41  to  43  and the guide posts  41 ′ to  43 ′ are not limited to the above functions and they may have other functions. The second gear  35  also works to drive the cassette transfer mechanism for driving the cassette frame on which a tape cassette is placed (not shown) between the tape cassette load/unload position and the reel base position provided on the chassis  31 . 
     At the other end of the slider  38 , a second linearly formed rack  44  is provided. A small diameter gear  45   a  configured as a pinion to be engaged with the second rack  44  and a fourth gear  45  having a large diameter gear  45   b  coaxial with the small diameter gear  45   a  are also laid out there. Engaged with the large diameter gear  45   b  of the fourth gear  45  is a loading gear  46   a , with which the other loading gear  46   b  is engaged. To the rotational axes of the loading gears  46   a  and  46   b , fixed arms  47   a  and  47   b  are mounted and fixed so that the fixed arms  47   a  and  47   b  make rotations in the circumferential direction together with the rotations of the loading gears  46   a  and  46   b . At the ends of the fixed arms  47   a  and  47   b , an end of a swing arm  48   a  and an end of a swing arm  48   b  are rotatably mounted respectively. On the other ends of the swing arms  48   a  and  48   b , magnetic tape pull-out poles  49   a  and  49   b  are planted toward the surface of the chassis  31  (toward the back of the figure) respectively. These magnetic tape pull-out poles  49   a  and  49   b  make sliding guided by loading guide grooves  50   a  and  50   b  formed on the chassis  31 . Placed at the center of the loading guide grooves  50   a  and  50   b , a cylinder  51  on which the magnetic tape is to be wound. 
     In other words, when the electric motor  32  makes rotations for driving and slides the slider  38 , the fourth gear  45  engaged with the second rack  44  makes rotations and the loading gear  46   a  engaged with the fourth gear  45  makes rotations clockwise in the figure, and the loading gear  46   b  engaged with the loading gear  46   a  makes rotations counterclockwise in the figure. This causes the fixed arms  47   a  and  47   b  to make rotations clockwise and counterclockwise. The rotations of these fixed arms  47   a  and  47   b  lead the magnetic tape pull-out poles  49   a  and  49   b  at the end of the swing arms  48   a  and  48   b  to make sliding guided by the loading guide grooves  50   a  and  50   b . In this procedure, the magnetic tape of the tape cassette (not shown) is pulled out by the magnetic tape pull-out poles  49   a  and  49   b  and wound on the cylinder  51 . 
     Reference numeral  52  in FIG. 4 indicates a capstan motor, whose revolution speed, number of revolutions and revolution direction are controlled by operation mode setting means (not shown). This is the driving source which drives the magnetic tape to run and drives the reel base of the tape cassette to make rotations. A small diameter belt pulley  53  is fixed to the rotational axis of the capstan motor  52  and a belt  54  is stretched on the belt pulley  53 . The rotational driving of the capstan motor  52  is transmitted to a reel base driving belt pulley  55  by the belt  54 . The reel base driving belt pulley  55  provides the rotational driving to either the tape feeding reel base or the tape winding reel base of the tape cassette (not shown) corresponding to the operation mode. 
     In short, when the magnetic tape driving mechanism of FIG. 4 carries and puts the tape cassette to the certain position on the chassis  31 , the rotational driving of the electric motor  32  causes the second gear  35  to make rotations, which drives the tape cassette transfer mechanism (not shown). Thus, the cassette is placed onto the reel base, the magnetic tape is pulled out by the loading poles  49   a  and  49   b  from the tape cassette placed on the reel base and wound onto the cylinder  51 . Further, the rotational driving of the electric motor  32  slides the slider  38  so as to adjust the speed of the running tape and stop the tape by driving the reel base brake to brake the reel base by the second guide groove  42  and the guide post  42 ′, to press the pinch roller against the capstan by driving the pinch roller driving lever with the first guide groove  41  and the first guide post  41 ′, or to drive the tension lever (not shown) using the third guide groove  43  and the third guide post  43 ′, so that various operation modes of the VTR can be set. 
