Shure 488T User Guide

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	The internal switch connections of the Model 488T				2.	The top (dashed) line of Figure 3 determines the
	are shown in Figure 1 and a typical external operating
						maximum allowable current (dissipation limit) for
	circuit is shown in Figure 2.	The design parameters of				various combinations of supply voltages and load
	the operating circuit (supply voltage, load resistance,					resistances.
	current, and sensitivity) may be		selected using the		3.	Make certain proper polarity is followed for			the
	curves shown in Figures 3 and 4.		If, for instance, the
						phone plug RING (+) and SLEEVE (-)		circuits as
	available supply voltage is 16 Vdc and a current drain of
						shown in Figures 1 and 2. In mobile and aircraft
	8 mA is desired, Figure 3 shows that a load resistance
						equipment	installations, vehicle battery polarity
	(RL)of 1.5 k is necessary. In Figure 4, it can be seen that
						verification is extremely important.
	with a supply voltage of 16 Vdc and a load resistance of
	1.5k, the relative sensitivity	is approximately +6 dB					MOUNTING
	higher than the specified output level of -5			dB with 10	The Model 488T is supplied with a mounting bracket
	Vdc and a 500-ohm load (see Specifications).
					to provide a snap-in hang-up for the microphone when
	MICROPHONE				not	in use. The bracket has mounting		holes	with
	CARTMOOE
					clearance for No. 8 screws (see Figure 6).
	AMPLIFIER
	RED(+)		4.7pF
	BLUE(-)		R E s I s T w E &?PuTOPERATION						are
	mT 1	m:-			No special		precautions beyond ordinary care
	I				necessary in operating the 488T microphone. It will
			-VOLTAGE		operate satisfactorily under most conditions of
			-	OCOMMON	temperature and humidity. Dropping the microphone or
	I			0 TRANSMIT	other severe mechanical shocks should be avoided.

TYPICAL OPERATING CIRCUIT

FIGURE 2

SUPFVY VOLTAGE iVs) IN VOLTS

MAXIMUM VOLTAGE AND CURRENT LIMITS

FIGURE 3

I

1

SUPPLY VOLTAGE (VS) IN VOLTS

SENSITIVITY AS A FUNCTION OF SUPPLY VOLTAGE

AND LOAD RESISTANCE

FIGURE 4

CAUTION: The following should be observed to avoid damage to the Model 488T:

1.Supply voltage (Vs) must be between 2 and 40 Vdc. Voltages in excess of 40 Vdc may substantially reduce the life of the microphone amplifier. Voltages less than 2 Vdc are not sufficient for microphone amplifier operation (although no damage will result).

MODULATION LEVEL ADJUSTMENT

For optimum operation of your transmitter, it is important to have proper modulation. Proper modulation will give undistorted transmissions, greater range, and less interference from static.

To obtain proper modulation, the following points should be considered:

1.TALK CLEARLY AT NORMAL SPEAKING LEVELS. TALK DIRECTLY INTO THE GRILLE OF THE 488T, WITH THE LIPS AS CLOSE TO THE GRILLE AS COMFORT WILL PERMIT, A DISTANCE OF APPROXIMATELY 6 MM ('A IN.) GIVING THE ULTIMATE IN DISCRIMINATION AGAINST UNWANTED ACOUSTIC NOISES. USE THE RUBBER LIP GUARD FOR PROPER PLACEMENT OF MOUTH.

2.In the majority of applications, the 488T will work properly when simply plugged into the transmitter. If you want to check modulation or if you suspect improper modulation level, read points a and b.

a.When using the 488T to replace another make or model microphone, it is desirable to check the modulation to make sure it is set to the transmitter manufacturer's specifications. This is because the 488T may have more or less output than the previous microphone, depending upon the specific transmitter. The modulation control in aircraft transmitters will always provide proper adjustment when using the 488T. This check should always be made when installing any new microphone and should only be made by qualified persons.

b.If the 488T has excess output, resulting in overmodulation or distortion, the gain of the 488T transistor amplifier may be reduced 6 dB.

To modify the 488T for reduced output, proceed as follows:

1.Remove the three No. 5-40 round head screws and washers from the back of the microphone case. (Caution: the microphone case is spring-loaded and care must be taken to hold the two halves of the microphone case firmly together.)

2.Separate the case front and back. See Figure 5.

3.Note transistor amplifier board at the bottom of the front case half. Clip leads of the indicated resistor at the upper right hand side of the transistor amplifier board. This reduces the output level approximately 6 dB.

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