April 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 Kennedy Superheterodyne Short-Wave Converter 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.
Short-wave converters and receivers have recently been sold in such large numbers as to require the expert attention of the average Service Man. The Kennedy Model 54 Converter, manufactured by Colin B. Kennedy Corp., South Bend, Ind., for instance, contains numerous points of ·particular interest to Service Men who previously have only attended to the peculiarities of standard broadcast receivers.
The Model 54 converter operates on the superheterodyne principle, and therefore includes a first-detector - a '24 tube, and an oscillator - a '27. Filament power is obtained from a built-in transformer; the "B" potential is obtained from the standard broadcast receiver, which also furnishes the remaining portions of the superheterodyne circuit - that is, the I.F. amplifier, second-detector, audio amplifier, main power pack; the "C" potentials are derived in the usual manner-as the potential across cathode resistors.
Of course, some broadcast receivers may already incorporate superheterodyne circuits. In such instances, the combination of converter and receiver constitutes a "double superheterodyne," in which the converter's tube arrangement would be referred to as first-detector V1, and first-oscillator V2; and the subsequent tubes in the broadcast receiver then will be designated, respectively, as: first R.F.; second-detector; second-oscillator; I.F. amplifier; third-detector; and audio amplifier.
Parts replacement values: Resistor R1. 10,000 ohms; R2, 400; R3, 25,000; R4, 50,000; R5, 5,000. Condenser C, oscillator tuning; C1, main tuning; C2, I.F. trimmer; C3, C4, 0.1-mf.; C5, C6, 0.25-mf.
The plate current obtained from the standard broadcast receiver for operation of the converter is very slight, and adds no noticeable burden to the power supply of most sets. Two wires, for supplying this power, come out from the side of the short-wave unit, and are to be connected to the long-wave (broadcast band) set. One is a black lead which is connected to the "ground" binding post on the long-wave set; the other is a red lead, which is connected to the negative side of the reproducer field coil (dynamic reproducer), to the speaker wire or connection carrying a filtered "B" supply, or (inside the chassis) to the positive end of the voltage-divider resistor.
Any source of "B" voltage from 150 to 250 volts is suitable. It should be obtained from some point in the long-wave receiver speaker or filter system, where it will receive fairly good filtering and be relatively free from hum. A lower voltage, well-filtered, is more to be desired than a higher voltage with a large proportion of A.C. modulation.
Obtaining this plate supply is very simple on many receivers, such as the Kennedy Models 210, 310, 220, 320, 1030, 632, 426, 526, 726, and 826. In these, the "B" supply may be taken from the tip-jack terminating the black speaker wire. In Kennedy Models 42, 50, and 52, it may be obtained at the speaker terminal panel from the side of the field winding which is common with the speaker-transformer primary. Similar points for connection, on other makes of receivers, are usually located without any difficulty. The correct terminal may usually be quickly found by testing from chassis, or ground, to the various speaker terminals. It should rarely be necessary to go inside the chassis to find a source of "E" power.
If difficulty is had in getting the unit to operate when initially hooked up, and the "B" source is suspected, 90 to 135 volts of "B" batteries may temporarily be tried. The red wire, of course, goes to the plus "B" lead, the black wire to the long-wave receiver ground post as before, and the negative "B" lead to the same ground post.
The antenna and ground wires are entirely disconnected from the long-wave receiver. They are to be connected to the posts marked "antenna" and "ground" of the short-wave unit, and left there permanently.
A wire is also connected from the antenna post of the long-wave set to the binding post on the rear of the short-wave chassis marked "L.W. Ant." This wire should be as short as practical to lessen any chance of broadcast stations being picked up and heard in addition to the short-wave signals.
It will be noted, facing the rear of the chassis, that on the left-hand side a wire has been brought out which may be connected to either one of two small binding posts near the end of the base. The purpose of this is to adjust the output impedance of the unit L2 to that of the antenna input circuit of the receiver with which it is to be used. The Kennedy models previously mentioned have "high impedance" antenna circuits and therefore require this wire to be on the upper binding post. In doubtful cases, this wire may be tried first on one and then on the other, with the unit operating, and permanently left where best results are obtained. These connections are indicated on the diagram and the photographic illustration.
The output of the short-wave unit is tuned. It is set, at the factory, for best operation at about 1,525 kc. Naturally, the long-wave receiver dial must be set at this point for short-wave reception, and left there. The R.F. portion of the long-wave receiver becomes the I.F. tuner and amplifier of the "short-wave superheterodyne." Just past 1,500 kc., a point easily reached on almost all broadcast receivers, has been found most desirable. The volume control on the long-wave receiver also acts as the volume control for short-wave reception; while finding stations, this control is to be turned full on.
In the event the long-wave receiver will not tune past 1,500 kc., or a strong local broadcast station interferes at that point, the output frequency tuning may be altered slightly to avoid the difficulty. An adjusting screw for this tuning may be reached through a hole in the rear of the chassis. It is located near the impedance adjusting wire and binding posts, and is to be adjusted with an insulated screw driver.
Set the main band-selector switch SW.1 on the position marked "15-25 meters," tune in a station (music or code) at about 50 on the right-hand dial. Then adjust the screw described until the left-hand dial also reads approximately 50 when correctly tuned in. This adjustment then holds for all wave-bands.
For best results, particular care should be taken in the installation of the antenna for short-wave reception. For instance, it should be clear of metal and grounded objects; and its lead-in should not be enclosed in metal conduit. Also, the antenna and ground leads should not be twisted together, or run tightly parallel for any considerable distance.
The right-hand dial of the short-wave unit (looking at the front of the instrument) might be termed the "station-finding dial," operating variable condenser C. It should be adjusted very slowly in order not to pass over the signals of the desired station. At the same time, the left-hand knob, controlling variable condenser C1, should be rotated so that the dials read approximately alike.
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.
Posted August 7, 2015