Figure 1 Measuring Circuit of RCA WV77.

Power Supply.

One prong of the AC line plug goes to the bottom of the power transformer primary. The other prong goes to the swinger of a 5 position simple switch. The five positions are off, AC, minus DC, plus DC, and Ohms. It is labeled S2eh. The switch is shown in the off position. The contact at the off position has nothing connected to it. The remaining 4 contacts for AC, minus DC, plus DC, and Ohms, are all tied together and go to the top of the primary of the power transformer. Connected across the primary is the series combination of a 220 k ohm resistor and a neon lamp. The transformer has two secondaries. One is a 6.3 volt winding for heaters and the other is a 125 volt winding for B plus. The two sides of the 6.3 volt winding are connected to the two heater pins of the 6AL5, 3 and 4. The two sides are also connected to the heaters of the 12AU7, pin 9 on one side and pins 4 and 5 on the other. One side of the 6.3 volt winding connects to common. The top of the 125 volt secondary connects to the anode of a semiconductor diode. The cathode connects to the positive end of a 10 u f 200 volt capacitor. The negative end connects to the bottom of the secondary winding, not common. The positive side of the capacitor is marked plus 88 volts. The negative side is marked minus 42 volts. The positive side of the capacitor connects to one end of a 47 k ohm resistor. The other end of this resistor connects to the left end of a 10 k ohm pot labeled AC balance. The right end goes to common. The wiper goes to circuitry that will be described much later. The right end of the pot also goes to one end of a 27 k ohm resistor. The other end of this resistor goes to the negative end of the 10 u f capacitor.

Meter amplifier and meter connections.

The meter amplifier consists of a dual triode, 12AU7. The cathode of one triode goes through a 330 ohm resistor to one end of a 15 k ohm resistor. The other cathode goes through another 330 ohm resistor to the same end of the 15 k ohm resistor. The other end of the 15 k goes to minus 42 volts. The grid of the right hand triode goes through the parallel combination of a 6.8 meg ohm resistor and a 0.005 u f capacitor to circuit common. The grid of the left hand triode goes through a 0.005 u f capacitor to common and to one end of a 3.3 meg ohm resistor. The other end of the 3.3 meg connects to circuitry which will be described much later. One end of a10 k ohm fixed resistor connects to the plate of the left hand triode. The other end connects to one end of a 10 k ohm pot. The other end of the pot connects to another 10 k ohm fixed resistor. The other end of this resistor connects to the right hand plate. The wiper of the pot connects to the plus 88 volt point. We will return to the tube in a moment. The positive terminal of the meter movement connects to the swinger of switch S2B and the negative terminal to the swinger of S2C. As you might have guessed these two are ganged with the switch that controls the power. Both switches have positions labeled off, AC, minus DC, plus DC, and Ohms. The off positions of S2B and S2C are tied together to place a short on the meter movement when power is turned off. The right hand plate of the tube connects to the AC and minus DC contacts of S2C and the plus DC and Ohms contacts of S2B. the left hand plate connects to three rheostats. One is a 15 k ohm and is labeled ohms adjust. The two remaining are 10 k ohm, one is labeled AC cal and the other is labeled DC cal. The other end of the ohms adjust rheostat connects to the ohms contact of S2C. The other end of the DC cal rheostat connects to the minus DC contact of S2B and the plus DC contact of S2C. The other end of the AC cal rheostat connects to the AC contact of s2B. This completes the description of the metering circuit.

Figure 2, AC Voltage Circuit of RCA WV77.

