Fixing a multimeter based on VC9805 schematic

[poker]

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?

[redwire]

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.

[stj]

Re: Fixing a multimeter based on VC9805 schematic

auto-off is a pain in the ass anyway.

[poker]

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?

[poker]

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?

[poker]

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?

[stj]

Re: vc9805 multimetre arızası

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

[poker]

Re: vc9805 multimetre arızası

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

[stj]

Re: Fixing a multimeter based on VC9805 schematic

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

[redwire]

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.

[poker]

Re: Fixing a multimeter based on VC9805 schematic

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

[poker]

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.

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]

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.

[poker]

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.

sir, If the multimeter will measure the battery at 20 volts dc, which ways do the contactor wires touch in this diagram?

[poker]

Re: Fixing a multimeter based on VC9805 schematic

I solve the problem. I change the u2 integrated diagnostic tl062 in the diagram and the multimetrem started working as it old. If anyone experiences the same problem with me, remember this and benefit from this information. Thank you so much to the good people who answered my questions here...

[eccerr0r]

Re: Fixing a multimeter based on VC9805 schematic

So does the transistor test and temperature ranges are working?

vref+-vref- =1

So you mean both these two pins are 1V WRT GND or is the difference between the two 1V? If it's the former, we need to look at how these voltages are being generated. If it's the latter, throw away the meter and buy another one...

------

Ah okay looks like you fixed it, excellent. If it's easier just to replace parts until it works, it's still much faster than doing systemic debug... hence replacing whole meters is sometimes easier.

[poker]

Re: Fixing a multimeter based on VC9805 schematic

So does the transistor test and temperature ranges are working?



So you mean both these two pins are 1V WRT GND or is the difference between the two 1V? If it's the former, we need to look at how these voltages are being generated. If it's the latter, throw away the meter and buy another one...

------

Ah okay looks like you fixed it, excellent. If it's easier just to replace parts until it works, it's still much faster than doing systemic debug... hence replacing whole meters is sometimes easier.

I couldn't find a fault anywhere, then I wanted to replace this part and yes it works as before now, temperature, voltage, all... thank you very much for your help...

[poker]

Hi there, recently I got a not that cheap chinese multimeter based on VC9805 schematic.
Unfortunately it doesn't measure correctly.

Here is an example:

Range | ~33mv | ~100mv | ~503mv | ~2V | ~3V | ~10V
200mv | 33------ | 101-------| - ---------| -------| ----- | -
2v------| 0.030---| 0.98------ | 504----- | 1.94 | - ---- | -
20v----| 0.01-----| 0.08 ----- | 0.48----- | 1.7-- | 3 ---| 10
200v---| 0 --------| 0 -------- | 0.03 ----- | 0 --- | 2.8 | 9.8
1000v-| 0---------| 0 -------- | 0 --------| 0---- | 1 --- | 8

range ohms
200---- | ---------| --------| ------- | ---------| 1.2
2k------ | --------| --------| ------- | 0.981 | 0
20k---- | 4.89---| --------| 2.15 | 0.96----| 0
200k-- | 4.7-----| 29.5--| 1.9-- | 0.8------| 0
2m---- | 003--- | 0.027 | 0------| 0-------- | 0
20m-- | 0------ | 0.01-- | 0------| 0-------- | 0

Both V and Ohm measures go through RS2 RS3 RS4 and RS5. All that resistors measure as shown on the schematic - 1k, 9k, 90k and 900k. RS6-9 are 1.75m, 2.95m, 1.75m and 2.95m or total of 9m ( while it should be 10m, but even with added 1M resistor, the measure pattern still stays the same ).

I also have measured the resistance of the paths for all V ranges between Rin and the corresponding resistor ( for instance range 20V - Rin <-> RS5xRS4 ) and all paths have almost the same resistance ( +/- 0.2 ohm ). These are the paths which go through the range selector knob. Also, the multimeter is almost brand new and no damage is visible on the contacting surfaces.

Rin is 1M

Any ideas what else to check?


p.s.

range | 1uf | 22uf | 10- | 1 ----| 0.47 | 10
200u--| 1.1 | 23.4 | 10.9| 1.1 --| 0.5 -| 0
20u---| 0.94| 19.3 | 8.52| 0.93--| 0.44 | 0.01
2u----| 0.95| ---- | ----| 0.945 | 0.448| 0.009
200nf-| ----| ---- | --- | ----- | -----| 9.1
20n---| ----| ---- | --- | ----- | -----| 9.15

The current measurements seem fine ( of course need calibration ).

Dude, I have the same multimeter. I confused the wires around the position selector switch. Can you send me your photo?

[poker]

Re: Fixing a multimeter

Sorry guys, I didn't have the time to work on this. Probably tomorrow will be able thoroughly check your replies and do some more tests.
Meanwhile here are some photos to get an idea about the particular DMM.


p.s. 6R3 = Rin

Dude, I have the same multimeter. I confused the wires around the position selector switch. Can you send me your photo?