YFE-3120 Multimeter

[masster]

I am looking for the schematics of this Made in Taiwan digital multimeter. The fuse blew up after measuring a battery voltage(!) on 20A DC scale... After replacing the fuse, DC current indication on 20A scale is 10x more than correct value. All other functions work OK, including on 200mA DC scale.

[ElektronikLabor]

It's a very interesting idea to measure voltage in current mode! 
Can you measure the resistance between inputs "COM" & "20A"?
If it's more 1Ohm, than something is wrong. A resistor could be damaged. (R1 doesn't look good?)

[masster]

It's a very interesting idea to measure voltage in current mode! 

Yeah... lending my multimeter to a neighbor was not a good idea.
Resistance between COM and 20A inputs is 0.2Ohm. I measured R1 and it is OK, it has 9MOhm (white-black-black-yellow-green)

[ElektronikLabor]

Some time ago I had a Multimeter with the same issue.
Somebody installed a wrong fuse (with much bigger current rating) and measured to big current.
Because of that the current shunt resistor damaged and got a much bigger resistance. The Multimeter indicated bigger values than measured.

I don't think that the 20A current shunt (R11) could be damaged by an 9V Battery. I think it's too big and has too small resistance.
I believe that the voltage across the R11 is measured with the same circuit, that is used for the 200mV Range (R11 = 200mV/20A = 10mOhm).

Does the 200mV range shows right values?

[masster]

I don't have a milli - ohmmeter, but R11 shunt resistance is very low, in the milliohms.
The multimeter is measuring well on 200mV DC range.
It is very frustrating not having the schematics, by now the multimeter would have been repaired. Showing a current 10 times higher on 20A DC range is not a good hint to find the component that failed?


Any similar schematic out there? I guess it is built around ICL7106.

[retiredcaps]

I don't have a milli - ohmmeter, but R11 shunt resistance is very low, in the milliohms.

For these type of meters, I expect the shunt resistance of 20A range to be around 0.005 ohms or 0.01 ohms.

Showing a current 10 times higher on 20A DC range is not a good hint to find the component that failed?

Is it exactly 10x higher?

Any similar schematic out there? I guess it is built around ICL7106.

Yes, it is likely based on the Intersil/Fluke 7106.

Have you tried looking and cleaning the range switch? 

The 20A looks like it is has a connection to R9?  Is R9 okay?

PS. We prefer that your photos be hosted here.  When I clicked on your links I got a pop-up spam opening in another window with loud music and some financial pitch! 

[retiredcaps]

I wrote Intersil/Fluke 7106 in the above post.  That reminded me of something in this thread by drtaylor (Fluke 8060A) at

https://www.eevblog.com/forum/testgear/old-fluke-multimeters/15/

" The only difference between the 8020 converter and the ICL7106 was that the Fluke chip electronically switches between 2V FS and 200mV FS, whereas the 7106 requires circuitry changes to accomplish the same."

So if your 20A reading is exactly 10x the correct reading, then something on your pcb is not switching from 2V FS to 200mV FS?  So whatever that circuitry is and where may not be functioning properly?  I wonder if the range switch has any bearing with the selection of the 2V FS to 200mV FS?

It is late here (1:30AM) and I may not be thinking clearly so I'm off to bed.  Hopefully, with that hint you might find something?

[EMUD]

Hi,
when you look to the connections of the 20A socket you see a wire wich is connected to the negative socket. This is the 20 A shunt resistor. It is normally made of constantan and has a value of less then 0.1 ohms. I think this resistor is your problem. It is damaged (resistance raised). try to check this resistance with a precision resistance meter. I would try to use a wire resistor (for the 20A range) of another lowcost
meter, it should be 0.01 ohms  (20A * 0.01 Ohms = 200mV !, Ohm`s law).

Good luck

Rudi

 

[masster]

Thank you for your replies, I will investigate further and update. Meanwhile I have 2 more questions:
- Can the shunt change permanently its resistance EXACTLY 10 times more in that short period of overcurrent till the fuse blew up? Or is it just a coincidence that 10x factor between the real current and the current displayed on the 20A DC range only? How is that permanent material resistance alteration working?
- Can I use the schematic below as an approximation for my multimeter? It is also built around ICL7106.

PS
@retiredcaps about hosting the photos on a foreign image hosting server:
My first impulse was to attach the photos to my post. But you can't attach high-resolution photos in a measly maximum allowable 2 (two) Megabytes  How could I get help from viewers if they don't see well the PCB of my multimeter since I don't have the schematic?
Because you are an old contributor, I recommend lobbying for a 10 MB quota. On the other hand, for any annoying ads while browsing you can use Adblock Plus.

[lowimpedance]

Umm... the 20 A socket appears to be not fused, are you sure the 20 A range was used ??.
Also is this meter really worth the effort.
It looks like its a 7106 based meter, so you should be able to see a scaled value of you current at the 7106 itself pins 31 and 30. If the dc volts  and other ranges are good then you can assume the 7106 is okay.
Time to trace the pcb from the 20 A jack. Being a manual range meter check the range switch for damage etc.
 Had a look at the photos but cannot tell if there is any damaged traces or vias.

Just a thought similar to what  retiredcaps indicated.
If the reading is exactly 10 times perhaps the cct that controls the decimal point is damaged or what drives it, probably the cd4070 check that area.

[ModemHead]

As mentioned previously, the 20A shunt should be 0.01 ohms, yielding a 200.0mV drop at 20A. If the 200mV range is accurate, I can only think of two ways the meter could read 10X the expected reading: 1) the shunt resistance has gone up, or 2) the wrong decimal point is on.



The current path for the 20A range is very short, straight from the 20A jack, across the shunt and back to the COM jack. So if the voltage drop has gone up, there aren't too many causes other than the shunt, the copper, and the solder joints.  The rest of the circuit should be a fairly straight path to the 200mV F.S. input of the 7106.

The best way to measure the shunt is to pass a known current through it and measure the voltage drop between the black wire (marked "20A" on the PCB) and the COM jack.  You'll need at least 500mA to get a good 5.0mV reading.

In 20A range, the display should be configured to display "19.99" max. The 7106 does not have decimal point drivers, that would be handled externally by the range switch.  It is likely that the 4070 CMOS IC beside the 7106 is the DP driver.

Edit: I see lowimpedance had the same DP idea...

[retiredcaps]

Thanks to lowimpedance and ModemHead for clarifying what I was fuzzy thinking earlier this morning.   

[Shock]

[masster]

I took out the shunt and measured it with a milliohmeter. It has precisely 10 mOhm. On 200mA range the meter works well. From your replies (lowimpedance and ModemHead) I understand I am left with the wrong decimal point error caused by 2 possible situations:

1) Bad selector switch traces / contacts on PCB
2) HEF4070BP circuit (quad XOR gate) malfunction

My question is this: wouldn't affect a bad  HEF4070BP the decimal point display on ALL other ranges?

And I ask again another unanswered question: how can a shunt modify its resistance permanently?? What is the physics behind it?