UT61E drift and recalibration

[Rick Law]

UT61E owners – What is your experience with the UT61E's first year drift?

My UT61E is near 1 year old now.  I am curious as to what the typical drift may be.  I am considering getting the DMM Check Plus, but want to compare notes with fellow owners.

I also noted from another post on this forum on “replace dc calibration potentiometer on ut61e” that if/when calibrating the UT61E using VR1, even closing the case back will cause a few digit difference, and that the UT61E is rather temperature sensitive.

That post was about replacing VR1 and not about (re)calibrating the UT61E in general.  What are your thoughts with recalibrating using DMM Check+?  Worth the trouble?

Thanks
Rick

[Wytnucls]

Here is a drift comparison between my UT71B/D and 61E, which was done about 6 months ago. The meters all have the original factory calibration. The 71D was used as a reference. You will notice that the 71s are very close to each other, while the 61E had drifted quite a bit.

[Lightages]

The drift problem is probably why I never replaced my blown UT61E. I liked it at first but I found I had to keep tweaking it and that really isn't the way a good meter should behave.

[Wytnucls]

Just did another drift comparison between meters. The UT61E seems to be getting worse, whereas the UT71s are pretty much together. The 71s have 5ppm/c precision film 0.02% resistors, hence less drift I assume.
The table lists the LSD differences, with the 71D as the standard. All readings were very steady, down to the last digit, with a linear power supply from 2 to 14V DC.

[Rick Law]

Just did another drift comparison between meters. The UT61E seems to be getting worse, whereas the UT71s are pretty much together. The 71s have 5ppm/c precision film 0.02% resistors, hence less drift I assume.
The table lists the LSD differences, with the 71D as the standard. All readings were very steady, down to the last digit, with a linear power supply from 2 to 14V DC.

Thanks for the info!  From your post, it looks like I am looking at about 10 count drift.  My requirement (with accuracy) is not high.  10 count drift doesn't please me, but it is (unhappily) acceptable to me.

The drift problem is probably why I never replaced my blown UT61E. I liked it at first but I found I had to keep tweaking it and that really isn't the way a good meter should behave.

While I am curious about the drift, at the same time, I feel a little silly spending $62 (DMM Check Plus) to check on my $60 DMM.  You "tweak it" and get it back in line.  Are you tweaking VR1 alone?

Thanks, both for the info.  You are very helpful.

Let me rephrase the question this way:
The DMM Check Plus gives me a +5v +- 0.1% to 5 digits.  If I can re-calibrate VR1 (alone) with the 5V reference, I am hoping to bring it back to say reduce the error by 1/2 (say from 10 digit off to 5 digit off).  Am I dreaming or is that realistic?

I still feel a little silly spending $62 to adjust a $60 DMM, but that would be at least more than merely satisfying my curiosity...

Thanks again, both Mr Wytnucls and Mr Lightages.  I appreciate your help.

Rick

[Lightages]

A DMM Check, or DMM Check Plus are good investments. You mad a small mistake. The DMM Check Plus gives you 5VDC at 0.01% accuracy. This is what is needed to provide a calibration reference for something on the order or 0.1% accuracy. So, it is the perfect thing to use for the UT61E.

It is hard, if not impossible, to get a better spec'd meter than the UT61E for the price. If you buy another meter then you might end up having two meters you are not sure of. If you buy another meter in the future the DMM Check will still be useful and can help confirm the accuracy of both.

If you are considering buying a second multimeter, I would suggest you need to spend around $150 to get much better functionality than the UT61E. What you should be looking for is a safer meter for higher energy circuits to compliment UT61E.

[Monkeh]

If you are considering buying a second multimeter, I would suggest you need to spend around $150 to get much better functionality than the UT61E. What you should be looking for is a safer meter for higher energy circuits to compliment UT61E.

Assuming he's working on high energy circuits.

[Lightages]

If you are considering buying a second multimeter, I would suggest you need to spend around $150 to get much better functionality than the UT61E. What you should be looking for is a safer meter for higher energy circuits to compliment UT61E.

Assuming he's working on high energy circuits.

Yes of course. 

