Through years of experience, techniques of inspection and testing have been developed which are largely fool-proof and quick, yet test for all the essential qualities necessary for satisfactory performance. These same methods of inspection must be used by both the manufacturer's and the customer's inspection departments if the two points of inspection are to be in reasonable agreement on rejections.

Figure 64 illustrates a suggested, and recommended, test equipment arrangement and circuit for an inspection department. This embodies the same general arrangement as is used by some vibrator manufacturers, and is flexible enough to permit making all needed tests. The upper portion of the illustration, (above the dashed lines), shows the components assembled as a rather elaborate vibrator power supply. The two lower blocks illustrate two suggested sources of power for the operation of the power unit.

The primary circuit is shown at the left of the transformer, with connections shown in heavy lines. Three vibrator sockets are shown, these being for types of vibrators in wide use at the present time; the left-hand 4-prong socket is for an interrupter vibrator, while the center and right-hand sockets are for self-rectifying vibrators with different base connections. Other sockets with their corresponding wiring connections can be installed if desired, or, adapters correctly wired can be used for extending the range of types that can be tested. A 6-prong and a 7-prong (small diameter) socket are suggested additions. If adapters are used, care must be taken to keep the resistance of the interrupter circuits low in order not to change the output readings.

All of the primary circuits shown in heavy lines on the drawing, must be wired with a large size of wire in order to avoid large values of primary-circuit resistance. It is suggested that No. 12 copper wire or larger be used for all of these connections. A low-resistance type of fuse-holder and fuse is required, to avoid incurring an excessive voltage drop at this point. One suggestion is the use of an ordinary AC screw-base fuse for this purpose because of its low resistance. However, newer types of radio fuse-holders and fuses are usually satisfactory. A 10-ampere fuse would be sufficient for protection for the average 6-volt tester, but the resistance is higher than the 15 ampere size. The latter should "blow" with a short-circuit, and therefore, should be satisfactory.

Measuring instruments in the primary circuit are shown as an ammeter in the battery lead and a voltmeter across the input circuit, together with a cathode-ray oscilloscope. The inclusion of the ammeter is optional, and since the knowledge of the value of input current is of little interest in the tests to be conducted, its omission is recommended. This simplifies the control and measuring panel, and removes a considerable amount of resistance from the input circuit. The voltmeter on the input is shown with a multiplier which can be omitted if the circuit is to be used at one nominal input voltage. The location of the connections of the voltmeter leads to the primary circuit is important. These should be connected to the center-tap of the primary and to the central-portion of the buss connecting the reed-terminals of the sockets together. The locations shown on the diagram were used for simplification of the drawing only. The vertical deflection plates of the oscilloscope should be connected by means of a concentric cable to the interrupter terminals of the sockets.

The secondary, or high voltage, circuit is shown to the right of the transformer in comparatively light lines. The timing capacitor circuit of capacitor C2 and resistor R2 are shown connected across the entire secondary of the transformer. Should the application be for a higher input voltage, and require a portion of the capacitance to be placed on the primary side, switch SWi can be closed, connecting Ci in the circuit. The leads to this switch and capacitor must be kept short and be fairly heavy.

The three leads from the secondary of the transformer are connected to a three-circuit, two-position switch, SW2, having high voltage insulation, which controls the distribution of the secondary output to either the tube rectifier socket or to the self-rectifying vibrator sockets. The switch is shown thrown to the tube rectifier position. Another switch, SW3, is optional, but may be provided as a "reversing" switch in the self-rectifying vibrator high-voltage leads to provide for the correct polarity of output voltage. SWi, if used, must also have high-voltage insulation. A suitable socket is provided for the tube rectifier, and an off-on switch SW, can be provided in the heater circuit. However, removal of the tube is usually a better procedure as this prevents possible internal arc-overs when operating with a cold heater.

An adjustable load resistor is provided at R3. This should preferably be of a 50-watt, or higher, rating, not only to keep the operating temperature low, but to assist in making accurate adjustments with ease. The value of resistance will be largely dependent upon the application, etc., but a 7500 ohm (maximum) unit would be a reasonable value for most conditions. A tank capacitor, C3, is placed across this load and reflects on the vibrator in the same manner as the first-filter capacitor in the normal power supply. This capacitor should be of the electrostatic type, rather than of the electrolytic type, for best results since the leakage current of the latter would represent an added load current and any ageing effects would affect the accuracy of the measurements. A capacitance value of 4 mfd., with a rating of 600 volts DC, should be the minimum value used, and except for very high current loads, this value should be sufficient.

Measuring instruments in the secondary circuit consist of the output milliammeter and the output voltmeter. A voltmeter multiplier R6 controlled by switch SW6) can be eliminated if the range of outputs to be measured is limited. For simplification purposes, the load resistor can be set to a known value of resistance, and the voltmeter eliminated. Thus, the measurement would be recorded as so many milli-amperes at a load of so many ohms.