Radio and Television Today (Jan-Dec 1940)

SERVICE NOTES Compact Uses Direct Coupled Audio Tubes This 6-tube two band superhet has several interesting features from the serviceman's standpoint. The slight difference in the two models is shown in the circuit diagram. Model DP does not have the RF filter between the common B — bus and the chassis. This filter is made up of R8 and C25. The B — is ground directly to the chassis in DP and not in DPI. The RF and oscillator coils are shown connected in the Broadcast position, No. 1, which covers 540 to 1730 kc. No. 2 is the short wave range of 5.6 to 18 mc. A series wave trap in the antenna circuit tunes in the vicinity of the IF of 455 kc. This trap should be adjusted for minimum output indication at 455 kc, or to an interfering station operating near the ' IF. The signal generator should be connected to the antenna, through 0.0002 mfd. condenser, and its voltage adjusted to give some output indication even with the trap adjusted for maximum rejection. This will permit alignment to the "dip." IF ALIGNMENT AT 455 KC The first IF transformer is located on top of the chassis and is mounted with snap-on fasteners. The transformer may be removed by unsoldering the leads and pinching the clips together. The trimmers are located in the top of the can. The second IF transformer is mounted on the rear wall of the chassis and its trimmers may be adjusted through holes in the back. Set the band switch for broadcast position and put tuning condenser at minimum capacity. Adjust the signal generator to 455 kc. and connect its output to the grid cap of the 6K8GT mixer through a 0.02 mfd. capacitor. The grid cap is not removed from the tube. Start with the secondary trimmer of the second IF transformer and work back to the primary of the first IF transformer adjusting for maximum output indication in each case. The signal level used should be as low as possible and still give usable indication. The output meter should be connected across the voice coil. RF ALIGNMENT The trimmers for the antenna coil are mounted on the assembly behind and to the right of the variable condenser. The trimmer for the b-c band is in the middle and for the short wave range, it is on the bottom. The oscillator coil assembly is mounted on the rear wall underneath the chassis. The series padder for the b-c band is located in the center of the assembly with the short wave trimmer nearest the end of the set. The trimmer on the end of the coil nearest center of the set is for broadcast range. With the wave switch still in the broadcast position, set the tuning condenser at 1600 kc, and feed a signal of the same frequency through a 0.0002 mfd. capacitor. Adjust, first, the broadcast trimmer and second, the antenna trimmer (connected between the primary and secondary of the antenna coil) for maximum response. Set the signal generator and receiver at 600 ke. and adjust the series tracking padder for peak response while rocking the tuning condenser. The short wave band is aligned at 16 mc. with the signal generator connected to the antenna through a 400ohm carbon resistor. Adjust, first, the oscillator trimmer, and second, the antenna trimmer for maximum response. The voltages indicated on the schematic are for AC operation, 117.5 line volts. DIRECT COUPLED AUDIO The direct coupled audio amplifier stages using the 6AE5GT and 25AC5GT tubes is novel in its operation. The normal grid current drawn by the 25AC5GT is the same as the plate current of the 6AE5GT. The grid bias for the 6AE5GT is developed across the grid-cathode impedance of the 25AC5GT. Since the grid of the 25AC5GT operates in the positive voltage region, its internal impedance which is the cathode load of the 6AE5GT, is at any instance the grid voltage divided by the grid current. Since this impedance varies with the signal, a degenereation action takes place tending to reduce harmonic distortion. The same effect is sometimes obtained by using an un-bypasseed cathode resistor which increases the negative bias on the tube when the signal current in the plate circuit increases. EMERSON DP-332 DPh332 6SQ7GT 8 a C25 ARE OT USED ON DP 10DEL AND B3 GROUNDED. )P-I MODEL JSES R8 8C25 rflTH B NOT GROUNDED. SPEAK. 25AC5GT SOCKE o .05 -r * M Tone )>Switch r * "o^ — r s s T ■5 i * N — L .0042 f 25Z6 25AC5GT 6K7GT 6SQ7GT EBTOIMMMMMm^I 6AE5GT 6KBGT tia!