problem measurement part frequency victor vc97

electronium

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Hi guys...
Good time everyone
I have a vc97 multimeter whose frequency meter does not work, what should I do? I used the power transistor of the hot part to measure the peak city electricity. I wanted to measure the switching frequency from the transistor, but the device had a problem in the frequency meter part.


 
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It's not very clear what's done, but it sounds like you don't have a clue what you're doing - and you've probably blown something.
 
Where is the frequency meter circuit of the vc97 multimeter and how does it measure the frequency, there was a schematic on Google, I didn't understand
 
Frequency is calculated by the processor in that meter, the big chip starting with part number "DM"
If it does not read frequency, then something is blown. What were the specifications for frequency measurement? Things like maximum voltage and maximum frequency are usually specified in the manual.
 
Found the info:

So maximum input voltage is 250VAC, and it says "peak", whatever they mean. 240VAC line voltage has a peak of over 330V. So if you measured a 240V AC line voltage, there is a chance it blew something.
I also notice the 10A fuse is missing. You blew something before...
 
electronium said:
No, the device is healthy, only the frequency meter does not work
Click to expand...
Which means it's not healthy

The sensitivity on Frequency is 1.5V - sticking mains across it is likely to have damaged it. The manual 'claims' it's protected to 250V peak, is you're got 220/230/240V mains, then the peaks higher than that (and I'm dubious about such protection claims).
 
I don't have a scope, and I want to measure the frequency of the hot part of the bjt power collector and the power mosfet, unfortunately, I didn't know about the limitation in the service manual cataloge section... I put the relevant fuse in place, now why do you think the frequency meter part doesn't work?
I put pwm power supply to this power MOSFET to monitor the switching frequency
 
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It's a different version of the meter PCB to the one in that schematic; that looks to be for this version:

Repairing a Broken Multimeter - Bartosz Nitka

In December 2017 my multimeter broke. It no longer measured resistance correctly. For low resistances, the numbers looked alright, but with bigger resistances, the value didn’t go higher than about 40kΩ. I only had one multimeter, so I didn’t have anything to compare against, so how did I...
niteria.github.io

Note the OPs photo shows a 64 pin IC, DM1106. That, from what I can find is a newer alternative to a DTM0660, which does have quite a bit of info available - eg. IC datasheet:

DTM0660 Datasheet | Manualzz

This datasheet provides information about the DTM0660 multimeter, a 14-bit noise-free sigma-delta ADC with built-in true RMS processor and external EEPROM. Find out about its features, specifications, and applications.
manualzz.com

There is a VC97 multimeter schematic in this article which may be a bit closer, as it uses the 64 pin IC, although the earlier 0660 version?

Мультиметр victor VC97 true rms

Китайские мультиметры бывают 4 типов. 1. Без защит от неправильного включения: DT830, DT182, M890 - внутри микросхема ICL7106. Производят до сих пор. Пик популярности был в 90-х 2. С автоматическим переключением диапазонов, внутри микросхема fs9721. Есть защиты от неправильного вкл. Уже не…
zepete.livejournal.com

The only components dedicated to frequency measurement are close to the centre of that, connecting through the PTC and clamp? to a coupling cap and an RC network, then IC pin 25.
 
As I've already said, because you've connected it to somewhere incredibly stupid - pretty well guaranteed to blow the meter.

I presume you're trying to measure the frequency of a switch-mode PSU (why?), which will be far too high for the frequency range of the meter anyway, which is essentially for mains type low frequencies.
 
دیتاشیت:
DS-HY12P66- TC
DTM0660
With the schematic that you kindly provided, is the pin connected to the frequency?



 
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Well, I don't have a scope, why shouldn't I measure the switching frequency, what is the solution? As you know, frequency is an important part of switching power supplies
 
electronium said:
Well, I don't have a scope, why shouldn't I measure the switching frequency, what is the solution? As you know, frequency is an important part of switching power supplies
Click to expand...

Again, why would you want to know? - and while switching speed 'may' be important (under some circumstances), that doesn't mean you have any need to know what it is. Depending on the type of supply, it could be very possibly be changing all the time anyway, and you wouldn't get any sensible frequency reading.

I've repaired MANY thousands of switch-mode power supplies, and don't remember ever trying to measure the frequency of one?.

But a multimeter isn't the way to do it, nor is a frequency counter - a scope would be the best option, for a 'reasonable guess'.

However, you'd probably kill the scope as well - switch-mode power supplies aren't things for beginners to mess about with, and as well as destroying incorrectly used test equipment, they can also be fatal.
 
I need to know what's going on? And that the frequency of the switch is important in the repair of switching circuits...power supplies...led drivers...engine starting drivers...audio circuits and amplifiers...radio circuits...why with minimal facilities from Shouldn't I benefit from my activity? And do not use? I am not a beginner, I am very knowledgeable in electronic repairs, but my level of knowledge is not at the level of Victor VC97 multimeter design, that's why I created my problem in the form of a topic.
 
This is the relevant part of the circuit, from the normal input socket to pin 25 (frequency counter) on the IC.

It does connect through via the Q1 + Q2 clamp circuit which should limit it to probably something like +/- 6 - 8 V, and the transistor are rated at 500mA, so it should stand 750V peak at the input, as far as the clamp and beyond that are concerned.


With only 1500 Ohms series feed to that, the initial dissipation through that PTC thermistor will be quite high at 240V AC - around 35W instantaneous, which may have blown it before it had time to heat up and it's resistance increase sufficiently to protect itself??

It's definitely not suitable for measuring frequency on high voltage sources.


Does it still work correctly on the millivolt range? It may be that is the only other one that would be affected by the thermistor failing?


 
Re. frequency measurement in general, multimeters are only suitable for lower frequencies and fairly non-critical sources, where the capacitance and "antenna effect" of the leads will not have any detrimental effect or change how the circuit operates.

That's why oscilloscopes are normally used with "x10" probes, that have a high impedance divider built in close to the probe tip - as just the lead capacitance of a directly connected probe can upset many high frequency circuits.

Many good, high frequency capable, frequency meters also use oscilloscope probes as the input - for exactly the same reason.

However, despite designing them in the past, I've not used a frequency meter for anything above around the audio range for over a decade, since I stopped building (amateur radio) transmitters - a scope is just more versatile.

 

Sorry, but using a multimeter in a completely inappropriate way and killing it is a very rookie mistake - I wouldn't consider anyone doing that 'knowledgeable in repairs'.

The frequency is unimportant for repairs anyway, all you need to know is if it's running or not - if it's running it's likely to be at some kind of correct frequency (which could be from 10KHz to a few MHz anyway).
 
q1 and q2 and ptc
are healthy and the path of pin 25 ends with the selector
 
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