Refinement of the DTI82 multimeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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The DT182 is a small 100x50x20mm multimeter, very handy for measurements in the field. Such multimeters are not protected from damage when voltage is applied to the input in resistance measurement mode. In this case, the conductive layer on the low-resistance resistors of the ohmmeter-milliammeter divider burns out, and their ratings remain unknown. Possible failure of the ADC IC. It is often made using open-frame technology. Even if there are contact pads on the board, it is extremely difficult to find a suitable analogue for replacement. The multimeter can only be thrown away.
Sometimes the device itself is not so much expensive as its absence. To repair such devices, it is not necessary to know their circuit, it is enough to remove the back cover of the case after purchasing the device, draw the location of all elements on a piece of paper and put down their ratings (Fig. 1). It is advisable to glue the sheet with the layout diagram and the ratings of the parts to the back cover of the multimeter.
Fig. 1
The ADC input is protected by diodes. To eliminate the error in reducing the readings, it is required to install three diodes in anti-parallel - VD1-VD6 (Fig. 2). The IC easily withstands an input voltage of 1,5 ... 1,8 V. For microcircuits with an unknown pinout, the input can be found by the strapping details or by the next measurement of the voltage between the common wire of the device - the "COM" probe (Fig. 1) - and each of the outputs of the IC. In the modes "Measurement of continuity diodes" and "200 Ohm" with open probes, this voltage is 1,10 V, in the modes "2k" - "2M" - 130 mV. When the probes of the measured device are shorted, the input voltage of the IC becomes equal to zero. The values given are obtained with a voltmeter with Rbx 10 MΩ. Diodes in glass cases can introduce a noticeable error due to the parasitic photoelectric effect.
Fig. 2
In devices of the M-93, DT92xx and other series, in the event of an unpackaged IC failure, it can be replaced with ICL7106 in a DIP package, which can be glued to the free space of the board and soldered to the existing contact pads with PEV-0,1 winding wire. Figure 3 shows the correspondence of the conclusions of these microcircuits.
To control the supply voltage, it is necessary to lead the wire from the negative pole of the battery to the bolt with a countersunk head on the instrument case. When you connect your own probe "V; Ohm" of the device to this contact at the limit of "20 V", the display will show the value of the battery voltage minus 3 V of the internal stabilizer, i.e. at a DT182 supply voltage of 12 V, the readings are -9 V. It is allowed to reduce the supply voltage to -7 V, the instrument readings to -4 V.
Fig. 3
After completing the refinement or repair of the multimeter, the board from the side where the elements are located can be varnished. To prevent varnish from getting on the contact surfaces of the mode and limit switch, the varnish should be dried by placing the device with the elements down.
Author: V.A. Zhukovsky, Krasnoarmeysk; Publication: radioradar.net
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