Popular Mechanics/Volume 49/Issue 1/Selectivity Feature of Aero-Dyne Six
By FRANK L. BRITTIN
Selectivity, achieved with a minimum of controls, has been the aim of circuit designers for some time, in a set of the tuned r.f. type. In the six-tube set described in this article, selectivity and sensitivity have been attained by the use of specially designed coils and a well-balanced circuit without adding any more controls than absolutely necessary. These coils are similar in construction to the low-loss coils employed in the 15 to 550-meter set. described in the November, 1926, issue, but are of the stationary instead of the plug-in type.
{{c|Rear View of Completed Set, Showing Simple Arrangement of Parts and Method of Wiring above Subpanel
The set employs three stages of r.f. amplification, and has two controls of the modern drum type, facilitating easy operation of the set. A separate tuning condenser is used for the first r.f. stage, permitting perfect tuning, regardless of the length of aerial used. The detector and two stages of transformer-coupled a.f. amplification, with power-tube equipment in the last stage, complete the circuit. If all tuning controls were grouped for operation with one dial, an ideal condition would be realized, but, if this is done, it would be difficult to obtain perfect matching of the tuning units over the entire waveband. It would be possible to bring the receiver into perfect resonance at one wavelength on the dial, but on turning the dial to other wavelengths, this would not be true. Another point taken into consideration by the designer was the matching
of the coils. It was found that in the common practice of testing coils at only one wavelength of the broadcasting band, two or more coils might be perfectly matched at the wavelength at which they were tested, and yet, at some other wavelength, would be hopelessly out of tune. By testing the coils at both the upper and lower ends of the band, the coils so matched would be in close agreement over the entire band, therefore these coils are supplied in matched sets, tested at both 250 and 500 meters. The condensers used are constructed in such a way that they can be assembled in the receiver and either a long control shaft, or insulated, flexible couplings substituted for the regular shafts without disturbing the accuracy of the units. Small neutralizing or balancing condensers, made by the same manufacturer are very useful for equalizing the units of the multiple-tuning condenser, and may be fastened directly to the socket binding posts or to the condensers, simplifying the wiring connections. The audio stages consist of two heavy-type a.f. transformers, together with a power tube, making possible beautifully rounded reproduction of the bass notes. Drilled and engraved panels are available; however, for those who wish to drill their own, full-size drilling templates of the diagrams appearing on page 129 have been prepared.
Above, Schematic Diagram; Lower Left, Front Panel Drilled, Showing Holes for Drum Dial Windows; Right, Front View of Assembled Panel
The wiring diagram on page 126 shows the simple arrangement of the instruments with the coil bases and sockets marked for position. Note, in the photo on page 129, that the subpanel is cut away at front to make room for the rheostat-switch and the potentiometer. Mount the instruments on the front panel first, as shown in the photos on this page. The brackets are then screwed to the underside of the subpanel, and the subpanel assembly completed. No. 14 round, tinned-copper busbar wire is used for wiring throughout the set. It will be noted that the last coil at the right, mounted close to the front panel, has a hinged primary which allows a very close adjustment of the coupling for aerials of various electrical characteristics, and different selectivity requirements. In each case, the No. 6 terminal of the coils is mounted as close as possi- ble to the grid terminal of the tube socket to which it is to be fastened. It will also be noted that, in many cases where a lead should go through the subpanel, a special hole has not been drilled, but the lug has been joined to a soldering lug placed under a near-by mounting screw and the circuit picked up from the screw under the subpanel, thereby simplifying the operation.