     Thus, the currently set operation mode can be learned by detecting the sliding position of the slider  38 . Further, when this operation mode changes to a next operation mode, the required sliding amount and traveling direction of the slider  38  (Number of revolutions and revolution direction of the electric motor  32 ) can be learned. To detect the operation mode position where the slider  38  currently is, the rotations of the electric motor  32  are transmitted, via the first to third gears  34  to  36 , to the detection gear  37 , which slides the slider  38 , and the rotation position of the detection gear  37  can be detected by a device  56  provided on the detection gear  37 . 
     Referring to FIG. 5, the configuration of the device  56  is described below. FIG. 5 shows the outline of the detection gear  37  and the device  56 . 
     The surface of the large diameter gear  37   a  in the detection gear  37  is provided with a circumferential groove  37   d  around a rotational axis  37   c  and a circular hole  37   e  is formed to the flat plane at the outer periphery of the circumferential groove  37   d.    
     On the other hand, the device  56  comprises a rotor section  57 , a stator section  58  and a terminal section  59  connected to the stator section  58 . Projected at the center of the surface on the rotor section  57  is, a cylinder  57   a  to be mated with the rotational axis  37   c  of the detection gear  37 . The rotor section  57  is further provided on its surface with a pillar  57   b  at the position corresponding to the hole  37   e  of the detection gear  37  so that the pillar  57   b  can be inserted into the hole  37   e.    
     Specifically, when the central cylinder  57   a  of the rotor section  57  is mated with the rotational axis  37   c  of the detection gear  37  and the pillar  57   b  of the rotor section  57  is inserted into the circular hole  37   e  of the detection gear  37 , the rotor section  57  makes rotations together with the rotations of the detection gear  37 . 
     Next, referring to FIG. 6, detailed configuration of the device  56  is described. FIG.  6 ( a ) is a perspective view showing the surface of the rotor section  57 , FIG.  6 ( b ) is a perspective view showing the back (inside) of the rotor section  57  and FIG.  6 ( c ) is a perspective view showing the back (inside) of the stator section  58  and the terminal section  59 . 
     As shown in FIGS.  6 ( a ) and  6 ( b ), the rotor section  57  is provided with a disc-shaped projection section  57   c  on its outer periphery. Further, as shown in FIG.  6 ( b ), the central cylinder  57   a  extended from the surface is formed on the back (inside) of the rotor section  57  and a plurality of movable contact member pressure projections  61   a  to  61   d  to be pressed against a plurality of switches  62   a  to  62   d  laid out on the stator section  58  (to be described later) is formed on circumferences with different diameters around the central cylinder  57   a . The movable contact member pressure projections  61   a  to  61   d  are positioned so that, when the rotor section  57  makes a rotation together with the rotation of the detection gear  37 , they can turn on and off the switches  62   a  to  62   d  of the stator section  58  corresponding to the angle of such rotation. 
     Referring next to FIG.  6 ( c ), the stator section  58  and the terminal section  59  are described. Planted at the center of the stator section  58  is a central axis  5   a , with which the central cylinder  57   a  of the rotor section  57  is to be mated. On the plane around the central axis  58   a , four switches  62   a ,  62   b ,  62   c  and  62   d  are laid out at positions in different distances from the central axis  5   a.    
     The switches  62   a  to  62   d  comprise movable contact members  62   a ′ to  62   d ′ and fixed contact members  62   a ″ to  62   d ″ respectively. The movable contact members  62   a ′ to  62   d ′ are commonly connected with a common connection conductor  63  and at the same time fixed to the plane on the inside of the stator section  58 . The fixed contact members  62   a ″ to  62   d ″ of the switches  62   a  to  62   d  are, via the conductors led to the terminal section  59 , connected to the connection terminals  64   a  to  64   d  and further connected to the control circuit (with reference numeral  70  in FIG. 7) for operation mode detection. 
