Audio power meter schematic.
We will begin at the input which is a type BNC female connector. A length of shielded cable has its shield grounded at the connector. The center conductor goes to the center contact of the BNC. At the other end of the cable the center conductor connects to the wiper of a 5 position switch and also to position PR5. The switch is labeled power range. Position PR5 connects through a resistor labeled R1 to position PR4. Position PR4 connects through R2 to PR3. PR3 connects through R3 to PR2. PR2 connects through R4 to PR1. PR1 connects through a 4700 ohm 1% resistor to pin 13, inverting input of a TLO74 quad operational amplifier . Pin 13 also goes to the cathode of one diode and to the anode of another diode. The other ends of both diodes are tied together and to ground. The diodes are type 1N4148. Pin 12, noninverting input is grounded. Pin 4 of the quad op amp IC goes to plus 12 volts and is bypassed to ground through a 0.1 u f capacitor. Pin 11 of the IC goes to minus 12 volts and is also bypassed to ground through another 0.1 u f capacitor. These power connections provide power to all four amplifiers in the IC. Pin 14, output of the first amplifier goes to the wiper of a 3 position switch and to the 4 ohm position. This switch is labeled load resistance. The 4 ohm position goes through a 2753 ohm resistor to the 8 ohm position. The 8 ohm position goes through a 1947 ohm resistor to the 16 ohm position. These two resistors have the note see text. The 16 ohm position goes through a 4700 ohm 1% resistor to pin 13, inverting input of the op amp. The output, pin 14, goes through a 10 k ohm 1 % resistor to pin 2, inverting input of the second op amp in the IC. Pin 3, the noninverting input is grounded. Pin 1, the output, goes to the anode of a diode 1N6263, the cathode connects to pin 2. Pin 1 also goes to the cathode of another 1N6263 diode. The anode goes to one end of a second 10 k ohm 1% resistor. The other end of this resistor goes to pin 2, inverting input. The junction of the diode anode and the second 10 k ohm 1% resistor goes to a third 10 k ohm 1% resistor. The other end of this third resistor goes to pin 6, inverting input of the third op amp in the IC. The noninverting input, pin 5 is grounded. The output, pin 7 goes to one end of R13, see text. The other end of R13 goes to pin 6. Now going back to pick up a loose end, pin 14, output of op amp 1 goes through a 20 k ohm 1% resistor to pin 6, inverting input of op amp 3. The output of op amp 3, pin 7 goes through a 1 meg ohm resistor then through another 1 meg ohm resistor to pin 10, noninverting input of op amp 4. Pin 10 also goes through a 0.1 u f capacitor to ground. Pin 9, inverting input is tied to pin 8, output of op amp 4. Pin 8 also goes through a 0.1 u f capacitor to pin 10 noninverting input. Pin 8 also goes through a 10 k ohm resistor to analog input 0 of the Arduino. The power pin of the Arduino that is labeled plus 9 volts goes to plus 12 volts. This is permitted because the 9 volt power input is connected to the input of a three terminal regulator and no harm will be done. The ground of the Arduino board goes to circuit ground. Analog output 3 of the Arduino goes through R17 then through R18 to the positive of the meter movement M. R18 is a 20 turn trimming potentiometer. The wiper connects to the junction of R17 and R18. R17 and R18 have the note see text. The end of the pot that connects to the meter is labeled CW indicating the clockwise end of the pot. There is a 1000 u f capacitor in parallel with the meter. The plus side of the capacitor goes to the plus side of the meter. There is a note which reads, quote, meter movement liberated from HP 400 ACVM or RCA, Heathkit, Eico, etc. VTVM. End quote. This completes the verbal description.
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