UT61E drift and recalibration

[MosherIV]

Hi

So I fianlly had time to run the experiment properly.

I first moved my Thurlby 1905a 5.5 digit dmm and the AD594JN voltage reference to the patio door.
I decided to measure the UT61E, Keysight U1232a and the clasic DT830B
I measured the reference with all the dmm under test (26°C and 60% relative humidity):
1905a 2.5V=2.4987v, 5V=5.0039V, 10V=9.9968V (control)
UT61E 2.5V=2.498V 5V=5.003V 10V=9.997V
U1232A 2.5V=2.498V 5V=5.003V 10V=9.99V
DT830B 2.5V=2.49V 5V=5.00V 10V=9.98V

[MosherIV]

I then moved the box with the 3 dmm under test into my hot conservatory. Left them there for over 1 hour to acclimatise.

Then turned on the meters and measured again :
1905A 2.5V=2.4987V 5V=5.0039V 10V=9.9968V(control 26°C)

DMM under test, measured 37°C sorry no humidity measurement in conservatory.
UT61E 2.5V=2.499V 5V=5.005V 10V= 10.001V
U1232A 2.5V=2.498V 5V=5.004V 10V=9.99V
DT830B 2.5V=2.50V 5V=5.00V 10V=10.00V

Not sure what to conclude, yes the temperature did affect the reading but only by 3 counts worst case but that is within specification.   

The Keysight shows no change what so ever, but then I would expect that.

The DT830 only chnages by 2 counts worst case. Amazing when you consider how cheap it is 

(Sorry the patio door is very dirty  )

[Rango]

An Amprobe AM-270 for high voltage use or a used Fluke.

GDT = Gas Discharge Tube. You can use those or spark gaps or MOVs. Spark gaps and GDT operate at 1KV and above usually and I have no plans to take this meter anywhere near there so I'll stick to a 275V MOV (clamps at 600VDC).

For input protection adding 3 of those MOVs would be sufficient. They are also 30pF so it would not bump capacitance that much but one can always Delta capacitance anyway correct? I posted more indepth question here but don't want to repost in tow post so will just post a link here
https://www.eevblog.com/forum/testgear/uni-t-ut61e-multimeter-teardown-photos/msg1349602/#msg1349602

Vishay VDRS - E Series Metal Oxide Varistor 30pF 5A, Clamping 1200V, Varistor 750V (178-059)
http://uk.rs-online.com/web/p/metal-oxide-varistors/0178059/?searchTerm=178059&relevancy-data=636F3D3126696E3D4931384E525353746F636B4E756D626572266C753D656E266D6D3D6D61746368616C6C26706D3D5E5C647B362C377D7C5C647B31307D7C28283F69292852537C5253207C52532D293F5C647B337D285C73293F5B5C732D2F255C2E2C5D285C73293F5C647B332C347D292426706F3D3126736E3D592673743D52535F53544F434B5F4E554D4245522677633D4E4F4E45267573743D313738303539267374613D3031373830353926

[Per Hansson]

To get rid of the last of the temp drift I have resorted to a thermistor (Vishay NTCLG100E2104JB Farnell 116-4823) in series with 1Mohm across R16 (9.09k on the schematic but measured at 7.55k). This gives a count of 2 drift at the moment, I need to fiddle more, dropping the 1M to 947k should do the trick.

As the details for the external Vref are listed earlier I have left that bit out. The 780k ohm resistor (calculated value without external Vref) would be 680k + 100k in series and would probably need adjusting to match each meter, but it would be a good starting point. With the external Vref I am using 947k but again this would probably need adjusting for each meter. If you can get hold of the thermistor without paying a fortune for postage it would be a very cheap way of improving the meter.

As the input protection on my TUV / GS rated UT61E died I'm ordering some parts on Elfa.
I noticed the LT1790BCS6-1.25 voltage reference is on sale on their site so I ordered the necessary components as my meter is quite old and has the requisite pads on the PCB...
As can be seen from the quotes by stephenlm324 above this will really require a thermistor too, so I ordered that as well however it's on backorder.
That will give me a good way to see the before and after response though so I do not complain
But one thing has me confused: in the first quoted message above stephenlm324 says to put the thermistor and resistor in parallel with R16.
But in the second quoted message there is a schematic attached (hotlinked below) because someone asked for clarification, and here the thermistor and resistor is shown in parallel with R16+VR1.
Can someone that has done this modification please clarify which it is?

[Serg65536]

My new UT61E in all AC modes very slowly returns to 0. It takes 2 minutes to drop down from several volts AC to 0. Is this normal or should I return it? (probes are short circuited while waiting)

[Per Hansson]

My new UT61E in all AC modes very slowly returns to 0. It takes 2 minutes to drop down from several volts AC to 0. Is this normal or should I return it? (probes are short circuited while waiting)

They do behave this way, here is an old thread about it with more examples:
https://www.eevblog.com/forum/testgear/uni-t-ut61e-does-not-show-zero-in-ac-range-with-shrten-test-leads/msg2766422/#msg2766422

[VE2UM]

BAD START...

