THE RCA MODEL 103 LOUDSPEAKER
By Kenneth Ericksen
Reproduced from the November 1990 SCARS Gazette
The RCA Model 103 Loudspeaker
Selling at $37.50 in 1928, the RCA Loudspeaker 103, in its day, marked a distinct advance in loudspeaker design. The beautifully modeled octagon frame, with its roses and vase tapestry grille enclosure, was a radical departure from the conventional loudspeakers of the day.
As the pioneer in every marked improvement to broadcast reception in the home, it is fitting that the Radio Corporation of America, some seventy-five years ago, created a loudspeaker which not only was an exceptional reproducer, but also was an ornamental masterpiece. The artistic appearance of the loudspeaker was then, as it is today, as completely satisfying to the eye as its superb performance was to the ear.
Every resemblance of a mechanical device was effectively eliminated from the encasement by its designers. The wooden frame and pedestal are molded to resemble hand-carved oak. Although exceptionally rigid, comparatively light in weight, its molded frame is many times stronger than one actually carved out of wood and it is not susceptible to variations in temperature or moisture which might crack or warp an ordinary wood frame. A beautiful tapestry medallion conceals the mechanism of the loudspeaker. The use of a soft felt pad which is fastened to the bottom of the pedestal prevents the possibility of marring or scratching any highly polished surface on which the loudspeaker may be placed. These facts strongly appealed at once to those who were considerate of their home furnishing, allowing the loudspeaker to become very popular throughout America and foreign countries.
The RCA-103 loudspeaker consists essentially of a molded frame with a tapestry grille, a baffle board, cone support, cone, motor mechanism and filter unit. A fiber canopy is provided to cover the entire rear mechanism and a silk cover is tied over this canopy to add to its artistic finish and still provide easy access to the mechanism when necessary.
Reproduction by the RCA-103 loudspeaker is unusually clear and distinct. Voice is clean-cut and natural, and musical notes are reproduced with remarkable fidelity. Although it is very sensitive, it will handle substantial volume without overloading.
The loudspeaker was an improved design of the extensively used RCA-100A loudspeaker, and it is especially apparent that when used with a RCA superheterodyne broadcast receiver such as the Radiola 60 that it provides particularly fine quality of reproduction. The maximum capabilities of the loud-speaker are obtained when it is connected to a radio receiver, or an amplifier unit, in which power-amplifier Radiotrons UX-120, UX-112-A or UX-171-A are used in the last audio stage.
To prevent mechanism damage, some method of coupling the output of the receiver to the loudspeaker should be employed when the loudspeaker is used in conjunction with receivers using output tubes passing current in excess of ten milliamperes through the loudspeaker windings. Although RCA provided a 1 to 1 ratio coupling transformer especially designed for this purpose during the late 1920's, a choke and condenser arrangement will also give satisfactory results for this purpose when properly connected. This effectively bypasses the plate current so that it does not pass through the loudspeaker. Such a device is absolutely necessary with Radiotron UX-171-A or with other power tubes when, as previously noted, the plate current exceeds ten milliamperes. With Radiotrons UX-112-A and UX-120, an output device is desirable though not absolutely necessary. Some receivers, such as Radiolas 17 and 18, are equipped with output devices. Figures 2 and 3 show the correct values and connections of either a transformer, or choke and condenser to the loudspeaker.
Fig. 2. Loudspeaker Transformer Coupling Method
Fig 3. Loudspeaker Choke and Condenser Coupling Method
SERVICING THE LOUDSPEAKER
Common service problems related to the loudspeaker range from weak reproduction to distortion, noise and rattle. These conditions and their attending causes, while not limited to the 103 loudspeaker, still warrant discussion.
1. Receiver Output. Before inspecting the loudspeaker for imperfect reproduction, check the receiver output with headphones. Any distortion in the receiver will be faithfully reproduced in the loudspeaker. If a signal of good quality and volume is being delivered by the receiver, the loudspeaker must be examined for the cause of any imperfect reproduction that may occur. This can easily be accomplished by removing the silk cover and fiber canopy to expose the mechanism of the loudspeaker.
2. Foreign Material Interfering with Armature Action. An inspection of the armature will generally disclose any foreign matter interfering with the armature action, resulting in poor reproduction. A small piece of heavy paper or a piece of copper or brass, not over .010 thick, may be used between the armature and pole pieces to remove dirt, dust, or other interfering substance.
3. Armature Striking Pole Pieces. Distortion and rattle may be caused by the armature striking either or both of the pole pieces. This is generally determined by inspection, however in some cases the contact may be so slight that it appears not necessary to adjust the armature. In any case an adjustment of the armature should be done. Adjustment of the armature is best accomplished by the use of two phosphor bronze strips .010 inch thick and .25 inch wide, tapering to a width of .15 inch at their extremities. One tool is placed in the space between the armature and pole piece of the motor mechanism at the end next to the filter unit. The other tool is placed at the end of the armature to one side in order to clear the drive pin located at the end of the armature. By loosening the two armature mounting screws, any tension in either direction, that may have been on the armature is released. Before removing the spacer tools apply a hot soldering iron to the drive pin thrust lever connection and heat the solder sufficiently to allow the drive pin to find its normal position with regard to the thrust lever. The iron is then removed, mounting screws are tightened and the spacer tools removed. This adjustment correctly aligns and balances the armature so that no abnormal strain is imposed upon it in any direction.
