July 1938 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.
If you do a search for "acoustinator," you get a certain model of acoustic guitar speaker manufactured by the UK-based company, Eminence. That, however, is not the same as the "Acoustinator" developed by Motorola for their "Golden Voice" series of car radios. It was essentially what we refer today to as a tone control, or as an equalizer. Not satisfied with creating just one futuristic sounding techno-term, Motorola also came up with the 'wobbulator' for use in tuning the receiver IF response. From what I gather in the Radio Service Data Sheet instructions, a wobbulator is a sweep frequency generator for testing a filter response shape. In case there never was a trademark associated with or a current trademark or trade name assigned to 'wobbulator,' (or alternately 'wobulator') let it be known that I hereby formally and legally serve notice of claiming the term. Now if you use it, you'll have to pay me a royalty ;-)
Push-Button Tuner Adjustment
To set the stations, proceed as follows: (1.) Turn the set on and let it play for not less Than 10 minutes, to assure all electrical circuits reaching a constant operating temperature. (2.) Select 6 stations to be "set" and arrange the 6 magnets to the approximate station frequencies as indicated on the scale, locking them in position. Do not "set" weak stations. (Fig. 1.) (3.) Press the first button. The motor will bring the mechanism to the first magnet. Loosen the lock nut. (4.) Tune manually to the exact peak of the station, using the tuning knob in the control head. (5.) Press the first button half-way in (far enough to energize the magnet, but not far enough to start the motor or mute the set.) (6.) Slide the magnet in the direction of error until a "click" indicates that the latch bar has engaged in the stop. (7.) Tighten the lock nut. (8.) Proceed to "set" the other 5 stations.
Spot Tuner Adjustment
(1.) Slip the spot tuner housing up the flexible control shaft far enough to expose the spot tuner mechanism, which consists of a small ball bearing, a brass raceway in which it moves, and a slotted retainer clip which acts as its guide. (2.) Tune in the desired station as accurately as possible. A tuning meter is recommended, except for Model 8-40, which has no Acoustinator receptacle. (3.) With a pair of slip jaw pliers, one jaw of which rests firmly on the small ball bearing, the other jaw of which rests on the bottom of the unit, apply enough pressure on the ball to force an indentation in the brass raceway in which it moves. (Fig. 2.) Proceed to set other favorite stations in the same manner. The raceway has a double track. Therefore, should you desire to reset the spot tuner to a new series of stations, it can be done by moving the ball bearing over to the second track. To do this, turn the condenser gang to full mesh (ball bearing at extreme end of raceway) and with a pointed instrument, force the ball bearing into the adjoining groove. A third setting can be made only by installing a new raceway. Order part No. 1X4487.
Plug in the Acoustinator, if the set undergoing alignment is thus equipped. Set Acoustinator at "Country" and "Voice" positions. Connect the speaker to the chassis and plug the "A" lead into its receptacle. Turn the volume control to maximum and leave it in that position throughout the alignment, reducing the signal generator output if necessary.
The trimmer labeled 600 kc. in the R.F. coil can must not be adjusted in the field. It is the key point in the entire R.F. alignment and was carefully set in the factory by means of an accurate capacity bridge to its correct capacity (840 mmf.). Before shipment, this trimmer was covered by a strip of black Scotch Tape, which should be left in position to eliminate any possibility of shifting its capacity through error.
(1.) Connect signal generator to control grid of the Osc.-Mod. tube (6A7 or 6A8G) through a 0.1-mf. condenser, having first removed the grid cap from the top of the tube. (See Fig. 3.) Connect a 500,000 ohm resistor from the grid of the tube to the grid cap on the lead just removed from this tube. Turn condenser gang completely out of mesh. Connect output meter across speaker voice coil. (2.) Set signal generator at 262 kc. and carefully adjust the trimmers in the diode coil can to the point showing highest reading on the output meter. (3.) Adjust the trimmers in the I.F. coil can to the point showing highest reading on the output meter. (4.) Go over I.F. and diode adjustment several times to secure maximum accuracy.
Setting the Range
(1.) Connect signal generator to the control grid of the R.F. tube (78 or 6K7G) through a 0.1-mf. condenser, having first removed the grip cap from the top of the tube. Connect a 0.5-meg. resistor from the grid of the tube to the grid cap on the lead just removed from this tube. (See Fig. 3.) (2.) Set signal generator at 1,560 kc. and, with condenser gang completely out of mesh, adjust for maximum deflection on the output meter the trimmer in the oscillator coil can labeled 1,400 kc. (3.) Set signal generator at 535 kc. Turn condenser plates completely in mesh and adjust for maximum deflection on output meter, the trimmer in the oscillator coil can marked 600 kc.