     FIG. 7 shows an example of the circuit configuration for the device  56  in FIGS.  6 ( a ) to  6 ( c ). Among the connection terminals  64   a  to  64   d  of the terminal section  59 , the terminals  64   a  to  64   c  are detection terminals to detect ON or OFF of the switches  62   a  to  62   c  and the terminal  64   d  is a common terminal (grounding terminal) and is connected to the reference potential (to the chassis  31  shown in FIG.  4 ). The movable contact member for detection terminal  62   a ′ and the fixed contact member for detection terminal  62   a ″ constitute the switch  62   a  and the movable contact member for detection terminal  62   b ′ and the fixed contact member for detection terminal  62   b ″ constitute the switch  62   b , the movable contact member for detection terminal  62   c ′ and the fixed contact member for detection terminal  62   c ″ constitute the switch  62   c  and the movable contact member for common terminal  62   d ′ and the fixed contact member for common terminal  62   d ″ constitute the switch  62   d . The movable contact members  62   a ′ to  62   d ′ are commonly connected by the conductor  63 . The fixed contact members  62   a ″ to  62   d ″ are, via the connection terminals  64   a  to  64   d  respectively, connected to the control circuit  70 , which comprises a microcomputer, for example. As understood from the connection configuration of FIG. 7, the control circuit  70  can detect various operation modes by turning on of at least one of the switches  62   a  to  62   c  with the switch  62   d  turned on, i.e. by combination of the low level of the switch  62   d  and the low level of at least one of the switches  62   a  to  62   c.    
     Further, referring to FIGS.  6 ( a ) to  6 ( c ), in order to fix the rotor section  57  rotatably to the stator section  58  after the central cylinder  57   a  of the rotor section  57  is mated with the central axis  58   a  of the stator section  58 , the stator section  58  is provided with projections  65   a  to  65   c  on its outer periphery. The projections  65   a  to  65   c  are further provided with fixing pieces  65   a ′ to  65   c ′ to fix the projection section  57   c  of the rotor  57 . Further, the terminal section  59  is provided, on its both end faces, with fixing pieces  66   a  and  66   b  to fix the device  56  to the chassis  31  or other casing. 
     Referring to FIG. 8, the relation between the switches  62   a  to  62   d  of the stator section  58  and the movable contact member pressure projections  61   a  to  61   d  of the rotor section  57  is described below. FIG. 8 shows a sectional view models of the movable contact member pressure projection  61   a  and the switch  62   a.    
     Reference numeral  67  in the figure indicates an elastic piece to drive the movable contact member  62   a ′. It is formed from insulating material and is designed so that its head  67   a  can contact with the movable contact member pressure projection  61   a  and the bottom  67   b  is in contact with the movable contact member  62   a ′. When the movable contact member pressure projection  61   a  contacts with the head  67   a  of the elastic piece  67  and presses down the elastic piece  67  downward of the figure, the movable contact member  62   a ′ is lowered and comes into contact with the fixed contact member  62   a ″. A switch with such a structure is called a piano-touch type switch. For the switch contact members  62   a ′ to  62   d ′ and  62   a ″ to  62   d ″, silver or ordinary metal plates (brass, for example) plated with silver is used. 
     FIG.  8 ( a ) shows a case where the movable contact member pressure projection  61   a  of the rotor section  57  is positioned not in contact with the elastic piece  67 . In this status, the movable contact member  62   a ′ is at a distance from the fixed contact member  62   a ″ because of the hardness of its material and is not in contact with it, i.e. the switch  62   a  is in OFF status. When, from this status, the rotor section  57  moves from the right to the left as shown with an arrow in the figure and comes to the position as shown in FIG.  8 ( b ), the movable contact member pressure projection  61   a  of the rotor section  57  rides onto the head  67   a  of the elastic piece  67  and presses the movable contact member  62   a ′ downward in the figure so that it comes into contact with the fixed contact member  62   a ″, i.e. the switch is turned on. 
     Further, when the rotor section  57  makes rotation and the movable contact member pressure projection  61   a  leaves the head  67   a  of the elastic piece  67 , the movable contact member  62   a ′ leaves the fixed contact member  62   a ″ because of its own elasticity and goes back to the position in FIG.  8 ( a ). 
     Thus, the rotation of the rotor section  57  in the device  56 , together with the rotation of the detection gear  37 , detects various operation modes from combination of ON and OFF among the switches  62   a  to  62   d.    
