The heart of the Model 224 is the 12-lever switch. With This switch, any tube pin can be connected to any of three circuits. The individual switches, numbered 1 through 12, correspond to the twelve possible tube pins. That is, switch 1 connects to pin 1 of all sockets, switch 2 connects to pin 2 of all sockets, and so on. Switch 10 connects to pin 10 of all sockets as well as to the top cap connector. Note that all sockets do not require all twelve switches, in fact, the compactron socket is the only one having 12 pins, thus requiring all 12 switches. The other sockets will use the first 6, 7, it, 9 or 10 switches, The five pins of the nuvistor sockets, however, do not connect to switches 1 through 5, The tube manufacturers have chosen to call these five pins 2,4, 8, 10 and 12, so this socket uses switches 2, 4,8, 10 and 12. When any of the twelve switches is in the TEST (middle) position, an ac voltage is applied to the corresponding tube pin through the neon lamp. If there is a conductive path of 1 megohm or less between the selected tube electrode and any other electrode, the neon lamp will glow. In this way inter-element shorts and undesirable leakage between electrodes may be discovered. This same circuit can be used to check heater continuity, in which case the lamp should glow if the tube heater is good. When the switch corresponding to the tube heater is placed in the C position (upper), heater voltage is applied to the tube by the B selector switch. Any of 17 separate heater voltages may be chosen by the B switch (position 18 is not used). With the proper switch set to C, the tube healer will light. All other tube elements except the cathode and the other heater connections are placed in the TEST position by setting the appropriate switches. This connects the tube as a diode. When the Press-To-Read-Meter switch is pushed, the neon lamp is disconnected and a voltage and load resistor combination selected by the A switch is connected instead. At the same time, the meter is disconnected from the Adjust Line circuit and reconnected to read the tube current. The current can be read either on the 0 to 100 scale or the REPLACE — ? — GOOD scale.