NOTE: The adjustments above set the range so the receiver will track with the calibrations in the control head.
R.F. And Antenna Alignment
(1.) Connect the signal generator to the antenna lead through a 0.00015-mf. condenser and to chassis ground. Set signal generator at 600 kc. and turn condenser gang until signal is heard. Adjust trimmer on the antenna coil can labeled 600 kc. for maximum deflection of output meter. (2.) Set signal generator at 1,400 kc. Turn condenser gang until signal is heard. Adjust for maximum deflection of output meter, the trimmer in the antenna coil can marked 1,400 kc. (3.) Adjust for maximum deflection of output meter, the trimmer in the R.F. coil can marked 1,400 kc. (4.) Recheck steps 1, 2, and 3, for accuracy.
Use of Oscilloscope in Aligning I.F.'s
Equipment Required: Cathode-ray oscilloscope and a frequency-modulated signal generator. (Note: if your signal generator is unmodulated, a frequency modulator will be required to adapt it for use with the oscilloscope.)
(1.) Align I.F. and diode trimmers in the regular manner as outlined in preceding paragraphs. (2.) Connect "wobbulator" to control-grid of Osc.-Mod. tube (6A7 or 6A8G) through a a.1-mf. condenser, having first removed the grid cap from the top of the tube. Connect a 0.5-meg. resistor from the grid of the tube to the grid cap on the lead just removed from this tube. (Fig. 3.) (3.) Connect oscilloscope to the top or high side of the diode load resistor, which, in this case, is the volume control. (4.) Adjust the "wobbulator" frequency to 262 kc. and observe the picture of the I.F. resonance curve as shown on the oscilloscope "screen." Correct alignment will result in a flat top curve, as shown in Fig. 4A. (5.) Should the curve appear sharp at the "nose" with a shelf on either side of the peak, as shown in Fig. 4B, adjust the plate trimmer of the I.F. transformer slightly, until the curve approaches the condition shown in Fig. 4A. If the regular I.F. alignment has been properly carried out, it will be necessary only in rare instances to adjust other than the I.F. Plate trimmer.
"X" indicates socket terminals used as dummy tie points.
(Acoustinator, lower-left, is not mounted on set.)
All readings except recto plates are from chassis ground to socket terminal indicated. Measurements made with 1,000 ohms/volt meter. Voltage at Battery - 6.3 V. Voltage at Receiver - 6.0 V.
Current Consumption - 8.5 amps. - Output (max.) 13 W.
*Note: The numerals heading the columns refer to socket terminals (see schematic diagram, Fig. 5).
Automatic Service Notes
Motor Fails to Start
(1.) Motor Contacts in Acoustinator Not Closing. Open the Acoustinator and inspect the motor contacts. If the gap is too great, contact will not be made when the button is pressed. Adjust by bending carefully.
(2.) Poor Contact at Acoustinator Plug. Inspect the contacts between the Acoustinator plug and the receptacle on the chassis.
(3.) Defective Reversing Switch. A defective switch would prevent the voltage from reaching the motor winding.
(4.) Open-Circuit in Motor. Check all connections to motor and check motor winding for continuity.
(5.) Motor Brushes Not Making Contact. Check contact between brushes and commutator.
(6.) Low Battery Voltage. A weak or defective battery in the car would not deliver sufficient voltage to start the motor.
(7.) Flexible Tuning Shaft Binds. Binding in the flexible tuning shaft places an additional load on the motor. If this load is too great, it will prevent the motor from turning the mechanism.
(S.) Magnet Fails to Release. If the magnet which has previously been energized, fails to release the latch bar for any reason, the motor cannot turn the mechanism.
Fails to Stop at Magnet
(1.) Open Magnet Winding. Check for continuity and replace if necessary.
(2.) Magnet Contact in Acoustinator Not Closing. Open Acoustinator and inspect contacts. Adjust or clean if necessary.
(3.) Rounded Head On Magnet Core. The head of the magnet should have sharp corners. Rounded corners may cause the latch bar to slip going in one direction, although it will usually catch in the reverse direction.
(4.) Latch Bar Defective. Inspect latch bar to make sure that it has not been damaged. Replace latch bar and gear assembly, if required.
(5.) Poor Contact at Acoustinator Plug. A poor contact here means a voltage drop which reduces the pulling power of the magnet.
(6.) Improper Spacing of Latch Bar. Check the spacing between the latch bar and the magnet. It should be somewhere between 0.01- and 0.02-inch. If the spacing is greater the pulling power of the magnet is reduced.
Posted April 28, 2015