From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical bulletin detailing the design highlights of a Philco chassis and service guidelines for radio receivers.
| Identifier | ExFiles\Box 63\3\ scan0305 | |
| Date | 1st March 1935 | |
| PHILCO SERVICEMAN and RADIO MANUFACTURERS' SERVICE NEWS HIGH SPOTS OF PHILCO CHASSIS DESIGN Apart from the three fundamental requirements of sensitivity, selectivity and fidelity of the radio chassis, probably the next most important characteristic is that of mechanical and electrical stability. Casual observers have often wondered why a Philco chassis has a singular appearance quite different from that of contemporary manufacturers, having, as it does, an individuality all of its own. This characteristic is largely governed, first of all by the type of chassis, and secondly by the electrical and mechanical lay-out of the mechanical components in logical sequence to the electrical performance, which each of these components is designed to perform. The chassis base, for instance, is usually of cold rolled mild steel, cadmium plated for the reason that cadmium plate uses low radio frequency resistance paths and has all the same eddy currents which are picked up by the various shields covering the coil components of the receiver. Wherever a coil is housed in some form of shell or shield, currents are induced in the power shields by virtue of the magnetic field set up by the current traversing the turns or layers of the coil inside, and one of the most important functions of a shield on the chassis is to convey the minute currents to ground with the most conductive path possible, and considerable research has evolved the type of shield and valve covers which Philco invariably employ. This, coupled with the logical sequence of allocation of parts, naturally results in the “Individual Personality” of a Philco Chassis, characteristics which can be noted regardless of whether the chassis contains three or twenty-three valves. The stability and constant performance of a radio chassis depends upon a number of factors, the two most important of which are the changes in electrical constants which take place with temperature and humidity changes, and the freedom from mechanical feed back and “microphonics” inherent in original design. The changes in electrical constants, due to temperature is a problem upon which the Philco Company have spent many weeks of work, and it has resulted in a few small component parts of great importance, seldom noted by the casual observer. For instance, under the adjusting nut of each padder condenser or trimmer there is a small bakelite washer having special composition and dimensions. The electrical and mechanical contents of these washers do not vary more than two parts in one million with each degree of temperature rise or fall, and since these trimmers or padders are largely responsible for the condenser calibration, the scale of Philco Chassis will always indicate the true frequency to which the receiver is tuned, similarly the position and design of the intermediate frequency tuning condensers which are composed of specially designed and moulded units, which will remain constant in capacity regardless of temperature or humidity changes over very wide ranges. The design of Philco Chassis for the English market requires an entirely different set of rigid component tests than chassis which are designed for the American and Canadian markets, owing to the fact that a much higher percentage of moisture is experienced in the air of the British Isles, thus great attention is paid to small details, such as the insulation of the grid leads which project from the valve shields of the chassis and connect to the grid cap of the high frequency valves. Microphonic “feed-back” is seldom experienced in Philco Chassis, due to the fact that the variable condensers are always mounted on rubber cushions, and, in addition, every Philco Chassis itself floats on four large rubber cushions in its Cabinet, thereby preventing heavy vibrations from the speaker shaking the chassis (or components on it) in unison or synchronism, which would result in a very disagreeable howl. Philco Chassis never embrace useless “gadgets” which are to be found on many chassis of competitive makes, which are supposed to contribute to sales appeal, but have little or no use for the actual user. Anything which is essential to the proper operation is incorporated internally, and apart from the actual tuning knob and wave-change switch, the volume control (and on/off switch) is the only essential requirement. The underside of any Philco Chassis is a revelation even to those initiated in radio matters, and the simplicity with which parts can be changed (should servicing be necessary) is a point which is never forgotten by our engineers. This is particularly true in the design of receivers for use on cars, where cramped quarters for giving any type of service is the rule. On mains operated sets a whole radio frequency coil can be removed instantly with one bolt or screw, after unsoldering the coil connections. Every contact in the chassis is silver-plated, including each contact socket which engages the individual pins on the base of each valve. Every Philco fixed condenser is built to tolerances so exacting that in many cases prominent condenser manufacturers have practically declined to build condensers for us since we required such close attention to detail that they would have to deviate from their standard methods of production. This also applies to mains and low frequency transformers. Every single component or part of a component delivered to the Philco factory is fully inspected and tested prior to its incorporation in the chassis. THE GOLDEN RULE OF SERVICE The previous article has shown you that due to care in design and manufacture the possibility of breakdown in a PHILCO receiver is reduced to a minimum, but a radio receiver, like everything mechanical, is subject to a certain amount to wear. In a radio the first part to wear out is, normally, the valves. Valve life is dependent on so many external factors that it is always a matter for conjecture. It is therefore the golden rule of service that the valves be the first suspect. Always try the valves by substitution before attempting to seek the fault elsewhere in the circuit. In the case of a car-radio, first check the fuse, then substitute the valves, and finally change or test the vibrator. You will find that by doing this it will be very seldom necessary to remove the set from the car. PAGE 2. MARCH, 1935. | ||