Fixing a multimeter based on VC9805 schematic

[stj]

how much is this worth??
just asking because 6000, and 10,000 count meters are very cheap now.
advancements in parts means they can make a good meter with only 2 chips and no calibration pots.

[madan1]

I bought this in the local "Chinese market" for the equivalent of 20 USD. The same DMM in amazon is 60 USD ( https://www.amazon.com/AMZVASO-Multi.../dp/B07F5FC27N ).



by the way... can anyone explain why a lithium 2032 battery would measure ~3V on that DMM and ~1.5 on another one? At least 5 other ( non-rechargeable ) batteries show exactly the same value on both DMMs

[stj]

no idea, but some meters have a battery-check mode where it puts a load across the battery.

[madan1]

After the calibration I was able to get close enough readings b/n the ranges:

ref.---200mv---2v---20v---200v---1000v
33mv---36.4---.032---0.02----0---0
130mv---142---.130---0.12---0---0
.500---1---.501---.49---0.4---0
1.005---1---1.010---.0.99---0.9---0
1.505---1---1.515---1.50---1.4---1
3.00---1---1---3.00---2.9---2
3.71---1---1---3.71-3.6---3 <-- battery
4.04---1---1---4.04-3.9---3 <--- battery
5.00---1---1---5.00---4.9---4
10.00---1---1---10.03---9.9---9
15.00---1---1---15.05---15.0---14

bench psu; ref is measured on another DMM

What I've done is to remove/bypass 6R5.
Do you think there is any point in changing all components in the BUF-A/Z-INT circuit to the recommended values?

Also, will it be possible to increase the 200uF range to a higher value?

[eccerr0r]

not sure it really looks any better, again the values don't matter but the quality do.

[poker]

hello, I have the same multimeter and the multimeter failed. only ohm and diode mode (Buzzer) measures, other voltage and current does not measure only 1 number on the screen can you help? what could be the problem?

[eccerr0r]

probably prescaler has a broken resistor?

[redwire]

First check the input-protection transistors for shorts. VC9805A+ there are 5 SOT-23 NPN types mainly, and a 2 for freq. counter and one for something else.
Never power a multimeter from a bench power supply, always use the battery. This is because the input COM(-) is NOT power GND and shorting those can cause damage.

On DCV 20V range, measure the A/D voltages: IN HI, IN LO, REF HI+, REF LO-, COM wrt V-.

I literally drove myself nuts fixing a similar multimeter that read -ve ohms. Leads shorted, reads -100 ohms. 1k resistor reads 900R.
It was not the protection transistors, PTC, A/D etc. - the crapola beeper comparator op-amp had a leaky input and was loading down the A/D input. That multimeter was 1" away from the garbage, I was so pissed before I found it.

[poker]

thank you for your answers. I will check your suggestions and reply.

[eccerr0r]

Note that when measuring IN HI/IN LO pins, you need to use a meter with 1GΩ input impedance, else you will get significant error in the reading. If you only have a 10MΩ input impedance DMM it's still worth to report what you get, but it will contain significant measurement error.
Granted if it really is saying just "1" overload, it may not matter, though it would matter if the meter stopped displaying "1" when you connected the test equipment.

BTW does it report 0 when you short the probes together, even? How about with IN HI/IN LO shorted?

[poker]

When I short-circuit the probes in the voltage section, -1 only becomes 1, only '' -'' disappears, only 1 remains.

[poker]

Hello again, I detected a short circuit in the m6 (s9015) transistor in the multimeter. I can't find the same transistor, which transistor can I use instead? can't find the same transistor, which transistor can I use instead?

[eccerr0r]

Not exact but a 2n3906 would probably will make a workable replacement...

[poker]

sir, I think this transistor is responsible for short circuit protection at the input. Can I bypass this transistor, which is specified as m6 in the diagram of the friend who opened the topic, what do you think until I find the transistor?

[eccerr0r]

M6 is a power switch transistor. I'd imagine it is used to shut off the meter if the battery is bad. If it's shorted and the battery is weak, then perhaps this is why you're getting strange readings.

[poker]

yes, the problem is that the m6 is short circuited, but now I don't have the m6 transistor. How can I bypass this transistor in the circuit? until i find the transistor?

[eccerr0r]

what I'm saying is even if it's shorted, it has no affect on the readings you're getting - unless if your battery is dead, so you do not have root cause and there's no need for bypassing.

[poker]

you are right then we have another problem that i need to find ?

[eccerr0r]

Just to be thorough, you should measure the voltage at emitter and collector, if they're about the same or within a half a volt or so and both near the voltage of your battery, then it's not the rootcause of problems...

Again as redwire suggests in post 28, check the protection devices Tt TF1 TF2 QC1 QC2 QC3 QC4 DI1 DI2 and then the input to the op amp pin 3. Also again as said, the voltages at the AME7106 pins would help pinpoint what's causing the problem.

[redwire]

Which transistor is shorted? poker, do not confuse the power-on transistors with the protection transistors.
On OP schematic for VC9805+ Tp1, Tp2 are for battery power switch. SMT marking L6 (is KSC1623L6/C1623) NPN and M6 PNP is part number S9015.

QC1/QC2, QC3/QC4, TR, Tt - are the protection transistors.
The multimeter should work with no protection transistor, if that's what shorted and you have removed it. But you need a new one there.