4. Filter Unit and Magnet Coil Test. A defective filter unit or a filter unit not properly connected in the circuit will cause distortion. A defective magnet coil will also cause imperfect reproduction. The circuit diagram and correct connections are shown in Figure 1. The reference letters in the circuit diagram refer to the filter unit itself. A simple VOM continuity test will indicate if an electrical defect either in the coil or the filter unit exists (See Figure 4). A short across the coil can be determined by checking its resistance. The correct resistance for this coil is 230 ohms. The magnet coils may be checked for an open by testing from one lead to the other. An open indicates a defective coil which will require replacement before the loudspeaker can become operational.
5. Loudspeaker Cord and Connections. A defective connection, either in loudspeaker cord or coil connections may cause distortion, noisy or no reproduction. As there is not much wear and tear on the coil connections, the most likely place to find trouble of this nature is in the connecting cord. The point where the cord enters the loudspeaker housing and the ends on which the pin terminals are located may become frayed and worn, causing a possible short or open circuit. If these points prove OK, and there are no indications of any defects external to the speaker housing, the cover should be removed and the lugs of the cord soldered to the filter unit examined. If there is no apparent defect the cord should be disconnected and tested by means of a VOM. Its leads should be tested for end-to-end continuity and also for a short between the leads. Shake the cord while conducting the continuity test to disclose any break. which will be indicated by the meter reading.
Figure 1. Schematic and drawing of the RCA 103 Filter Unit
FILTER UNIT CONTINUITY TESTS
Remove all connections and refer to Figure 1 above
|Correct Effect||Test||Incorrect Effect Caused By|
|Open||N to M||Shorted condenser|
|Closed||N to L||Open coil|
6. Rear Silk Cover. This cover should not be removed permanently since its purpose is to prevent accumulation of dirt and dust in the speaker mechanism. The draw cords on the silk cover may be made inconspicuous by placing them inside of the opening, but should be placed in a position such as not to interfere with the vibrating mechanism of the loudspeaker.
Manufacturing the Tapestry Grille Cloth in the 1980's
The first time I looked upon a 103 loudspeaker was during a SCARS meet in 1982. It was the sorriest loudspeaker I had ever laid eyes on. The tapestry grille cloth, including the rear silk cover was in rags, it leaned backward on its pedestal, and its original finish had long since disappeared. From that time on I decided to dedicate my efforts in SCARS toward doing something for that loudspeaker.
Not long afterwards I met another member, Harry Grossman, who willingly gave me the rare tapestry grille cloth from his loudspeaker to experiment with. Not many people would do that even today, and for this I hold great respect for him. That same year my wife Evelyn and I carried the grille cloth to the Peoples Republic of China where we met with Mr. Chen Wen-Shuen and his associate, Mrs. Xia Zhil-Lin, in an effort to determine the feasibility of reproducing the cloth which had been manufactured some fifty-six years earlier. Many hurdles had to be overcome such as determining the type of weave and pattern, loom requirements, thread color and our ability to meet or exceed the original RCA specifications. By 1986 after experiencing numerous failures and near look-alikes, we were able to recreate a grille cloth that appeared to meet all of RCA's original specifications. For this effort, I hold both Chen Wen-Shuen and Xia Zhil-Lin in the highest esteem possible, for without their technical efforts the limited amounts of new grille cloth in existence today could never have been achieved.
This loudspeaker frame and base can be beautifully restored with a little patience. Restoration is best accomplished by the use of a good stripping agent, a stiff brush, and the use of lacquer thinner to wash away any stripping residue left. Since the frame is of a wood fiber composite, allow it to dry for approximately three weeks before applying a stain. The frame will accept only an oil base stain (a dark walnut color is recommended) and is best applied by using a soft brush and brushing until no dark patches or streaks appear. Two to three coats are required, allowing a drying time between each of about four weeks.
Should the loudspeaker exhibit any cracks, fill smooth with a plastic putty before staining. If the loud-speaker has a severe backward tilt on its pedestal, sand the base with a belt sander at an angle just enough to correct the lean. lf a light sanding isn't going to be adequate to correct all of the lean, then very carefully pry the octagon frame away from the pedestal base (they are held together by a metal screw and two wood dowels) and sand each for correction. Last but not least, upon completing the restoration, be sure to put a new piece of felt cloth on the pedestal base and never allow anyone to lift the loudspeaker by the top of its octagon frame.
New Grille Cloth Replacement grille cloth for the RCA 103 is
available from the following Web site: