July 1932 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.
This Radio Service Data Sheet for the Sparton Model 40 6-Tube T.R.F. Automotive Receiver is an example of the dozens of similar schematic and alignment instruction sheets that have been posted on RF Cafe over the years. Obtaining technical information on most things, even readily available items, prior to the Internet era was often very difficult - if not impossible. Service centers had what was need provided by manufacturers and distributors, but if you wanted to find a part number or service data on a refrigerator, radio, lawn mower, garage door opener, etc., and did not have the original paperwork, you were usually out of luck. Nowadays a Web search will quite often get you what you need thanks to people (like me) who go to the trouble of making the information available. The stuff doesn't just magically appear or get posted by benevolent governmental entities. You're welcome.
Lafoy-type Automatic Volume Control; Remote Tuning Control; Electro-Dynamic Reproducer}
To maintain constant signal output, regardless of the intensity of the incoming signal (within practical limits), to overcome the reduction in signal intensity which will occur in a given locality (due to metallic structures, ore deposits, etc.) it is necessary to incorporate some form of volume control which will operate to vary the gain in the amplification of the receiver in proportion to the loss in carrier signal strength. Most automatic volume controls or A.V.C. circuits operate to vary the control-grid of the amplifier tubes, in accordance with the A.F. modulation of the station's carrier; the "Lafoy" system, however, varies the control-grid bias more nearly in accord with the intensity variations of the station's carrier itself, the A.V.C., tube V6 in the diagram, functioning more nearly as an R.F. amplifier than as a detector.
High amplification in this set is obtained through the use of a three-stage R.F. amplifier incorporating screen-grid tubes of the "automotive" type, the output of this section feeding a screen-grid detector. The audio circuit comprises a single pentode, which is impedance- and resistance-capacity-coupled to the detector.
The values of the components are as follows: Condensers C1 C2, C3, C4, tuning units; C5, antenna compensator; C6, C8, C10, C12, 0.2-mf.; C7A 0.3-mf.; B, 0.2-mf. C, 0.3-mf.; C9A, 0.3-mf., B, 0.2-mf., C, 0.3-mf.; C11A, 0.3-mf., C, 0.3-mf.; C13, C14, 0.00025-mf.; C15A, 0.3-mf., n, 0.2-mf.; C16, coupling condenser, 0.01-mf.; C17, 0.0005-mf.; C18, 0.006-mf.; C19, 1.0 mf.; C20, 0.1-mf.
Resistors R1, R2, R5, R7, 20,000 ohms; R3, R4, R6, 5,000 ohms; R8, 30,000 ohms; R9, manual volume control, 1/4-meg.; R10, 1/4-meg.; R11, 160 ohms; R12, 350 ohms.
Correct methods for installing and servicing antennas and interference suppressors have been described in past issues of Radio-Craft. However, a little additional data is available.
For the aerial in collapsible type tops, we recommend that the "false top" type be employed. This type of aerial is constructed in the following manner:
Fashion two pieces of drill cloth that are the same color as the top material, as long as, and approximately six inches narrower than the roof. On one section, lay a piece of light weight felt of the same dimensions, and then lay on top of the felt a piece of 16-mesh copper screen wire the same size. On top of this wire, lay another piece of light weight felt and over this the second section of drill cloth, then sew the edges of the combination together.
The top deck is removed from the roof bows and the aerial is placed on top of thorn. The top decking is then placed back over the aerial.
where it is desired to let the top down, it is advisable to connect the aerial lead-in wire to the aerial at the rear, and let the shielding on this wire, extend only for a distance of about three feet from the receiver end. In such cases, the lead-in wire is run through the floor boards back of the seat underneath the car, up to the receiver.
Note that in this receiver there are two fuses; one of them is of 1/8-A. rating, and is connected in the "B" battery jumper wire, while the other is a 5 A unit located in the receiving unit near tile ground binding post.
Interference may be distinguished by the sound: Generator noises (eliminated by bypassing the commutator) are tone frequencies quite different from the staccato tapping sound of high-tension spark interference ; high-tension interference is a sharp, raspy sound and can be eliminated practically 100 percent by means of spark suppressors (on the distributor and spark plugs); low-tension breaker point noise is not readily distinguished from high-tension interference, but will be the sound remaining after spark suppressors have been installed. Low-tension breaker interference is difficult to eliminate. Try reversing the two primary leads to the coil; install a bypass condenser connected from the engine to the ammeter and switch lead.
Sketch of the battery box showing the location of the cable and the battery.
The operating voltage and current characteristics of this set are to be measured with a set analyzer equipped with a voltmeter of the 1,000-ohms-per-volt type; the manual volume control must be turned to the full on position, and with no signal reception.
The filament potential of all tubes is 6 volts. Plate potential, V1, V2, V3, V5, 135 volts; V4, 132 volts ; V6, zero. Control-grid, V1, v2, V3, 1.5 volts; V4, 10 volts; V5, V6, 18 volts. Screen-grid potential,V1, V2, V3, V4, 67.5 volts; V5, 135 volts; V6, zero. Plate current, V1, V2, V3, 3 ma.; V4, 0.1-ma.; V5, 8 ma.; V6, zero.
The antenna compensating condenser C5 is to be adjusted at the time the receiver is installed. Tune in a weak station between 1200 and 1400 kc., turn the volume control full on, and then, using an insulated screwdriver, adjust C5 for maximum receiver output. Never adjust either the C5, or the remaining trimmers, with the cover removed.
Circuit oscillation can be caused either by tubes or the receiver itself. Check the contact surfaces between the partitions and the rotor shaft, making sure that a good ground is obtained. Do not under any conditions oil the shaft under the contacts. Make sure that the receiver chassis is well grounded.
For best results it is essential that the receiver unit be located so that the remote control flexible-shaft runs direct (that is, without sharp bends).
The battery circuit for this receiver is unusual, as indicated in the diagram; an additional figure illustrates the connections.
The "A" battery consumption of this set is about 2 1/2 A.; the "B" requirements, about 20 ma.
Complete schematic circuit of the Spartan model 40 receiver using the new Lafoy system of automatic volume control.
Posted September 15, 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.