February 1933 Radio-Craft[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles from Radio-Craft.
In a continuing effort to provide archival material for researchers and for anyone seeking information on a particular radio restoration project, this Radio Service Data Sheet for the Zenith Model 430/440 radio from a 1933 edition of Radio-Craft is being posted. An Internet search will show that there are many people engaging in such activities. Restoring my Crosley Model 03BC console radio would have been more difficult if not for others who have done similar work to assist the 'community.' I generally despise the phrase "giving back" because it is usually uttered by people that really owe nothing to anyone, but somehow feel obligated to do so or are conditioned to automatically say such things. This is a case where I benefitted from somebody else's work and there is an opportunity to return the favor.
(Dual reproducers, Zenith class AAA A.F. power amplifier; illuminated auto type full-vision control escutcheons; 540 to 1750 kc. - 121 channels, including police band; silent tuning; A.V.C.; low-volume tone compensator; "shadowgraph" tuning; tone control; 110 V. or 220 V. line.)
The newest product of Zenith Radio Corp. is the model 430 receiver; equipped with the Zenith automatic tuning device, it is the model 440. The outstanding feature of this set is the "class AAA" system of A.F. amplification used;
Line potential, 115 V.; all controls maximum.
All tube readings, except heaters, are socket to ground, using 1,000-ohms-per-volt D. C. meter.
Resistor R1, manual volume control. 0.5-meg., total, tapped at 0.4-meg. from ground end; R2, tone control, 0.5-meg.; R3, quiet-tuning control, 400 ohms; R4, R6, R10, 1,900 ohms; R5, 150 ohms; R7, R25, 1,500 ohms; R8, R17, 0.25-meg.; R9, R16, 24,000 ohms; R11, R12, 50,000 ohms; R13, 0.1-meg.; R14, R18, 500 ohms; R15, 8,000 ohms; R19, R20, 1 meg, ; R21, 2,500 ohms; R22, 18,000 ohms; R23, R24, 400 ohms.
Condensers C1, C2, C3, tuning gang; C1A, C2A, C3A, tuning trimmers; C4 to C7, I. F. trimmers; C8, L. F. padder; C9, H. F. coupling condenser; C10, C11, C12, 0.2-mf.; C13, 0.1-mf.; C14, C15, C16, C17, C19, C20, C24 (25 cycles, 0.4-mf.), C30, C31, C35, .1-mf.; C18, 0.05-mf.; C21, C22, C23, C25, C26, C27, 8 mf.; C28, 0.5-mf.; C29, 25 mmf.; C32, C33, 500 mmf.; C34, 0.002-mf.
The condenser gang in this receiver is aligned at 1,500 kc.; the oscillator padder is aligned at 600 kc.
Field coil No. 1 has a resistance of 1,600 ohms; field coil No. 2 has a resistance of 2,000 ohms for section A and 3,500 ohms for section B. Choke Ch.2 has a resistance of 400 ohms.
There are no circuit changes or switches in this receiver for the reception of police calls. The tuning dial, itself, actually takes in the police frequencies since it covers a total range of 540 to 1,750 kc.
"Shadowgraph" tuning is effected by means of a vane fastened to a meter movement connected in the plate-supply circuit to tubes V1, V2, V3. The incoming signal causes this "meter" to move in the usual manner, thus twisting the vane so that it either obstructs the light and casts a wide shadow (off resonance), or permits the light to pass, casting a shadow no wider than the thickness of the vane (exact resonance of the tuned circuits).
Class AAA A.F. amplifier is the Zenith designation for a system employing three power tubes, each of which are biased as class A amplifiers, one of which is used to drive the other two in push-pull. If the grids were overdriven with the usual type of coupling transformer, having a high secondary impedance, between the driver and the output tubes and the push-pull grids overdriven, extreme distortion would take place. This difficulty is overcome by employing a special transformer of step-down ratio having larger wire and much less resistance in its secondary. This transformer is a very important part of the system. With such an arrangement it is possible to drive the grids of the power tubes positive without introducing distortion, because of the driver being a power tube and capable of supplying the necessary current on such peaks. The primary of this transformer has a higher impedance than is normally used, which value is somewhat reduced as the impedance changes in the secondary, when the power grids are driven positive. Its impedance, however, at the lowest value, still matches the driver plate circuit. It should be remembered that in an ordinary class A amplifier distortion enters when the grids are driven positive by virtue of the previous tube not supplying sufficient power for grid excitation of the output tubes. In other words, in class A, voltage on the power grids is available as long as they do not go positive. As soon as they are driven past zero in that direction the previous amplifier can no longer supply the necessary power.
Above, Arrangement of the tubes in the "430." Right, Comparative figures indicating the effectiveness of A.F. amplifiers classes A, push-pull; B, push-push; pentodes in push-pull, and AAA, push-pull, Watts rating, respectively, 4.3 W., 0.8- to 24 W., 6.3 W., 14.2 W.
Summing up the above, the triple A amplifier is a combination of both class A and class B. The exception being that the tubes are biased as class A, but by virtue of the power delivered by the driver and the special coupling transformer, sufficient power is supplied for the power tube to draw grid current and still prevent distortion as in the class B amplifier.
Many set analyzers will not accommodate the new tubes, consequently, all voltage readings are to ground. Thus, for instance. the actual voltage on the plates of the power tubes is 295 V.
The two reproducers are similar in construction but one is peaked at 90 cycles and the other at 70 cycles. (This is done to cancel the natural period of each reproducer.) The paralleled voice coils result in four times the power obtainable from a single reproducer, states the manufacturer.
An overall sensitivity of less than one-half microvolt-per-meter is obtained with an undistorted power output of 15 watts. The total line consumption at 115 V. is 125 W.
Posted March 24, 2015
Radio Service Data Sheets
These schematics, tuning instructions, and other data are reproduced from my collection of vintage radio and electronics magazines. As back in the era, similar schematic and service info was available for purchase from sources such as SAMS Photofacts, but these printings were a no-cost bonus for readers.