International projectionist (Jan-Dec 1947)

The Triatic Signal Tracer CAREFUL analysis of the actual experiences of several hundred field service representatives indicated that a universal portable signal tracer would prove to be a most welcome and useful addition to their test instrument kit. Such an instrument, this analysis revealed, would save much time in identifying faulty components and in locating sources of noises in audio equipment. As a result, RCA Service Co. developed the compact Triatic Signal Tracer described herein. The development of miniature tubes contributed much toward producing this unique test instrument, for it is the small size of the tubes that permitted fitting a large array of components into a comparatively small housing. The Triatic Signal Tracer (meaning three-in-one) is furnished as standard equipment to all RCA theatre service representatives. It is used in locating trouble and in the servicing of electronic equipment in general. It consists of an a.c.-operated amplifier, probe voltage checker, and capacity bridge. A selector switch on the front panel can be set to any one of the three capacitance ranges, or to the signal tracer position. The amplifier has three stages of audio amplification with a gain of approximately 100 db. The power output is a little over one watt. The amplifier output is permanently connected to a twoinch p.m. loudspeaker built into the case. A jack, marked "output", is capacitance-coupled to the output tube anode for use when headphone listening is desired. The "input" jack is capacitance-coupled to the amplifier volume By EDWARD STANKO Manager, Technical Section Theatre Service Division, RCA Service Co. Detailed instructions for the use under actual field conditions, together with a schematic of its circuit, of the compact yet markedly versatile Triatic Signal Tracer are given in the appended contribution. Since previous releases anent this instrument touched upon only its constructional highlights, this article may be regarded as a basic contribution to the literature of field testing units. control and the slider arm of this control feeds the first tube grid. A strong signal at the input jack, therefore, may be reduced to a point where it will not overload the first tube grid. The probe input jack is directly coupled to a resistor, capacitor circuit and the "magic eye" indicator. With no voltage applied to the probe and with the selector switch in the "probe" position, the "magic eye" should remain approximately half-closed. A.F. Signal Tracing The capacitance bridge which has three ranges will indicate capacitance values between ten micro-micro-farads and 80 microfarads. Bridge balance is indicated by the opening of the "magic eye" shadow. Two pin jacks are provided for connecting the capacitor to be measured. A well filtered 90-volt supply is available at a pin jack located on the front panel. This is used to supply polarizing voltage for a photocell. An internal one-megohm load resistor can be connected from this 90-volt sup A schematic diagram of RCA Triatic Signal Tracer. ply to the amplifier input by turning on a toggle switch. A photocell can then be connected directly to the input jack and its audio output monitor, through either the loudspeaker in the signal tracer, or a plugged-in headset. A low-capacity shielded cable with an attached probe is used as the input lead. The case of the signal tracer is grounded to the circuit under test by means of a ground clip which is attached to the probe lead. Using a loudspeaker, or if necessary, a headset, the amplifier gain control is adjusted to give an audible signal when the probe lead is touched to the desired point on an audio frequency circuit. Usually, if the amplifier is completely inoperative, the first place to test should be the input circuit, to make sure that the signal is actually coming into the amplifier. When the signal has been located, the probe may be placed successively on the grid, anode, stage transformer high side connections, or coupling capacitors until the loss of signal indicates the defective part. The gain control must, of course, be reset to maintain the proper output as the probe is applied to successive stages. Probing for A.C. or D.C. Voltages When testing for a.c. or d.c. voltages the selector switch is connected to "probe" position. The probe lead is connected to the "probe" jack. The probe circuit consists of a resistor-capacitor and an electron-ray tube 6U5/6G5. This circuit provides a means of rapidly checking an amplifier or similar circuit to determine the approximate voltages present, the polarity, and whether the current is direct or alternating. The probe lead may be touched to any point in the amplifier or power supply circuits, to determine the presence of voltage there, without danger of injuring the signal tracer if the potential does not exceed 1000 volts. The indicator is not used as a voltmeter, but to quickly indicate presence of a.c, a.f. and d.c. voltages. If any voltage is present, 22 INTERNATIONAL PROJECTIONIST November 1947