My references are a 10.0000 Volt reference designed by VE2AZX and based on a Texas Instruments REF102C and my second is a 5.0000 Volt of my own design and based on a MAXIM Integrated MAX6350. Both are buried Zener devices and are correlent to eachother.

Upon reception of the UT61E ( aug. 4 2011 ), it was off by -4 count on 10.000.

Fortunately, it is easily calibrable using an on-board blue multiturn 2k trimpot (VR1).
You'll have to open the multimeter. I seriously think about adding a peep hole to calibrate it w/o having to open it.

And today (aug. 31 2011) it was off, again by -6 count. i recalibrated it.

More news later...

The UT61E has an "all-in-one" 22000 count multimeter IC (ES51922 by CYRUSTEK, a chinese manufacturer.) and unfortunately, does not have a separate voltage reference.

Also, unlike its little brother, the UT61C, the UT61E does not have any battery saver function. In this case, it might be a good point since it will aid the ES51922 to age.

Otherwise, the multimeter is well built for its price. AC / DC volts use a precision integrated resistor pack with carefully designed HV cutouts.

UPDATE ( sept. 2011 ):

I finally found the faulty component in the UT61C:

The DC CAL trimpot. VR1.

VR1's major problem is its way too high value (2000R) and it's ( ) Mandarium quality.

After a fresh calibration, I unsoldered it, measured it (with another DMM obviously) and got 528R.

Following that, i ordered a high quality 50R multiturn 3299 series trimpot from Bourns along with a (528 - 25R) fixed
resistor, in my case, 500R, .05 % 1206 SMD device.

The 25-ohm substraction is to allow the trimpot to be calibrated around its midrange. In my case, the resistor-trimpot combination cover a 500 to 550 ohm range, centered on 525R.

By changing the trimpot from 2000R down to 50R, i have a 25x narrower trimming range making the UT61E much smoother to calibrate and way more stable over time.

The trace between the trimpot and R16 had to be cut, soldermask removed and fixed resistor soldered through the cut.

BEWARE: Your multimeter's VR1 has to be measured before you order the series fixed resistor. It's value shall be close to your measured value minus 25 ohms, so the 50R trimpot will correctly calibrate your meter near its midrange. Also, when reassembling LCD / Zebra strip, make sure grounding spring is well seated on the LCD's shield.

I made this mod in 2021 and today, it isstill rock solid. Knowing that, i'd opted for a 25R BOURNS ( instead of a 50R ) trimpot and a SMD fixed resistor @ +/- 0.1 % tolerance  ( original trimpot value - ( 10 to 15R ) ).

[joeqsmith]

I was expecting new data, not a copy and paste from 2011.   It's been about a year since I tested mine.   It drifts around 3 counts in the 10V range after heavy mods.

[iMo]

A pity you have not captured the ambient temperature with all that measurements as well..

[joeqsmith]

A pity you have not captured the ambient temperature with all that measurements as well..

Maybe humidity as well.  These measurements were always taken in my home office.  The meter is also stored in my office to keep it's internal temp stable with the ambient air temp.   

My office temperatures vary maybe +/-2.5 deg C from winter to summer.   The UT61E had more temperature drift than other meters I looked at.  I had made an attempt to temperature compensate the reference.   This work was done prior to starting my long term study.    If you look at Oct to April, these will be the colder times in the lab.  The drift does not seem to track it.  It's also much drier during this time as well.   

[ptluis]

Well, since you're talking about the UT61E, my unit suddenly went crazy, and had the following problems:

1. Instability in mV mode, with or without the test leads inserted, showing random values between 20 and 180 mV, never showing OL.
2. Instability in Resistance mode, always showing random values on the MΩ scale (in kΩ, Ω, no problems). Never showing OL.
3. Instability in V mode (AC and DC). The measured value wouldn't stabilize and start counting down every second, for example, the measured value was 4.2133 V, then decrease by one digit every second on any scale, e.g., 4.21132V, 4.2131V, and so on... never stabilising

I also cleaned, inspected under an optical microscope, and tested the ES51922A's pin connections to the pcb, measured voltages, etc., and everything was fine, but the device didn't work properly.

After testing various parts, including the commuter, etc., I began to suspect the IC was damaged.

So, as a last resort, I decided to remove the ES51922A, clean the PCB, and solder it back on. And voila! The multimeter worked normally again! Stable measurements!

The only problem I noticed was the 200 MΩ scale, which displayed a measured value much lower than the actual resistance measured, around 20-30 MΩ.

Since this multimeter doesn't have a potentiometer for adjusting the resistance scale, the solution I found was to adjust the value of R39. In my case, I had to lower the original value. The resistance bridge had the correct accuracy. Only by adjusting R39 was I able to adjust the readings to the 200 MΩ scale.

Here's my adventure fixing this device, maybe it help someone with the same problem.

Solution: Remove, clean, and resolder the ES51922A, Also check and clean the commuter and check the plastic tabs that lock the plastic part containing the metal contacts. This part must stay clipped to the pcb not falling if you remove the pcb from the case. If this part become loose from the pcb you will have issues with the commuter and selected mode. It happened quite often on my unit.