In the case of battery receivers or old-fashioned sets provided with two terminals for the speaker connections, two separate wires or a length of flex can be used to join them to the jacks marked "P or B+"
OUTPUT METER
The output meter, at the right of the diagram, consists of a rectifier type A.C. voltmeter connected permanently across the 6,000 ohm section of the speaker transformer. The meter readings will thus remain accurate, regardless of the setting of the load Impedance switch.
Output voltage measurements convey very little in the majority of cases, so that the usefulness of the instrument is increased by calibrating the meter scale in milliwatts instead of volts, It should be clearly understood that the power indications of this meter are accurate for an input impedance of 3,000, 6,000 or 12,000 ohms, depending on the load impedance switch settings. The readings are accurate at both 50 and 400 cycles per second, but may vary somewhat at other frequencies because of the changing impedance of the loud speaker.
Three convenient power ranges and an "off" position are provided by the four position "meter range" switch. The ranges are 0-500 milliwatts for battery operated sets and sensitive alignment of A.C. sets, 0-5 watts for ordinary A.C. receivers with a single output tube, and 0-50 watts for high powered sets or amplifiers. Although the meter is capable of registering peaks of up to 50 watts, It should be borne in mind that the 8inch speaker will not handle anything like this amount .of power continuously. The power handling ability of the instrument is governed solely by the capability of the speaker itself.
In addition to the milliwatt scales, the meter face is also provided with a scale of decibels. The standard method of measuring the sensitivity of a receiver is to determine, with the aid of a signal generator, the number of micro volts of signal which must be applied to the aerial terminal, to produce 50 milliwatt of power output. Consequently, the zero reference level of the decibel scale coincides with the 50 milliwatt division. on the 500 milliwatt scale. The calibrations on the decibel scale cover a range from -10 to +10 db. With the meter switch on the 5-watt position, 10 should be added to the decibel indications so that the scale covers from zero to +20 db. In the 50-watt position of the switch, it is necessary to add 20 to the decibel scale reading, so that the scale covers from + 10 to +30 db. When the meter is not being used for actual measurements or alignment, the switch should be turned to the 50-watt range, or the "off" position.
GENERAL
The setting up of the instrument requires only three simple operations: setting the field resistance switch to the correct resistance, setting the load impedance switch to the right position, and plugging the coloured leads into the correct jacks In the patching system. The output meter can then he turned on if desired.
In some receivers, particularly those employing a system of inverse feedback, oscillation may occur at a frequency above audibility. This will be indicated by a large deflection of the output meter without any audible sound from the speaker. Should these conditions occur, the oscillation should be cured in the receiver, or the tone quality will be seriously impaired.
GUARANTEE
This instrument is guaranteed for a full period of 12 months against manufacturing defects and faulty components. Any alteration of the circuit or construction of the Instrument makes the guarantee void.
ANY SPARE PARTS FOR "University" TEST EQUIPMENT ARE
OBTAINABLE FROM THE MANUFACTURERS
RADIO EQUIPMENT PTY. LTD
BROADWAY, SYDNEY
"university"
UNIVERSAL SPEAKER AND
MODEL U.S.O.
INSTRUCTIONS FOR USE
RADIO EQUIPMENT PTY. LTD.
For all Servicemen's Requirements
SYDNEY
Using the "University" Universal
Speaker and Output Meter
Introduction
This much-needed radio service instrument combines a handy, calibrated output meter and a universal speaker designed to suit all types of receivers. It is designed with the object of saving time and giving the serviceman confidence in testing receivers or amplifiers regardless of their requirements in the way of speaker connection, load impedances, or field coil resistances. The calibrated output meter, when used with a modulated oscillator, makes accurate receiver alignment a very simple matter.
CIRCUIT
The use of this instrument is simplified if the operator has a knowledge of the circuit arrangement employed in it. Consequently, the circuit diagram is reproduced, and an examination of it will reveal that the instrument can be divided into five main sections, namely, a high quality 8-inch permanent magnet speaker, a tapped multi-impedance input transformer, and artificial field coil of variable resistance, a patching system to accommodate unusual speaker connections, and an output meter.
