EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: lionelkjh on March 19, 2017, 05:34:20 pm
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Hello; I am currently restoring a multimetre voltcraft vc650, I have the schema, but I can not figure out which VDD and VSS should be, because they are not detailed (is it: -5V / + 5V ??). Can you help me? (I join the complete Shema with) Thank you for your answers
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From a data sheet for the KS57C2616 (http://datasheet.datasheetarchive.com/originals/scans/Scans-000/Scans-0012621.pdf) CPU:
VDD = 2.7V to 6.0V
VSS = Ground
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The charge pump for VSS is at the top left on page 2 (negative voltage), and at bottom right on the same page is the voltage regulator for VCC (U16, presumably 5V).
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See picture attached
VDD = +5V
VCC = ~ +3V
GND = 0V
VSS = ~ -5V
(https://www.eevblog.com/forum/repair/multimeter-voltcraft-vc650/?action=dlattach;attach=300830;image)
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Hello; Thank you for this information. I did the measurements. VDD is at 5v, but vss is only -3.3v. I have checked the diodes D9, D10, they are good. I checked c11, c12 (replaced), and the voltage is still -3.3v. What do you think?? Can the circuit 74hc04 (u15) be involved ???
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-3.3V seems to be fine for Vss. The output voltage of such a charge pump can't go up to -Vcc. And the voltage drop by Q9 is maybe 0.2 or 0.3V, so Vcc is about 4.7V (in case Q9 is in saturation mode).
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Hello, the voltage of -3.3v is not correct, it must be around -5v, it is missing -1.7v. Only the ohmmetre and diode tester parts are faulty. It is either too high or too low on a caliber of the ohmmetre. The entire circuit around the operational amplifier has been checked and checked, and there is nothing abnormal about it. An idea?? :palm:
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Use another meter to measure the voltage of the voltcraft at the probes, red to red, black to black.
1.0 put voltcraft to diode mode - how many volt measured?
2.0 put voltcraft to ohm mode - how many volt measured?
I think -3.3v should be fine due to loading of the circuit.
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What exactly is the meaning of this "Only the ohmmetre and diode tester parts are faulty. It is either too high or too low on a caliber of the ohmmetre."??
Put a known ohm X ohm say 100 ohms or something, what is the reading?
Put a know diode X say 1N4148 or something what is the reading?