Multimeter not measuring AC

[Swell6]

I have a Vici VC97a multimeter, virtually new. Everything works fine except that it won't measure AC voltage at all, DC is fine. I've tried mains and a 9V AC/AC adapter. Is this a main chip issue or could it be a particular component?

 

[rjenkinsgb]

It almost certainly has a separate rectifier used for the AC ranges, and then measures the output from that, with scaling compensation.

 

[Swell6]

I can't see any obvious diodes - there are 4 transistors, Q 1 & 2 both Y1, Q 3 & 4 both J3, could any of those be rectifying the voltage?

 

[Swell6]

 

[rjenkinsgb]

Possibly under the switch knob??

 

[Swell6]

Just taken it off and there's nothing, nothing on the back either. I can't imagine this function could be integrated into the chip, like you say would have to be external to that, but just can't find anything. Only thing I can think of to try is replace those transistors in case they are playing some part - or just accept that it can't measure AC!

 

[Nigel Goodwin]

Just taken it off and there's nothing, nothing on the back either. I can't imagine this function could be integrated into the chip, like you say would have to be external to that, but just can't find anything. Only thing I can think of to try is replace those transistors in case they are playing some part - or just accept that it can't measure AC!

You declined to mention it's a true RMS meter, so the function is almost certainly inside the COB (Chip On Board), so there won't be any diodes.

 

[Swell6]

Ahhhh, didn't realise that. Bad chip then, not worth fixing. Thanks for all the advice!

 

[crutschow]

Does it give any reading if you apply a DC voltage when in the AC mode?

 

[rjenkinsgb]

Can you take photos of both sides of the PCB, without the switch knob in place?

 

[Swell6]

 

[Swell6]

 

[Swell6]

Does it give any reading if you apply a DC voltage when in the AC mode?

It auto selects AC/ DC based on the voltage being measured

 

[rjenkinsgb]

It auto selects AC/ DC based on the voltage being measured

???
The picture I've found shows a blue DC/AC select button, to change mode? If it's on DC it will likely not read anything with an AC input...

It's not unusual to want to measure AC ripple on a DC supply, or DC offset on an AC signal!

VICI VC97A 3999 Auto Range True RMS Multimeter backlight short buzz tester DMM : Amazon.co.uk: Large Appliances

VICI VC97A 3999 Auto Range True RMS Multimeter backlight short buzz tester DMM : Amazon.co.uk: Large Appliances

www.amazon.co.uk

 

[Nigel Goodwin]

???
The picture I've found shows a blue DC/AC select button, to change mode? If it's on DC it will likely not read anything with an AC input...

It's not unusual to want to measure AC ripple on a DC supply, or DC offset on an AC signal!

VICI VC97A 3999 Auto Range True RMS Multimeter backlight short buzz tester DMM : Amazon.co.uk: Large Appliances

VICI VC97A 3999 Auto Range True RMS Multimeter backlight short buzz tester DMM : Amazon.co.uk: Large Appliances

www.amazon.co.uk

I found the same picture (which is how I knew it was true RMS), as you say, if he's not selecting AC then it won't work.

 

[renato2]

Vici VC97A manual

I'm not sure this DMM really has the TRMS option built in. The manual quotes:

3.2.3. AC Voltage（ACV）(True RMS)
Range Accuracy Resolution
400mV ±(1.5%+10d) 0.1mV
4V
±(0.8%+10d)

1mV
40V 10mV
400V 100mV
750V ±(1.0%+10d) 1V
Input impedance: at 400mV range >40MΩ, at other ranges is 10MΩ.
Overload protection: 1000VDC or 750VAC peak value.
Frequency response: at 750V range: (40~1000)Hz, at other ranges: (40~2000)Hz
Displaying: True RMS response (calibration based on sine wave RMS)

As I understand, the above words in round brackets tell the user, that the displayed value of applied AC voltage is calibrated only for sine waves. If you apply square or triangular waves, the displayed value would deviate. So, this is not True Root Mean Square reponse!

A DMM with nearly the same name is available from a different manufacturer:
Victor VC97
This instrument has indeed the TRMS response, which is also written on the front side. I prooved it by applying sine, triangle and rectangular waves of the appropriate amplitude, while cross-checking the TRMS value with an oscilloscope.

Video review of two VC97 instruments part1
Video review of two VC97 instruments part2

cheers R.

 

[Swell6]

 

[Swell6]

The pic in that Amazon ad is of a VC97+ which doesn't match the actual product described, on mine the select switch is just labelled as diode/ resistance and continuity modes - HOWEVER - I've just tried hitting the blue switch in voltage mode and it does indeed toggle between AC & DC - I can't believe I didn't try it previously but I just thought that was just for that particular function as it's located next to it - HOWEVER again I can see now that it selects any function which is shown on the selector switch with a blue icon e.g. deg C / deg F, and voltage has that too. Feel a bit stupid now but at the same time very happy to have resolved this!!! Again, thanks for all the suggestions!

 

[renato2]

... and finally:
What's inside, how it works, and how to repair a VC97

 

[Nigel Goodwin]

... and finally:
What's inside, how it works, and how to repair a VC97

Nice find!!