Underside View of Subpanel, Showing Supporting Beam; Right, Assembling Coils on Subpanel: Below, Drilling Template for Front Panel and Subpanel Layout
The underside view of the subpanel, shown on this page, gives the method of wiring and the location of all the fixed condensers except the .00025 grid condenser, which is mounted directly on the socket terminal above the subpanel. This photo shows another detail, which should be mounted at this time—the bar which is mounted across the brackets at the rear as a supporting beam for the subpanel. It consists of a strip of panel material. 3⁄16 by 11⁄8 by 23 in., and is fastened to the rear ends of the subpanel brackets by means of four short machine screws. This bar increases the rigidity of the assembly, thus assuring the continued per- fect alinement of the condensers. Where the under-subpanel wiring runs from back to front, small holes are drilled to pass the wire, making possible permanent separation and neat arrangement of the various leads. After wiring as much as possible of the subpanel without the front panel, prepare the front panel for mounting. Care should be taken in the mounting that the driving mechanism of the drum dial is placed in the collar of the bracket before the panel is faced up to the subpanel assembly, after which the front panel may be pulled up with screws through the brackets. Now, with a screwdriver inserted to spread the lips of the drum driver, the dial may be slid along the condenser shaft and into the aperture and the drive mechanism released on it. At the same time the dials should be turned so as to read 100 with the condenser plates all the way in. and the dial locked in place on the shaft. The remainder of the wiring to the potentiometer and switch-rheostat may now be completed and the receiver tested.
All sockets of the receiver are to be supplied with UX-201A or CX-301A tubes, except the power-tube socket located nearest the back edge of the subpanel; this tube may be UX-112 or UX-171, as indicated in the schematic diagram. Note that the C-battery necessary for the UX-112 is 9 volts and, if the UX-171 tube is used, 27 volts of C-battery are required.
Due to the careful matching of the units, the set will not require any major adjustments for efficient operation over the entire waveband or, if any slight balancing is found necessary, this may easily be done with the small balancing condensers mentioned. Connect the A, B and C-batteries in the manner shown in the diagram on page 126, also the aerial and ground, and turn on the filament switch; this is done by turning on the rheostat, which also turns off all the tubes in the set when turned to the extreme left. When tuning for a station, keep both dials in step and when the station is located, adjust the left-hand dial independently for loudest and clearest signal; the volume is then controlled by the potentiometer.
An enlarged blueprint of the wiring diagram, shown on page 126, together with full-size drilling templates of the front and subpanel, may be obtained from Popular Mechanics radio department, 200 E. Ontario st.. Chicago, for 25 cents. Specify blueprint No. 125.
Material List
- 1 front panel, 3.16 by 7 by 24 in.
- 1 subpanel, 3.16 by 9 by 23 in.
- 1 Aero U16 kit of four coils.
- 2 SM drum dials.
- 2 SM, type 220, a.f. transformers.
- 4 Hammarlund .0005 mid. condensers.
- 1 Benjamin, or similar type, spring-suspended socket.
- 5 SM No. 511 sockets.
- 2 Tobe, or similar type, 1-mfd. fixed condensers.
- 1 Carter, or similar type, .001 mfd. fixed condenser.
- 1 .00025-mid. grid condenser, Carter or similar type.
- 1 Yaxley, or similar type, switch rheostat, 6 ohms.
- 1 Yaxley, or similar type, potentiometer, 200 ohms.
- 1 Yaxley, or similar type, fixed resistance, 1 ohm.
- 1 Polymet EZ, or similar type, gridleak mounting.
- 1 Tobe, or similar type, 2-meg. gridleak.
- 11 XL, or similar type, lettered binding posts.
- 10-in. round brass rod, 1⁄4 in. in diameter, or 3 Hammarlund insulated flexible couplings, busbar, No. 14 round, tinned-copper type, machine screws, nuts, etc.
- 5 doz. soldering lugs, long, round hole type.
Snap Terminals on Battery Leads
Good electrical and mechanical connections are the features claimed for the "snap" type of battery terminal shown in the illustration. Small round-head studs are provided on the receiver instead of regular binding posts, and the terminals snap tightly on these studs. A multi-terminal can also be obtained. for use when two or three leads must be connected to the same stud. Vibration does not loosen these fasteners, as is the case with leads that are attached to the common type of binding post.