I'm just a starter.
I have a problem with Excel DT9205A meter and it is showing lesser ohm than real resistance in every range of ohm scale, only 96 ohm for 100 ohm, 968 ohm for 1k, 9.52k for 10k, 99.1k for 100k, etc., tested with 5% resistors.
I checked the series resistors of ohm scale with an analog meter (because I don't have another DMM) and all seem fine, and it has 100mV at the input of 10K resistor of main IC Ref+ pin, like described in the schematic, so it seems ok.
According to what I read, it seems Ref- voltage is getting less or Ref+ voltage is getting higher than normal. I checked componets (except capacitors) related to Ref+ and Ref- lines without removing from the board, and all seem fine, but the PTC thermistor connected at the input of VOhmDiode line, part code 4B DMZ, is unknown because I could not find that ptc code on the internet, also local stores don't have it and so I don't know its normal resistance, just tested by removing and heating with soldering iron and checking for resistance and it seems fine but not sure, I wonder if it gone less than rated value.
Can anybody help me to find the fault or tell me the normal value of the ptc if you have this model DMM?
Because my country is rather hot, please also take a note for the room temperature when you measure the ptc normal resistance, if you are not in an Air-Con room then °C shown in the weather app may be enough.

 

Well, everything has a tolerance. Your resistors are ±5% and your readings are coming off less than 5% off.

You might improve your understanding of the accuracy of your multimeter if you obtain some 1% or 0.1% resistors.

 

Why would you expect the meter to read better values than the 5% resistor tolerance?

 

I also have that DMM and a few other 9205A's not all exactly the same but the same name and number. Therefore I already measured the PTC thinking only that component could be different compared to my good meters. The green PTC in my good DMM measured 1449 Ohm but only measured about 650 Ohm in my defect 9205A. And is with one lead directly connected to the V/Ohm input.
I also know why I had partly destroyed a good meter which was when I accidentally tried to measure the 230V AC on the Ohm resistor measuring scale. Hopefully my and your meter is fixed with another good PTC ! My livingroom temperature is now 25 degrees Celsius. But when I measured those PTC's a month or so ago it may have been different. Anyway I measured those at about the same day and time.

 

I also have that DMM and a few other 9205A's not all exactly the same but the same name and number. Therefore I already measured the PTC thinking only that component could be different compared to my good meters. The green PTC in my good DMM measured 1449 Ohm but only measured about 650 Ohm in my defect 9205A. And is with one lead directly connected to the V/Ohm input.
I also know why I had partly destroyed a good meter which was when I accidentally tried to measure the 230V AC on the Ohm resistor measuring scale. Hopefully my and your meter is fixed with another good PTC ! My livingroom temperature is now 25 degrees Celsius. But when I measured those PTC's a month or so ago it may have been different. Anyway I measured those at about the same day and time.

See attached schematic that confirms the 1,5Kohm PTC resistor!

 

Sorry for the late reply, I was sick and completely forgot about this. Thanks a lot for your replies and helps @DickCappels @crutschow @Alectronic . Alectronic's schematic is rather different from my meter, anyway I could not find these PTC in my country and now I gave up. And I also found info that these meters are not quality meters (like 200M scale is also botched and always show 1M higher) and of course they are cheap. So this may be because of the meter's accuracy like DickCappels said. Thanks again for your help.

 

I was able to replace the PTC with an exact new part ordered from Aliexpress, but it didn't fix my defect meter. So it must be the controller itself that got damaged accidentally. And replacing that is not worth it because these meters new are cheaper than those replacements cost. Anyway finding the right pin configuration for the 42 pin or alike smd controller used on these printed circuit boards is also difficult to be able to do just that. Why I keep my defect Digital meter now just for scrap parts.

 

Just curious, do you (or anybody for that matter) know where in the circuit the PTC is located and what its function is?

 

Yes sure, it is the green component that is with one lead attached to the V/Ohm input. Here they can be ordered: 20Pcs Ptc Positieve Temperatuur Thermistor MZ5 MZ6 MZ8 100 300 Ohn 300 500 Ohn 500 800 ohn 1K 1.5K 2K Ohn|Weerstanden| - AliExpress

 

Thank you.

The only reason I can think of for it being there is that it is for some temperature compensation. I never noticed a PTC in any of the VOMs or VTVMs I have taken apart.

 

That will be what it is used for I guess DickCappels because it didn't help protecting my DMM when I accidentally measured 230VAC on the Resistor measuring scale. And because nothing else was defect it only leaves the controller that must be the guilty part.