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Voltcraft VC940 (alias Unitrend UT71E) problem - false readings

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csadam:
Hi Guys!

I've bought a defective VC940. The symptoms are the following:

* in ohm range,  everything is OK
* in DC Volt range, wrong values displayed, but linearity is OK
* in DC mA range, similar wrong values as in DC volt
When I say "wrong" values, I mean the following:

Input valueDisplayed valuedifference to the previous step0 V0.0000V - 1 V0.1593V0.15932 V0.3194V0.16013 V0.4799V0.16050 mA0 mA - 1 mA0.142mA0.1422 mA0.303mA0.1613 mA0.464mA0.16110 mA1.584mA -
It seems that the value of one unit (1 V) is taken as 0.16V by the multimeter. So if I divide the displayed values by 0.16 then I get the right value. I hope it's understandable :)

I have the schematics and the calibration steps posted in the other topics but none of them helps. The displayed value does not change in Volt mode by turning the VR2.
There are no visible damages on the PCB or on the components. The fuses are OK.
Could you please help me? What could be the problem?

Thanx,
Adam

Lightages:
If making adjustments to the trimmers do nothing, then you have:

1. one or more defective components
2. one or more bad solder joints
3. a defective circuit board
4. burned traces
5 maybe all of the above

If this is new, send it back. If you purchased used and as defective then maybe you can start checking component values and see if you can trace down the bad part. If you don't know how to do this without help, I can offer some in the way of guiding you to the obvious places to look.

csadam:
Thank you, I've started checking the components, and all of them were OK.
Then I searched the components which responsible for the reference voltage. I've found that the reference voltage is on GND.
And finally I've fixed it!
While I was measuring it for a time, at once the ref voltage magically appeared and the displayed values are correct since then.
I don't really understand it,  maybe I have magic repair powers...  :D Or bad solder joints under the VR2 or the R31.

I attached a picture. The critical components for the reference voltage are the

* C22: 1uF
* R31: 4.7k (upper dot)
* R32: 25K  (bottom dot)
* VR2: 200 (blue pot)
The voltage between the GND and the upper dot is -196mV, and at the bottom dot is -204.5mV. The VR2 makes the -200mV reference for the contol IC from these values.

Lightages:
I am glad you got it working, but don't be too happy. As any owner of the UT71X series will tel you, they aren't reliable.

csadam:
New problem appeared, or old problem but I just noticed it now :)

I measure 1.000 volt. It displays 1.000 volt. OK.
But when I activate the PEAK mode the displayed value changes to 1.400 Volt. Then I deactivate the PEAK and it shows 1 volt again.

So someting is wrong with the peak calculation. I dug deep into the internals of the multimeter, and I've sniffed the communication between the measuring IC (Cyrustek ES51966 F) and the controller IC (MSP430F149). Surprisingly the PEAK function does not use the measuring IC-s peak capabilities, because no such command goes into it when the peak mode is enabled. Moreover the measured counts, which are sent by the measuring ic to the controller, are not changed at all during the peak mode.

So I've narrowed the problem to the control IC. I can think two possibilities:
1. there is problem with the firmware
2. the calibration data in the EEPROM is damaged

I'm not sure how can I step towards from this. I would be grateful if someone could send the data in the eprom of his VC940 or UT71E. The data is read from the eeprom via i2c bus at the boot, so it is relative easy to get the data with a logic analyzer.

I can help with the details of this if someone have enough courage to do it :)

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