1.
DESCRIPTION
Application.
A.W.A. Voltohmyst type A56010 is a versatile instrument which has application in the measurement of positive and negative D.C. voltages, D.C. current, resistance and A.C. voltages.
The instrument features a circuit which measures directly the peak-to-peak values of sinusoidal and complex waveforms, the scales being calibrated directly in peak-to-peak values and also in R.M.S. for sinusoidal waveforms.
The instrument has a rated input for non-symmetrical wave-forms of 2,100V, but sinusoidal and symmetrical waveforms of up to 4,200V. can be measured. (Refer to figure 1 below for waveforms measurable with the instrument.)
Fig.1 Waveforms Measurable with the Instrument
When measuring voltages, with a source impedance of approximately 100 ohms, the A.C. ranges up to and including 0-500V. are frequency compensated to provide a flat characteristic from 30 cycles to 2.5 Mc. When used in conjunction with a crystal diode probe such as A.W.A. type 2R56020, the input capacitance is reduced to a low value and the A.C. voltage ranges are made accurate to within ±10% from 50 kc. to 250 Mc. (Refer to subsection 1.4.3 for frequency response figures.)
When using an associated D.C. probe, on all D.C. ranges, the instrument has an input resistance of 11 M.ohms, thus offering considerable advantages over conventional test equipment in a number of applications such as those listed below:-
2.
(i) Oscillator Grid-bias
The negative D.C. voltage developed at the grid of an oscillator is proportional to the amplitude of oscillation.
The instrument, when connected directly to the grid, measures this bias, with negligible effect on the conditions of oscillation.
When checking multi-band receivers, comparative readings should be taken, rotating the oscillator tuning control through its entire range during each check.
(ii) A.V.C. Voltage
The instrument has negligible effect on A.V.C. measurements, allowing them to be taken directly at the grids of the stages to which the A.V.C. is applied, or on the A.V.C. line or at the output of the diode load.
(iii) Output Meter
In the alignment of TV, AM or FM receivers the instrument can be employed as an output meter. The centre-zero feature is particularly useful in aligning the discriminators of the latter receivers (refer to sub-section 2.4 of this book).
(iv) Detection of Gassy Valves and Leaky Coupling Capacitors
The presence of an abnormally low value of negative bias or of a definite positive bias at the grid of a stage may indicate a gassy valve or leaky coupling capacitor; this fault is readily located with the aid of the Voltohmyst.
(v) Insulation Resistance Measurement
High insulation resistances such as those of capacitors, transformers etc. can be measured by applying an external voltage and connecting the instrument as indicated in fig. 4.(Refer also to sub-section 2.6.)
Additional applications of the instrument are:-
(i) Measurement of dbm
By using the instrument in conjunction with the dbm conversion chart (refer to fig. 5), it is possible to determine the dbm. values corresponding to R.M.S. values measured across a 600-ohm resistive circuit.
3.
(ii) Peak -to-peak pulse values
The peak-to-peak values of pulse waveforms may be measured, using the A.C. peak-to-peak scale of the instrument. A long time-constant is incorporated in the circuits to allow for measurement of signals having low repetition rates.
Further information on facilities and electrical characteristics is given in sub-section
1.2 Mechanical construction
The instrument is housed in a compact metal case, having meter, four controls and three terminals mounted symmetrically on the front. An engraved front panel is fitted, and a carrying handle provided at the top of the case.
Dimensions are as follows:-
Height:- 10"
Width:- 6 ¼"
Depth: 6 ¼"
Weight: 8 ½ lbs
1.3 Design Characteristics and Circuit Description
The circuit is designed to give a high degree of accuracy in all applications, making the instrument especially adaptable for work in television and industrial equipment.
Maximum peak-to-peak input rating for non-symmetrical complex waveforms is 2,100V. and for sinusoidal or symmetrical waveforms up to 4,200V.
In the case of sine waves, the instrument is calibrated directly for R.M.S. and peak- to-peak values (a feature made practicable by the constant ratio of 2.83:1 of these values). The peak-to-peak-reading feature has particular application in servicing television receivers when taking measurements of wave-forms as indicated in Fig.
It should be noted that in this instrument, as in all voltage measuring devices, certain precautions are necessary in interpreting the readings, and inaccuracies may be due to external conditions such as high impedance of the circuit being measured, high- frequency voltage source, or if the voltage waveform consists of short pulses with a low repetition rate. The error introduced increases with the ratio of the source
4
impedance to the impedance of the instrument, and varies with frequency. Refer to para. 1.4.3 (c). The error in short-pulse measurement when the repetition rate is low is due to the time constants involving the capacitors in the peak-to-peak rectifier in the instrument. These capacitors do not have time to become fully charged during the pulse, and in the relatively long period between pulses, are unable to retain the charge they have received. The range or conditions allowing satisfactory measurement of essentially rectangular pulses derived from a 50 ohms source indicated in figure 2 below. For sources of higher impedance error will be greater than 2%.
Fig.2 Pulse Response Capability
The instrument is frequency-compensated for A.C. voltage ranges up to and including 0-500V. On the higher range the response varies with frequency.
All resistors in the divider networks associated with the voltage and ohms applications are high stability carbon types, having a tolerance of ±1%, and in conjunction with the D.C. amplifier, ensure reliable service for a considerable period.
The fundamental element of the circuit is a D.C. bridge involving type l2AU7 valve (V3) and its associated network. - Refer to the simplified schematic in fig. 3 below.
5.
Fig. 3
Assuming the instrument is switched on, power is supplied from the power transformer (T1), rectified by one half of a type 6AL5 valve (V2) and applied to the bridge (V3).
Providing no readings are being taken, and the circuit has been previously balanced by adjustment to the balance resistor (R25), the cathode currents of the two triode sections of the valve (V3) are equal, and the microammeter connected between the anodes reads zero.
When the facility selector (s2) is set in any of the voltage or ohms positions, and a reading is taken, a voltage is applied via range selectors, etc., to the grid of the left-hand triode (as drawn on the simplified schematic) section of V3, this altering the current through this section and in turn changing the potential across the common cathode resistor (R25). The change in the latter voltage then acts as a reverse change in the bias on the second section, and the meter responds to the resulting changes in potential at the anodes due to the voltage drops across anode resistors R20 and R22, respectively.
When used as a milliammeter, the facility selector (S2) disconnects the meter from the electronic circuit, and connects the "+" terminal of the meter to the "+" mA-OHMS input terminal, and the "-" terminal to the "-" mA. EARTH input, which is then disconnected from
6
earth. Operation is then as for a conventional milliammeter, but it should be noted that all electronic protection for the meter is now out of circuit, and care must be taken not to overload.
1.4 Summary of Controls, Facilities and Electrical Characteristics
1.4.1 Facilities
Ranges
"+" D.C. Volts ) 0 – 1.5V. (On separate LO Scale for A.C.)
)
"-" D.C. Volts ) 0 – 5V.
) 0 –15V.
A.C. (R.M.S. volts ) 0 – 50V.
) 0 – 150V.
0 – 500V.
0 – 1,500V.
A.C. Volts, Peak-to-Peak; 0 – 4.2V (On separate LO scale) (read simultaneously with 0 – 14V.
R.M.S. values for sine waves) 0 – 42v.
on separate scale. 0 – 140V.
0 – 420V.
0 – 1,4000V.
0 – 4,200V.
Current Measurement - mA. Ranges
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