     In order to detect the operation mode, the switch  62   d  of the device  56  is used as the common switch (grounding switch connected by the fixed contact member  62   d ″ to the reference potential). In the cassette eject mode where the tape cassette is discharged from VTR and the magnetic tape driving mechanism is in the standby status waiting for loading of a tape cassette, the switches  62   a  to  62   d  of the device  56  are turned on and the connection terminals  64   a  to  64   d  are, for example, all set to the low level. Then, in the tape loading mode where the tape cassette is carried to the VTR and the magnetic tape is arranged to the cylinder  51  and the rotational axis of the capstan motor  52  of the VTR, the switch  62   b  and the switch  62   d  are turned on. Thus, by checking the combination of ON status of the common switch  62   d  and ON status of other switches  62   a  to  62   c , operation modes including PLAY, STOP, FF(Fast Forwarding), REWIND and REC(Recording) modes are detected. 
     If, in the VTR provided with an apparatus to detect the operation mode, the VTR operation power is turned off without input of any new operation mode or timer recording is reserved after the tape cassette is put into the VTR and the tape loading mode is completed, some of the switches  62   a  to  62   d  in the device  56  are turned off. Until input of the next operation mode or arrival of the recording start time, the VTR continues to be in the standby status. When the standby status is kept long, the switch contact members in OFF status are exposed to the air and react with substances in the air, resulting in sulfurization or oxidation of the surface or dust contamination on the surface. When the VTR proceeds from this standby mode to the next operation mode and the switch in the OFF status is turned on, the above sulfurization, oxidation or dust causes noises. Such noises are supplied from the contact terminals  64   a  to  64   d  to the control circuit  70  and lead to erroneous operation in operation mode detection or control of proceeding to the next operation mode. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an operation mode detection device of the magnetic recording and reproducing apparatus which prevents noise generation which may cause errors in the operation mode detection and control by removing sulfurization or oxidation or dust contamination on the switch contact members in the device when the device having the piano-touch type switch structure is used for operation mode detection of the VTR without affecting the operation mode judgement of the VTR. 
     The operation mode detection device according to the present invention is, when operation mode setting means to set a plurality of operation modes of the apparatus by driving the mechanical members arranges any driving situation, driven for rotations corresponding to such driving situation of the mechanical members and electrically detects the set driving mode. 
     In a first invention, the above device comprises a stator section having a fixed contact member for common terminal and a plurality of fixed contact members for detection terminals, as well as a movable contact member for common terminal and a plurality of movable contact members for detection terminals laid out oppositely to the fixed contact members and usually positioned in a distance from the fixed contact members, and a rotor section provided with a plurality of movale contact member pressure projections which are driven for rotation corresponding to the driving situation of the mechanical members and selectively come into contact with the movable contact member for common terminal and at least one of the movable contact members for detection terminals at the same time corresponding to the rotational position and cause such movable contact members to come into contact with the fixed contact member to output the electric signal showing the operation mode, and a dummy on projection which comes into contact with the movable contact member for common terminal alone or with the movable contact member for detection terminal alone corresponding to the rotational position so that the applicable movable contact member comes into contact with the fixed contact member. 
     In a second invention, the above dummy on projection is provided at a position where the magnetic recording and reproducing apparatus is in the way of proceeding from the operation standby status to other operation mode. 
     Further, in a third invention, the above dummy on projection is positioned between the position corresponding to the operation standby status of the magnetic recording and reproducing apparatus and the position corresponding to the first operation mode laid out next to the operation standby status. 
     In a fourth invention, the above device is designed so that, when the magnetic recording and reproducing apparatus is set to an operation mode other than the first operation mode from the operation standby status, the rotor section rotates, via the position corresponding to the first operation mode, to the position corresponding to the applicable operation mode. 