The use of a permanent magnet speaker enables the instrument to be used on battery and vibrator operated receivers in districts where A.C. power is not available, and on all A.C. or A.C./D.C. operated sets.
FIELD REQUIREMENTS
The permanent magnet speaker has no field coil, but most A.C. operated receivers make use of a speaker field coil as a filter choke. Consequently, it is necessary to provide an artificial field coil resistance of which can be varied to suit any type of receiver. This is shown at the left of the circuit diagram, and consists of a special, high inductance filter choke, together with a number of resistors, and a selector switch to adjust the total field resistance to any of the standard values shown on the engraved indicator plate on the upper left-hand corner of the front panel. The field resistance values provided, and the maximum current values which can safely be passed through them, are as follows: 500 ohms, up to 200 M.A.; 750 and 1,000 ohms up to 150 M.A.; 2,500 ohms up to 70 M.A.; and 7,500 ohms up to 35 M.A. These ratings provide a margin in excess of any currents likely to be met with in practice.
INPUT IMPEDANCE
It would be impossible to provide a constantly variable load impedance to exactly suit all of the vast number of power output tubes in common use. Consequently, four average values have been selected to suit the four main types of audio frequency power amplifiers.
The load impedance switch on the front panel has four positions marked S.T., S.P., PPT., PPP., representing respectively single triode, single pentode, push-pull pentode output tubes. In the single triode position an impedance of 3,000 ohms is provided between the jacks on the panel, marked d "P or B+" In the single pentode position, the impedance is 8,000 ohms between the same jacks. This value is suitable for A.C. pentode and all beam tetrode valves. In the push-pull triode position, the impedance is 6,000 ohms total, with a centre tap connected to the jack marked "C.T." In the push-pull pentode position, the impedance is 12,000 ohms center tapped. Single battery pentodes which require on impedance of 10,000 ohms or more are best suited by using the PPP. position and disregarding the centre tap. This provides a load impedance of 12,000 ohms.
PATCHING SYSTEM
There is no completely standardized method of wiring the loud speaker socket in radio receivers. Receivers may be fitted with 4, 5 or 6-pin speaker sockets, and the wiring of the various socket connections may differ
in some brands of receivers. To accommodate these variations, a simple patching system is provided by connecting the artificial field leads and the input transformer leads to the double row of jacks on the panel.
The six-wire cable connecting the receiver to the speaker is colour coded, and terminates in six plugs which can be inserted in the appropriate jacks on the panel. The 6 to 5-pin and 6 to 4-pin adaptors are also colour coded, the coloured dots around them conforming to the colours of the wires in the cable. For example, the most commonly used connections for a four-pin speaker are field connections to the two thick prongs, and input connections in the two thin prongs, in these circumstances, the two thick prongs are coloured black and blue, so that the black and blue wires in the cable should be plugged into the jacks marked "field." The two thin prongs may be coloured yellow and orange, and yellow and orange wires would he plugged into the jacks marked "P or B+." The remaining wires can be either left hanging loose or plugged into any of the empty jacks, as they are disused.
In the case of push-pull output tubes, a five-prong socket is generally provided. In this case, the fifth prong, connecting to the centre tap, may be coloured green, so that the green wire is the one to be plugged into the socket marked "C.T."
In a few cases, provision is made in the receiver to protect the electrolytic condensers should the speaker plug be withdrawn, by connecting the first electrolytic to a separate connection on the speakers socket. A short-circuiting link in the loud speaker plug is then used to connect the electrolytic to the filament of the rectifier or to the power transformer centre tap. In this case, it is simply necessary to examine the receiver and determine the coloured dots for the prongs which should be joined together. These two, coloured leads can then he plugged into jacks which are one above the other. This is the reason for providing the two rows of jacks which are directly joined to one another.
Any other non-standard connections can be easily followed cut by looking under the chassis to determine the colours of the prongs connecting to the field and input transformer contacts on the speaker socket, and plugging the corresponding coloured wires into the jocks on the instrument panel.
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