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).
Service news bulletin for radio receivers, covering oscillators, wiring modifications, and set testers.
| Identifier | ExFiles\Box 63\3\ scan0306 | |
| Date | 1st March 1935 | |
| PHILCO SERVICEMAN and RADIO MANUFACTURERS' SERVICE NEWS 091 OSCILLATOR With the increasing use of short-wave receivers it is becoming essential for dealers to be in a position to service this class of instrument. The Philco crystal-controlled oscillator is specially designed for balancing all types of short-wave receivers. The fundamental frequency of the crystal is 3.6 megacycles with an accuracy of plus or minus .1 per cent. at 60 degrees Fahrenheit. Sufficient output is obtainable up to the tenth harmonic (36 megacycles) to render this oscillator suitable for adjusting any make of short-wave receiver or converter. It is quite impossible to service any short-wave receiver correctly without using this type of oscillator. The list price is 10 Guineas. MODEL 233 We issued a “change slip” for this model on February 21st, showing how it was possible to reduce the High Tension consumption without detracting from the performance. This alteration is very simple and most efficacious; this is the method adopted:— A 15,000 ohm. ¼ Watt resistor Pt. No. 6208 is shunted by a .05 Mfd. tubular condenser Pt. No. 30-4020; this combination is inserted in series with the H.T. feed to the screen of the type 2101 output pentode, thereby reducing the effective applied screen potential. To fit, disconnect the three leads from the screen terminal on the 2101 valve socket: (a) buff lead from the output transformer primary, (b) yellow lead from battery cable, (c) lead connecting with junction of resistors 14 and 24. Remove the 1 meg. resistor 25 from the terminal lug carrier on condenser “7.” Transfer the green lead from the battery cable to the lug carrying one end of the resistor 26, and transfer the wire (d) leading from the condenser 22 to the control grid of the 2101 output valve socket. Note that resistor 26 is shown in Service Bulletin 20 as 51,000 ohms.; all production has employed a 490,000 ohm. resistor (yellow-white-yellow) in this position. This leaves one lug on the terminal strip (part number 38-5772) free. NOTE: This last change may already have been made before despatch from the Factory. Rotate the free lug on its mounting rivet to point towards the output socket, and connect the three wires (a), (b) and (c) thereto. Mount the 15,000 ohm. (brown, green, orange) resistor and .05 Mfd. condenser between this lug and the screen terminal on the 2101 socket. It is recommended that the lead (c) be dressed as far as possible across the rear of the chass’s together with the lead going to the midd’e lug on condenser 22, in order that stray couplings which might cause instability, might be obviated. [Diagram for MODEL 233 with the following labels] 1 MEG. RESISTOR 25 REMOVE COMPLETELY 490,000 ω RESISTOR 26 GREEN BUFF (a) YELLOW (b) (c) (d) 15,000 ω RES. .05 mfd CONDR. Type 2101 OUTPUT VALVE REAR OF CHASSIS Should you desire to change any receiver in stock or already sold, send serial number to your Distributors and material will be sent to you free. PAGE 3. DRESSING OF WIRING The importance of “dressing” the wiring of a receiver is a point which is often either overlooked or neglected by servicemen. With the modern super-sensitive receivers small differences of capacity and inductance may make a very great difference to performance. This is particularly true of short-wave receivers, consequently when a component is moved or changed it is essential that the original location of all the wires be noted and replaced with the utmost care, particular attention being paid to the proximity of the wiring to the chassis and to adjacent wiring, as there are parts of a circuit where quite a small movement of one wire may mar the performance. Low frequency oscillation may be caused by the coupling between wires, and may be cured by “re-dressing.” This “dressing” is largely a matter of patience and experience, and, unless a specific set is referred to, cannot be taught; it is nevertheless an important part of a serviceman’s work, and becoming more so as the short-wave receiver becomes popular. 048 SET TESTER For some purposes it is desirable to reduce the percentage modulation on the 048 and 042 Oscillator, and at the same time increase the frequency of the note. This is easily accomplished by lowering the value of the grid leak resistance. We recommend that this be done by paralleling another resistance across the leak, in order to reduce the value to approximately 200,000 ohms. This figure will give a note of, approximately, 1,000 cycles, with considerably reduced modulation. It is also useful to generate a silent carrier (for example, when trimming a very sensitive set with efficient A.V.C.). This may be done by mounting a single pole change-over switch on the top panel, the moving arm of which is connected to the control grid of the valve and the grid condenser—one contact with the existing grid leak, and the other contact with a 400,000 ohm resistance, the free end of which is connected to the cathode side of the grid condenser. This will enable the leak to be connected between grid and cathode or across the grid condenser. The latter will eliminate the self-modulation and give a silent carrier. Herewith a sketch which should make the foregoing clear:— [Circuit diagram for 048 SET TESTER with a 400,000Ω resistor shown] | ||