     Still in a fifth invention, when the movable contact member for detection terminal and the fixed contact member for detection terminal are continuously in contact for a certain rotational angle, the movale contact member pressure projection which causes the movable contact member for detection terminal to come into contact with the fixed contact member for detection terminal is provided with a dummy off part to cancel contact between the movable contact member for detection terminal and the fixed contact member for detection terminal while the movable contact member for common terminal is not in contact with the fixed contact member for common terminal. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     FIGS.  1 ( a ) to  1 ( e ) are switch operation development diagram for every operation mode illustrating an embodiment of the operation mode detection device in the magnetic recording and reproducing apparatus according to the present invention; 
     FIG. 2 is a conceptual schematic diagram illustrating roughly the constitution of the back (inside) of the rotor section of the device in FIG. 1; 
     FIG. 3 is a sectional view of a switch contact member of a device used in the present invention; 
     FIG. 4 is a plan view showing the magnetic tape driving mechanism of the conventional magnetic recording and reproducing apparatus; 
     FIG. 5 is a perspective view of a detection gear and a device used in the conventional magnetic recording and reproducing apparatus; 
     FIGS.  6 ( a ) to  6 ( c ) are developed perspective views of a device used in the conventional magnetic recording and reproducing apparatus; 
     FIG. 7 is a circuit diagram for FIGS.  6 ( a ) to  6 ( c ); and 
     FIGS.  8 ( a ) and  8 ( b ) are sectional views to explain operation of the device as shown in FIGS.  6 ( a ) to  6 ( c ). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the attached figures, preferred embodiments of the present invention will be described in details below. FIG. 1 is a switch operation development diagram for every operation mode illustrating an embodiment of the operation mode detection device in the magnetic recording and reproducing apparatus according to the present invention. Note that the operation mode detection device of the VTR according to the present invention is, similarly to the conventional one, laid out onto the detection gear  37 , which causes sliding of the slider  38  constituting the magnetic tape driving mechanism by driving the electric motor  32  and is provided with a piano-touch type movable contact members. 
     The ON/OFF operation of the device  56  for every operation mode of the VTR according to the present invention is made in the same way as that shown in FIGS.  6 ( a ) to  6 ( c ): the switches  62   a  to  62   d  are turned on by the arranged positions of the movable contact member pressure projections  61   a  to  61   d  of the rotor section  57 . FIG. 1 shows the operation statuses corresponding to the positions of the movable contact member pressure projections  61   a  to  61   d  laid out for every rotational angle of the rotor section  57  and the switches  62   a  to  62   d  of the stator section  58  are turned on and off according to the positions where the movable contact member pressure projections  61   a  to  61   d  are laid out. 
     FIG.  1 ( a ) shows the ON/OFF status of the switch  62   d , FIG.  1 ( b ) shows the ON/OFF status of the switch  62   a , FIG.  1 ( c ) shows the ON/OFF status of the switch  62   b , FIG.  1 ( d ) shows the ON/OFF status of the switch  62   c  and FIG.  1 ( e ) shows the operation mode determined by combination of ON statuses of the switch  62   d  and switches  62   a  to  62   c.    
     The switch  62   d  shows a common switch (grounding switch with the fixed contact member  62   d ″ in contact with the referential potential point such as chassis  31 ). From the combination of ON statuses of the common switch  62   d  and other switches  62   a  to  62   c , operation modes such as PLAY, STOP, FF(Fast Forwarding), REW(Rewind) and REC(Recording) can be detected. 
     In the cassette eject mode without any tape cassette in the VTR, the switches  62   a  to  62   d  are turned on by the movable contact member pressure projections  61   a  to  61   d . Then, in the tape loading mode where a tape cassette is put into the VTR and the magnetic tape is arranged onto the cylinder  51  and the rotational axis of the capstan motor  52  of the VTR, the movable contact member pressure projections  61   b  and  61   d  turn on the switches  62   b  and  62   d . Similarly, as shown in the figure, the rotor section  57  is provided, for each of its rotation angles, with the movable contact member pressure projections  61   a  to  61   d  so that the switches  62   a  to  62   c  other than the common switch  62   d  are turned on and off differently for different modes: REVIEW, PLAY, REC, STOP and FF/REW. Note that the magnetic recording and reproducing apparatus according to this embodiment is designed so that soft braking is applied to the reel base on the tape feeding side. In FIG.  1 ( e ), FF/REW 1 and FF/REW 2  show that two positions (or two modes) are required because the soft braking is applied to different reel bases corresponding to the winding direction. 
