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Old 06-17-2022, 03:47 PM   #41
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Default Re: Fixing a multimeter based on VC9805 schematic

Thank you for your reply. The m6 smd I marked in the photo and diagram was a short circuit. I don't have an m6 transistor right now. What should I do to operate the multimeter?
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Old 06-17-2022, 04:51 PM   #42
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Default Re: Fixing a multimeter based on VC9805 schematic

Ok, that transistor matches the schematic for Tp2, controlling battery power to the multimeter, for the auto-power off.
If it was shorted, the multimeter would stay powered on, it would not auto-shut off. The on/off switch would still work.
It would be rare for that transistor to fail short-circuit. It seems strange. Unless there is another shorted part on the board, like a C2 or an IC.

You can just short C-E with a jumper and the multimeter would power up and work, just no auto-shutoff feature.
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Old 06-18-2022, 02:54 AM   #43
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Default Re: Fixing a multimeter based on VC9805 schematic

auto-off is a pain in the ass anyway.
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Old 06-18-2022, 06:09 AM   #44
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Default Re: Fixing a multimeter based on VC9805 schematic

Yes, it doesn't turn off automatically. you guys are great. thank you. good people like you will save the world... so why can't I measure voltage and current? only ohm and diode (buzzer) are working. I couldn't find any other short circuit, contactor wires are in place. Could you please give an idea for this as well?
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Old 06-20-2022, 11:02 AM   #45
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Default Re: Fixing a multimeter based on VC9805 schematic

Now the multimeter is working but it doesn't measure voltage and current. Do you have any idea about it?
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Old 06-24-2022, 10:20 AM   #46
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Default vc9805 multimetre arızası

hello, I have the same vc9805 multimeter, whose photos and schematic I have attached below. After replacing the battery only ohm and diode (buzzer) measurements work, other measurements do not work voltage, current ... none. Only the number 1 remains on the screen. When I measured the components, I couldn't find any problem parts. If anyone has any idea, what should I do?
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Last edited by poker; 06-24-2022 at 10:24 AM..
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Old 06-24-2022, 11:18 AM   #47
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Default Re: vc9805 multimetre arızası

check all the transistors for shorts - some are used for input protection.
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Old 06-24-2022, 12:47 PM   #48
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Default Re: vc9805 multimetre arızası

I checked all of them, unfortunately I can't find it, I don't understand where am I doing wrong?
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Old 06-24-2022, 06:21 PM   #49
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Default Re: Fixing a multimeter based on VC9805 schematic

did you move any of the contacts in the rotary switch?
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Old 06-24-2022, 09:54 PM   #50
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Default Re: Fixing a multimeter based on VC9805 schematic

Post #28 I said to use a multimeter and measure some voltages "On DCV 20V range, measure the A/D voltages: IN HI, IN LO, REF HI+, REF LO-, COM wrt V-"
That is 5 readings to take.
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Old 06-25-2022, 01:52 AM   #51
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Default Re: Fixing a multimeter based on VC9805 schematic

I will measure and reply immediately. Sorry, I missed your message.
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Old 06-25-2022, 03:58 AM   #52
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Default Re: Fixing a multimeter based on VC9805 schematic

Quote:
Originally Posted by redwire View Post
Post #28 I said to use a multimeter and measure some voltages "On DCV 20V range, measure the A/D voltages: IN HI, IN LO, REF HI+, REF LO-, COM wrt V-"
That is 5 readings to take.
sir,
buzzer = 2.90 v
temperature = 38.1 millivolts
in low- in hi= 0.00
vref+-vref- =1
v- v+ = battery voltage 9.0
v- and com varies 179 mv, 190, 110
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Old 06-27-2022, 04:06 AM   #53
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Default Re: Fixing a multimeter based on VC9805 schematic

Display shows 1___ with those readings?
Pin 40 COM voltage is too low, it needs to be at least 1V above V-. On the schematic it would be QC4,QC3 to check- that would short COM below 1V. On your board check whichever two are the same transistors as those. 9Q1 maybe?

There should be two protection transistors between COM and V-. If your meter does not have them, the IC can get damaged on pin 40.
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