The following review is taken from the October 1937 edition of Television and Short-Wave World magazine.
|FIRST DETAILS OF|
| THE PHILIPS
|20-INCH PICTURES BY PROJECTION|
ALTHOUGH several manufacturers of television receivers have been carrying out research in the design of cathode-ray projection receivers, the Phillips' receiver publicly demonstrated at Radiolympia is the first to be produced as a commercial article and its advent created something of a sensation. H.M.V. also showed a projection type receiver, but this and also the Philips' receiver were removed early during the course of the exhibition and many visitors were disappointed in not being able to see them in operation.
In the new Philips' projection receiver the television image measuring 2 in. by 1.6 in. is formed on the end of a 4 in. cathode-ray tube and is brilliant enough to be enlarged 100 times when projected via a 45° angle mirror in the inside of the lid of the cabinet on to a screen measuring 20 in. by 16 in.
The screen is incorporated in the cabinet and comes into view when the lid is raised. This screen is of the back projection type and is made on the 3-ply principle with the etched surface enclosed between two sheets of plain glass to prevent discoloura- tion or soiling of the screen. A slow-motion device is provided to prevent damage to the screen by too rapid closing.
The cathode-ray tube has electro-magnetic focusing and deflection and an H.T. voltage of 25,000 volts is used on the anode. This high voltage supply is provided by two H.T. rectifiers in a voltage doubling circuit and the whole of the H.T. unit is entirely enclosed in a metal chamber with an interlocking door which breaks the supply voltage on being opened and earths the H.T. terminals of the condensers.
The television receiver is built up on three chassis which are bolted to a framework so constructed that the whole of the receiver can be removed from the cabinet for servicing. The three units are:
|(1)||The television sound and vision receiver and the synchronising amplifier.|
|(2)||The time bases for frame and line frequencies with the power pack for the amplifiers, but excluding the extra high tension supply.|
|(3)||The E.H.T. unit combining the H.T. transformer and con- densers, also .the cathode-ray tube comp'artment with the pro- jector lens unit.|
A broadcast receiver is included, but is separate from the television assembly frame and is similar to the standard 785AX type, a 5-valve high fidelity 3-waveband receiver with mono-knob control on the front of the cabinet.
The loudspeaker is mounted high up in the cabinet to give the impression that the sound accompaniment emanates from the screen.
Twenty-eight valves are used in the receiver, twenty-three for television and five for the broadcast set.
The circuit of the television receiver commences with a radio-frequency amplifier with a TSP4 valve for both sound and vision. Separate triode-hexode frequency changers Type TH4A are used for sound and vision.
The sound is taken to a TSP4 I.F. amplifier on 7.6 Mc and then to a 2D4A for detection and A.V.C. From here the sound is fed into the pick-up sockets of the broadcast receiver and controlled for volume and quality by the mono-knob on the front of the cabinet.
The vision after the frequency changer goes to four stages of I.F. amplification on 11.1 Mc with TSP4 valves and then to a 2D4A for detection and a Pen A4 output pentode. A further diode is used to obtain the D.C. control for the athode-ray tube.
The synchronising signal is taken from the output of the vision amplifier to a further TSP4 amplifier and a diode which supplies the synchronising impulses to the slow (frame) and fast (line) time bases. The line frequency or fast time base employs four valves; a Pen A4 feeding two Pen 428 valves in parallel. The frame frequency or slow time base has two valves, a gas triode, type GT4H, feeding into a Pen A4. The H.T. for the time bases and amplifiers is provided by two IW4 valves.
The main on/off switch for television and broadcast is operated by drawing forward the tilting dial which, in the off position, lies flush with the front of the cabinet. The wavechange and television switch is operated by the moulded ring which encircles the mono-knob on the front of the cabinet.
For television there are four main controls grouped into two concentric pairs located on a sunk panel under the lid and in front of the screen. The right-hand pair only need occasional adjustment, as they control the tuning (large knob) and the focus on the cathode-ray tube. The tuning control combines both sound and vision, the correct tuning for vision being the point where the sound accompaniment has maximum volume for a given setting of the volume control.
The left-hand pair are: A large knob for background, which controls the amount of light on the screen by varying the bias on the cathode-ray tube, and a small knob for contrast, which varies the gain of the vision amplifier.
The pre-set controls are located to the right of the main controls under a small hinged cover to prevent accidental mis-adjustment. The left-hand pair control time-base frequencies, the middle pair control size of picture on the screen, and the right-hand pair control centring of picture on the screen. In every case the small knob controls the vertical and the large knob the horizontal components of the picture.
Excellent pictures were obtainable with this instrument, and the picture being projected on to a flat screen is quite free from any distortion. The extra high sensitivity enables results to be obtained outside the normal service area, provided conditions of local interference permit.
|Interior of the projection set, taken from Wirless World magazine for 3rd September 1937.|
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