Philips PM6302 Instruction Manual page 31

31
3. Service data
3.1.
3.1.1.
CIRCUIT DESCRIPTION (see fig. 25)
The measuring bridge PM 6302 is made of:
— a switchable bridge arrangement
— a switchable zero indicator
— a supply voltage generator, 1 kHz/100 Hz and 1.5 V dic.
— a conversion facility for searching and measuring
— a power supply.
Bridge arrangements
There are three selectable measuring ranges, viz:
R measurements, C measurements and L_ measurements,
The basic set-up of the arrangements for C and L measurement (for reactances) is illustrated in figure 7.
In this case, the four bridge parts, the supply source and the indicator are illustrated for C measurement as far
as the potential is concerned.
The same arrangement applies for.L measurement, except for the fact that C515 and R713 are interchanged,
This iflustration can be used combined with fig. 8 for R measurement; assume C515 and R713 interchanged,
fixed resistor R634 instead of R713 and bridge supplied from a floating d.c. voltage source.
3.7.1.1,.8 measurement
'The Wheatstone bridge {Fig. 8) comprises resistors 701-711 for measuring ranges in decade steps. They
determine the coarse measuring range and form together with the sample {actual resistance) one arm of the
bridge.
The other one is formed by components 634 and 713 + 636,
Fixed resistor 634 is adjusted with 635 and 658, while the initial resistance of potentiometer 713 is adjusted
with resistors 637 and 659 and 636. Both adjustments determine measuring points 1 and 10 of the linear scale.
The bridge for R measurement is supplied with an internal d.c. voltage. The diagonal voltage.passes through
a de-ac converter before entering the input of the indicator amplifier,
Arrangement in searching mode for R measurement, see 3.1.4,
31.1.2, C measurement
The de Sauty-bridge (Fig. 9} comprises the same measuring range resistors 701-711 and mostly complex sample
Cy in the (left) arm of the bridge. The other arm comprises components 713 + {636//637//659) and 516 in
series with 712/A or 712/B. As a result of this both bridge arms are made of RC components which divide
the supply voltage equally and in the same phase, provided the time constants are equal,
The diagonal voltage between P and earth is then 0 V.
The results in adjusted and not adjusted state are vectorially shown in figure 10b and 10c. These figures apply
for toss-free samples.
For lossy capacitances a phase adjustment by means of tandem potentiometer 712/.. is required.
This adjustment simulates an attenuation of adjusting capacitor 515. For correctly adjusted phase, the
simulated attenuation equals that of the sample (Fig. 11-b) and its angle loss and tangent values are equal;
potentials 0 (-) and P compensate each other.
Tandem potentiometer 712/.. is marked in tand £D values; The attenuation range is switchable with push-
button D2 801/F,
The end vatues of the ranges are adjusted with resistors 633//712/A and 632//712/B. The frequency of supply
generator 312-314 is converted from about 1 kHz to about 100 Hz by depressing button D (see also 3.1.3.}.
'The calibration of 712/.. in D values depends on the frequency of the supply voltage to the bridge; therefore
pay attention to this in case af checking or external bridge supply. The calibration of adjusting potentiometer
713 is almost independent of the frequency of the supply voltage.
Two examples of a not-adjusted de Sauty-bridge are shown in Fig. 11-c by dotted and chain-dotted vectors.
The other vectors represent the voltage division of the bridge arm, in which the sample is present. In the
first case, the simulated losses are greater, while in the second case they are smalter than the losses {attenuation}
'of the sample.