Fixing a multimeter based on VC9805 schematic

[poker]

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?

[redwire]

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.

[stj]

auto-off is a pain in the ass anyway.

[poker]

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?

[poker]

Now the multimeter is working but it doesn't measure voltage and current. Do you have any idea about it?

[poker]

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?

[stj]

check all the transistors for shorts - some are used for input protection.

[poker]

I checked all of them, unfortunately I can't find it, I don't understand where am I doing wrong?

[stj]

did you move any of the contacts in the rotary switch?

[redwire]

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.

[poker]

I will measure and reply immediately. Sorry, I missed your message.

[poker]

Originally Posted by redwire

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

[redwire]

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.