     The VTR waits in the standby status for the longest time in the cassette eject mode without any tape cassette in the VTR. In this mode, the switches  62   a  to  62   d  of the device  56  are set so that they are all turned on as shown in FIG.  1 . Here, the movable contact member  62   a ′ to  62   d ′ and the fixed contact members  62   a ″ to  62   d ″ of the switches  62   a  to  62   d  are in contact and the switch contact members are not sulfurized or oxidized or contaminated with dust. However, if the VTR is left in the status with a tape cassette in the VTR and the magnetic tape arranged onto the certain tape running mechanism (including the cylinder and the rotational axis of the capstan) in the tape loading mode without input of the next operation mode or if the recording mode is reserved with the timer, one to two switches among the switches  62   a  to  62   d  of the device  56  are left in the OFF status or the standby status until the input of the next operation mode. During such standby period, the switch contact members are sulfurized or oxidized or contaminated with dust. 
     On the other hand, suppose execution of any operation mode after tape loading with a tape cassette put into the VTR. The mechanical system takes the PLAY/REC mode position, which is the same status as that when the VTR makes reproducing and recording operations, as the reference position. The system is designed so that, from the PLAY/REC mode position, it once proceeds to the REVIEW mode, which is the first operation mode, and then goes to other operation mode. In other words, the mechanical PLAY/REC mode position is used as the standby position and the system is arranged so that it proceeds from that position, via the first operation mode position, to other operation modes. Specifically, the rotor section  57  of the device  56  once makes rotation in a certain direction from the PLAY/REC mode position to go back to the REVIEW mode and then makes rotation in the reverse direction to go to the STOP, FF/REW or other operation mode. Thus, in the standby status above where the VTR is in the PLAY/REC mode (i.e. the mechanism is in the same status as the PLAY/REC mode but any playing or recording operation is not executed electrically), the switch  62   b  of the device  56  is left in the OFF status for a long time and the contact member of the switch  62   b  may be sulfurized or oxidized or contaminated with dust. 
     In this embodiment, a new mode to turn on the switches  62   a ,  62   b  and  62   d  of the device  56  as dummies (hereinafter called “dummy on”) is provided between the PLAY/REC mode and the REVIEW mode, for example. As shown in FIG. 2, dummy on projections  61   a ′,  61   b ′ and  61   d ′ for this dummy on mode are provided between the PLAY/REC mode position and the REVIEW mode position of the rotation angle on the rotor section  57 . The dummy on projections  61   a ′,  61   b ′ and  61   d ′ shown in FIG. 2 are provided in addition to the movable contact member pressure projections  61   a ,  61   b  and  61   d  as shown in FIG.  6 ( b ). Note that FIG. 2 just shows the outline of this embodiment in comparison with the schematic configuration of FIG.  6 ( b ) and does not show the exact switch structure. The operation mode timings corresponding to the exact switch structure are shown in FIG.  1 . The VTR is designed so that, when any operation mode is set, it leaves the standby status, goes to the dummy on mode and REVIEW mode positions and then proceeds to the set mode. For this operation, as the REVIEW mode is not set as the VTR operation mode, the VTR does not stop at the REVIEW mode position. 
     In other words, the device waits in the PLAY/REC mode status and, when the next operation mode is instructed, it proceeds, from the standby status, to the dummy mode and the review mode and then to the operation mode as input. During this procedure, the switches  62   a  and  62   b  of the device  56  only are turned on and then the switch  62   d  only is turned on (dummy on mode). Sulfurization or oxidation or dust contamination generated on the switches  62   a ,  62   b  and  62   d  are removed in this mode. Since the switch  62   d  is not turned on at the timing when the switches  62   a  and  62   b  are turned on, and the switches  62   a  to  62   c  are not turned on at the timing when the switch  62   d  is turned on, any operation mode signal is not supplied to the control circuit  70  connected to the connection terminals  64   a  to  64   d  and the switch contact member cleaning only is executed. 
