Heathkit QM-1 | Q Meter
The quality factor (or Q) of an inductor is the ratio of its inductive reactance to its resistance at a given frequency, and is a measure of its efficiency. A Q-Meter consists internally of a calibrated VFO (variable-frequency oscillator) capable of inserting a known voltage in series with a resonant circuit, and a VTVM (vacuum-tube voltmeter) which measures the voltage across the resonant circuit. Q-Meter reading will be E/e, where e is the inserted voltage, and E is the voltage read by the voltmeter. By maintaining e constant, the voltmeter can be calibrated directly in Q values. The higher the Q factor of the inductor, the higher will be the gain, ability to oscilate, and narrower the bandwidth in relationship to the central frequency. The QM-1 is designed to enable technicians and experimenters to simulate conditions actually encountered in practical circuits and measure the performance of the coil or condenser by itsef. Such measurements are made at the operating frecuency actually encountered in the practical circuits, and these measurements provide information about the Q (quality) factor of the L/C (inductance/capacitance) tank at selected frequencies.
Heathkit model QM-1 construction details + Schematic & Operation

 

Specifications:

 
FREQUENCY ranges A    150 Kc to 480 Kc
  B    450 Kc to 1600 Kc
  C   1450 Kc to 5.4 Mc
  D 4.8 Mc to 18 Mc
RESONANCE scales Inductance L (dep. test freq.) 1 µH to 10 mH
  Capacity CE (effective) 40 to 400 pF
  Capacity CT (total) 40 to 450 pF
VERNIER 7 pF  -3 pF to +3 pF
Q scale range Full scale X1 or X2 250
Test coil specifications At 790 Kc (range B) L=250 µH
CE=96 pF
Q=110

Notes:

CE: Effective capacity, the capacity of RESONANCE (this capacitance is in series with the 5000 pF injection padder).

5000 pF injection padder: Special capacitor connected between one binding post "L" and chassis, it is in series with the coil under test; the RESONANCE capacitor is connected between the binding posts "C", one of them is common to the other binding post "L", and the other is connected to chassis. This capacitor is sandwich mica design with wide and very short leads in order to eliminate any inductive reactances. There must be a circuit capable of resonance and there must be a signal to which the circuit can resonate. The signal is obtained from the generator section, and the resonant circuit comprises the RESONANCE capacitor connected between the "C" binding posts, the injection padder, and the coil under test. If the current flow from the generator through the injection padder is obtained from a constant current source, the voltage at resonance developed across the coil under test will be directly proportional to the Q of the L/C tank at a given frequency. Because the injection padder is much larger than the RESONANCE capacitor (450 pF), practically all of the voltage across the coil appears across this capacitor, where it is measured by the VTVM circuit.

CT: Total capacity, the capacity of the RESONANCE capacitor + the stray capacities in the wiring.

Tubes: 12AT7 (oscillator), 6AL5 (VTVM diode), 12AU7 (VTVM amplifier), 6X5 (rectifier), & 0D3 (voltage regulator).

SET LEVEL: This control injects voltage to oscillator tube, the level of RF output is measured on the meter when the CAL-Q switch is in CAL position, the X2 and X1 marks are measurement basis, if this control is settled counterclockwise the oscillator will not work due to lack of CC voltage, keep in mind this on frequency tests.

 
Practical uses:
To measure the Q of a coil (use "L" binding posts):
  • Set switch to CAL.
  • Select RANGE and FREQUENCY to the desired operating frequency.
  • Adjust SET LEVEL to X1.
  • Set switch to Q, make sure there isn't any resonance at that point (tune off RESONANCE).
  • Then turn SET ZERO for zero meter reading.
  • Adjust RESONANCE accurately for maximun indication on the meter.
  • Read the Q value on the meter. If the meter reads off scale, switch to CAL and adjust SET LEVEL to X2, switch to Q and double the indicated value.
  • The indicated capacity on the C scale is the capacity of resonance of the tested coil.
  • Q and capacity values refers to the operating frequency, at other frequencies Q and capacity will have other values.
To measure inductance of a coil (use "L" binding posts):
  • Set switch to CAL.
  • Select RANGE and FREQUENCY to operating frequency of 250 Kc, 790 Kc, 2.5 Mc, or 7.9 Mc.
  • Adjust SET LEVEL to X1.
  • Set switch to Q, make sure there isn't any resonance at that point (tune off RESONANCE).
  • Then turn SET ZERO for zero meter reading.
  • Adjust RESONANCE accurately for maximun indication on the meter.
  • Read the inductance on the L scale.
  • Place the decimal point properly by referring to the frequency-inductance tabulation. Use "L" scale with frequency set to:
250 Kc 1 - 10 mH
790 Kc 100 - 1000 µH (1 mH)
2.5 Mc 10 - 100 µH
7.9 Mc 1 - 10 µH