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Fig. 101. Electronic vocal chords have been devised using this Bell Lab oscillator
circuit and transducer.
A wonderful device that is finding increasing use is an electronic vocal chord device for individuals who have either lost vocal chords through surgery or had them paralyzed by a stroke or other type of medical problem. Amazingly, a person so stricken adapts quickly to the electronic voice and the instruments are finding wide use. Not wide enough for electronic repair stations to really concentrate on them, but wide enough to present a trying experience for a person who is dependent on one and has it go out on him. Fortunately, the circuits are simple and a discussion of the testing techniques should help locate a defect.
No. 159: POWER SUPPLY TEST-DC
Failure in an electronic vocal chord unit is most often due to power supply problems. In a Bell Laboratories circuit (Fig. 101, there is a single-ended output transistor that is attached directly to a transducer. A 10-volt battery puts a negative voltage on the collector through the transducer. The transistor (a PNP) drives the transducer and battery drain is high.
When the user is in a fixed location such as behind a desk at work, or in bed as a patient, the battery can be dispensed with and a small converter can be used. The converter changes the 117-volt wall outlet current into the plus and minus 10 volts DC needed to power the unit.
The battery voltage can be tested by placing a 100-ohm resistor across the battery before reading it. Even with the large load, a good battery will maintain a near 10-volt output.
If the battery voltage drops with the load, the battery should be discarded. In fact, if in doubt discard the old battery. How the EVC Works The Bell Lab circuit uses a finger-operated combination switch-variable resistor unit. The user presses the switch on, and with variable pressure from his finger can control the pitch of his voice to sound as natural as possible.
The idea of the unit to supply to the throat a sound that emanates from the transducer (like the one on the inside of the telephone earpiece) to the throat cavity, tongue, mouth, teeth and lips. Only the sound is gone from the stricken individual.
He can still form all the words in his vocabulary with the other parts of the anatomy.
The transducer is held against the throat. The on-off switch is pressed and a negative voltage is applied to one transistor, a PNP, while a positive voltage is applied to an NPN. The two transistors form a relaxation oscillator. The frequency of the oscillator can be varied with a resistor between the two bases. This is the pitch control in tandem with the off-on switch.
The output is a negative-going pulse. The frequency can be changed from about 100 to 200 Hz for men and 200 to 400 Hz for women. The oscillator output is fed to the audio output stage and then to the transducer. The transducer has a steady 10 volt DC supply across it. The oscillations modulate the DC. The transducer plates move in accordance to the electronic audio, and the audio frequency is transferred into the throat cavity. The user then mouths the words and the sound emanates from him.
No. 160: SIGNAL INJECTING A DEAD EVC
When the EVC goes dead and the power supply is judged satisfactory, a quick way to isolate the trouble is with an audio generator. Take a low-audio pulse <under 1000 Hz ) and inject it at the base of the audio output transistor, with the off-on switch on. If the note can be heard from the transducer, the audio output stage is good and the relaxation oscillator circuit is defective. Should, the notes not be heard, the transducer or output transistor is defective.
No. 161 : SPURIOUS PULSE ELIMINATION
The oscillator produces a pulse in the low audio range. The pulse leaves the oscillator at the junction of the NPN collector and the cathode of the diode. The diode is held at a high reverse bias between pulses and thus isolates the output stage from the oscillator between pulses. The isolation enhances stabilization and no spurious oscillations are produced from possible feedback from the output to the oscillator.
Should unwanted noises, buzzing or squealing start in a user's voice, it could be caused by breakdown of this diode.
Measure the diode's resistance in both directions. If the resistance is lowered for reverse bias, the diode could be at fault. Another reason for this trouble could be lowered reverse bias in the emitter-base or collector-base junctions of the output transistor. Analyze the transistor, too.
No. 162: SIGNAL TRACING THE DEAD EVC
The oscillator pulse in an EVC can be observed throughout the unit. With a scope or the peak-to-peak scale of the VTVM, the voltage present at the test points at collectors, emitters and bases can be analyzed. In the collector of the output, a pulse should be observed with a high peak-to-peak voltage, enough to drive the transducer, and a frequency for the man or woman patient as described before.
At the collector of the NPN oscillator transistor, the same pulse should be present, negative-going, but lower in voltage than the output, since it hasn't been amplified. A missing pulse, with the off-on switch on, at either test point indicates trouble in the transistor circuit under test.
No. 163: PITCH CONTROL TEST
The pitch control is a variable resistor placed between the two oscillator bases. The NPN transistor is attached to the battery bias through a 1500-ohm fixed resistor. The PNP transistor is attached to battery bias through the variable resistor. As the pitch control is varied, it changes the amount of bias on the PNP. This, in turn, varies the PNP collector current which changes the bias on the NPN. Therefore, the oscillator frequency changes as the pitch control is varied.
Attach a scope probe to the cathode of the diode and observe the frequency of the scope display. A good working pitch control will vary the frequency about a 100 or so Hertz in either direction.
No. 164: PULSE WIDTH TEST
The feedback in the oscillator determines the width of the oscillator pulse. The pulse width can be varied by adjustment of the blocking capacitor and series resistor from the collector of the NPN to the base of the PNP. A compromise has to be formed to produce a pulse as high as possible for strong audio output with a narrow pulse so the transistors will actually draw current during as small an interval as possible. The pulse can be observed on the scope. Pulse width can be adjusted with various values of the series capacitor and resistor. Due to the low frequencies used, high sound levels are needed from the transducer.
No. 165: PERMISSIBLE CURRENT DRAIN TEST
Since such large power needs are required, the current drain in a battery portable unit must be tested to be sure a powerful enough battery is being used. Attach an ammeter in series with the off-on switch using a fresh battery. Then turn on the unit and take a reading.
Suppose you find the EVC is drawing 35 milliamps. A good rule of thumb is not to use a battery with a maximum rating under ten times the current drain. That means the battery should have no less than a 350 ma per hour rating. This should give at least 10 hours and possibly as much as 15 hours of continuous operation. That means weeks and weeks of actual conversation. If the user is judicious in the use and keeps one finger on the button, letting the unit turn off between speaking segments, while he is listening, the battery will last a long time.
Since the unit is transistorized and a negligible amount of heat is generated, the reaction of the unit is instantaneous. The fact that the unit provides a man's voice tones, a frequency between 100-200 Hz, or a woman's voice tones, 200 to 400-Hz, has no bearing on current drain. It draws exactly the same amount of current in either case.