     Though dummy on operation of the switch  62   c  has not been described, it is easily possible to provide a dummy on projection  61   c ′ in addition to the movable contact member pressure projection  61   c  in FIG.  6 ( b ) and thereby cause dummy on status as in the case of other switches  62   a ,  62   b  and  62   d.    
     Then, referring to FIG. 3, removal of sulfurization or oxidation or dust contamination on the switch contact members by the ON operation of the switch  62   b  in the dummy on mode will be described below. The same reference numerals are given to the same parts shown in FIGS. 6 to  8 . The applicable section in the switch  62   b  of the device  56  is used for explanation. 
     The movable contact member  62   b ′ in the switch  62   b  of the device  56  is gradually pressed via the elastic piece  67  by rotation of the dummy on projection  61   b ′ of the rotor section  57  and the end of the movable contact member  62   b ′ slides on the surface of the fixed contact member  62   b ″ to the position shown with broken line in FIG. 3 in the direction of arrow A. In other words, the end  62   b ′ of the movable contact member  62   b ′ makes sliding with being in contact with the surface of the fixed contact member  62   b ″ for area B as shown in FIG.  3 . By the sliding friction between the end  62   b ′ of the movable contact member  62   b ′ and the surface of the fixed contact member  62   b ″, sulfurization or oxidation or dust on the switch contact member can be removed. 
     After removal of sulfurization or oxidation or dust on the switch contact members in this dummy mode, turning on of the switch  62   b  in other operation mode does not cause any noise generation because sulfurization or oxidation or dust has been already removed. 
     To sum up, according to this embodiment of the present invention, the switch structure of the piano-touch type switch for operation mode detection is installed in the magnetic tape driving mechanism to drive and control the VTR operation mode, and is provided with the dummy on projections formed at the movable contact member pressure projections to turn on and off a plurality of switches in the above piano-touch type switch by rotations linked with the magnetic tape driving mechanism. Upon input of a new operation mode after a long standby status with a tape cassette put in and loaded to the VTR, the dummy on projections once turn on the switches to clean the switch contact members by sliding friction. 
     If, as shown in FIG. 1, there exists a relatively long period C in which a switch is continuously turned on while the rotor section  57  makes a rotation for a certain angle such as the period between the PLAY/REC mode and the STOP mode for the switch  62   a , the VTR may be designed so that it goes off (dummy off) while the common switch  62   d  is not turned on during that period and then it comes on again. Thus, the contact members are cleaned to improve the electric contact status. To once turn off the VTR (dummy off), a part of the movable contact member pressure projection  61   a  may be cut (deleted) (i.e. a dummy off part  61   a ″ is formed in the configuration as shown in FIG.  2 ). 
     Though the dummy on position has been arranged between the standby PLAY/REC mode and the REVIEW mode in the above description, it is acceptable to have “dummy on” at other positions. Further, proceeding from the standby status to other operation mode has been described to be made via the dummy on position and the REVIEW mode position before going to the applicable operation mode, but the dummy on position may be provided at all positions where it is possible to have such dummy on mode. Specifically, within the rotation range of the rotor section  57  of the device  56 , dummy on projections may be provided at all rotational positions (periods) where the switch  62   d  alone goes off or at all rotational positions (periods) where the switch  62   d  alone is turned on, for example. In such case, the VTR can directly goes from the standby status to the selected operation mode without going through the REVIEW mode as described in FIG. 1 after leaving the standby status in the PLAY/REC mode. This is because the dummy on position can be also set between the standby status and the operation mode to which the VTR is directly proceeding. 
     As described above, according to the present invention, an operation mode detection device free from any erroneous judgment of the operation mode or erroneous control can be provided. Before input of any operation mode, the VTR has a tape cassette put in and loaded to it and is in the standby status. Upon input of a new operation mode, the switch contact members of the switches to detect the operation mode are set to the dummy on status and are cleaned so that noise generation during ON/OFF operations of the switch can be prevented in advance and the switch ON/OFF status can be exactly detected. 
     It is naturally understood that the present invention is not limited to the embodiment as described above. It can be modified in various ways without departing from the spirit of the invention.