The common input jack goes to meter circuit common. The common jack goes to chassis ground through the parallel combination of a 1 meg ohm resistor and a 0.001 u f capacitor. The AC OHMS jack connects to the rotor of switch S2E. The switch is shown in the AC position. The AC stator connects to one end of a 0.047 u f capacitor. The other end connects to 5 of the stators of switch S1D, range switch. These are, 1.5 v, 5 v, 15 v, 50 v, and 150 v. the 150 v stator connects through a 900 k ohm resistor to the 500 v stator. The 500 v stator connects through a 320 k ohm resistor to the 1500 v stator. The 1500 v stator connects through a 138 k ohm resistor to circuit common. This is the prerectifier divider which prevents voltages higher than 150 volts from being applied to the rectifier. The rotor is shown in the 500 v position. The rotor connects to one end of a 0.02 u f capacitor. The other end connects to the rectifier tube. The tube contains two independent diodes. The capacitor connects to the anode of diode 1 and the cathode of diode two. The cathode of diode 1 connects to circuit common. The anode of diode 2 connects to common through another 0.02 u f capacitor. The anode also goes through a 20 meg ohm resistor to the AC stator of mode switch S2D. This point goes through a 91 meg ohm resistor to the wiper of the AC BAL, a c balance pot in the power supply. The rotor of S2D goes to the 1.5 v stator of S1B, range switch. The 1.5 volt stator of S1B goes through a 7 meg ohm resistor to the 5 v stator. The 5 v stator goes through a 2 meg ohm resistor to the 15 v stator. The 15 v stator goes through a 700 k ohm resistor to 50 v stator. The 50 v stator goes through a 200 k ohm resistor to the 150 v stator. The 150 v, 500 v, and 1500 v stators are all tied together. This is the post rectifier voltage divider. The three stators connect through a 70 k ohm resistor, then a 20 k ohm resistor, then a 10 k ohm resistor to common. Three separate resistors are used because the connects between all of these resistors connect to the DC voltage divider which is shown in cyan. The rotor of S1B connects to the AC position of S2F. The rotor of S2F connects to the 3.3 meg ohm resistor which connects to the left hand grid of the meter amplifier tube. This completes the verbal description of the AC measurement portion of the meter.

Figure 3 DC Voltage Circuit of RCA WV77.

The common input jack of the meter connects to circuit common. It also connects through the parallel combination of a 1 meg ohm resistor and a 0.001 u f capacitor to the chassis. The DC input jack connects through a 470 pf capacitor to the common jack. The DC input jack also connects to the plus DC and minus DC stators of switch S2D. The rotor of S2D connects to the 1.5 volt stator of S1A. The 1.5 volt stator connects through a 7 meg ohm resistor to the 5 volt stator. The 5 volt stator connects through a 2 meg ohm resistor to the 15 volt stator. The 15 volt stator connects through a 700 k ohm resistor to the 50 volt stator. The 50 volt stator connects through a 200 k ohm resistor to the 150 volt stator. The 150 volt stator connects through a 70 k ohm resistor to the 500 volt stator. The 500 volt stator connects through a 20 k ohm resistor to the 1500 volt stator. The 1500 volt stator connects through a 10 k ohm resistor to circuit common. Some of these resistors are also used in the AC voltage divider. They are wired to both S1A and S1B. The wiper of S1A is connected to both the plus DC and minus DC stators of S2F. The wiper of S2F connects to one end of the 3.3 meg ohm resistor that leads into the measurement circuit. This completes the verbal description of Figure 3.

Figure 4 Ohmmeter Circuit of RCA WV77.

As described previously the common jack connects directly to circuit common and through a parallel RC circuit to chassis. The AC OHMS jack connects to the rotor of S2E. The jack also connects to the ohms stator of S2F. The rotor of S2F connects to the 3.3 meg ohm resistor that leads into the measuring circuit. The ohms stator of S2E connects to the rotor of S1C. This switch wafer selects the resistance range resistors. The RX1, read as R times one, stator connects to one end of an 8.2 ohm resistor. The r times ten stator connects to one end of a one hundred ohm resistor. The r times one hundred stator connects to one end of a 1.0 k ohm resistor. The r times 1 thousand stator connects to one end of a 10 k ohm resistor. The r times 10 thousand stator connects to one end of a one hundred k ohm resistor. The r times one hundred thousand stator connects to one end of a 1.0 meg ohm resistor. The r times one million stator connects to one end of a 10 meg ohm resistor. The other ends of all 7 of the above resistors are connected together and go to one end of a one half amp fuse. The other end of the fuse goes to the positive terminal of a 1.5 volt cell. The negative end of the cell goes to circuit common. A reminder. This circuit gets connected to the measuring circuit through S2F described near the beginning of this section. This completes the verbal description.
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