[Rick Law]

A DMM Check, or DMM Check Plus are good investments. You mad a small mistake. The DMM Check Plus gives you 5VDC at 0.01% accuracy. This is what is needed to provide a calibration reference for something on the order or 0.1% accuracy. So, it is the perfect thing to use for the UT61E.

It is hard, if not impossible, to get a better spec'd meter than the UT61E for the price. If you buy another meter then you might end up having two meters you are not sure of. If you buy another meter in the future the DMM Check will still be useful and can help confirm the accuracy of both.

If you are considering buying a second multimeter, I would suggest you need to spend around $150 to get much better functionality than the UT61E. What you should be looking for is a safer meter for higher energy circuits to compliment UT61E.

You are right! 0.01% and not 0.1%.  Typo error on my part.

I don't work on anything more than 20v-30v.  I am merely poking around trying to do something fun so it is hard to say I need it.

I am really itching for the darn thing (getting the DMM Check Plus), and I think I am close to talking myself into it.  Your "So, it is the perfect thing to use for the UT61E" is helpful in my decision making...

[BravoV]

What kind of voltage references that is used in this kind of DMM ? External or built-in at the main chip ?

[iloveelectronics]

You can also get one of those Chinese voltage reference devices with 4 voltages (2.5, 5, 7.5 and 10V) that has been discussed here at a lower price. In fact I have some of those too (just never had the time to list them on eBay) but I won't be shipping the lithium battery like other sellers. They are really just taking their chances to be honest, because both HK Post and China Post officially do not allow air shipment of lithium batteries.

A DMM Check, or DMM Check Plus are good investments. You mad a small mistake. The DMM Check Plus gives you 5VDC at 0.01% accuracy. This is what is needed to provide a calibration reference for something on the order or 0.1% accuracy. So, it is the perfect thing to use for the UT61E.

It is hard, if not impossible, to get a better spec'd meter than the UT61E for the price. If you buy another meter then you might end up having two meters you are not sure of. If you buy another meter in the future the DMM Check will still be useful and can help confirm the accuracy of both.

If you are considering buying a second multimeter, I would suggest you need to spend around $150 to get much better functionality than the UT61E. What you should be looking for is a safer meter for higher energy circuits to compliment UT61E.

You are right! 0.01% and not 0.1%.  Typo error on my part.

I don't work on anything more than 20v-30v.  I am merely poking around trying to do something fun so it is hard to say I need it.

I am really itching for the darn thing (getting the DMM Check Plus), and I think I am close to talking myself into it.  Your "So, it is the perfect thing to use for the UT61E" is helpful in my decision making...

[Wytnucls]

What kind of voltage references that is used in this kind of DMM ? External or built-in at the main chip ?

The band-gap voltage reference (-1.2V) is inside of the UT61E Cyrustek ES51922 chip (75 ppm/C).
So has the 71D, fitted with Cyrustek ES51966, with a slightly better 50 ppm/C.
However, the 71 series uses a Maxim external voltage reference with a 2ppm/c specification (max 5ppm/c).

[PedroDaGr8]

The UT61E has a smattering of unpopulated spots which make up the circuit for an external vref (IC, resistors and filter caps). It SHOULD be relatively easy to add if you can figure out the required dividers (mainly I couldn't find a value for the input impedence of the external Vref pin). This is a project that I have had on the back burner for a while now.

[Monkeh]

The UT61E has a smattering of unpopulated spots which make up the circuit for an external vref (IC, resistors and filter caps). It SHOULD be relatively easy to add if you can figure out the required dividers (mainly I couldn't find a value for the input impedence of the external Vref pin). This is a project that I have had on the back burner for a while now.

Would that be U4?

Please elaborate on the details here, I'm rather curious.

[PedroDaGr8]

The UT61E has a smattering of unpopulated spots which make up the circuit for an external vref (IC, resistors and filter caps). It SHOULD be relatively easy to add if you can figure out the required dividers (mainly I couldn't find a value for the input impedence of the external Vref pin). This is a project that I have had on the back burner for a while now.

Would that be U4?

Please elaborate on the details here, I'm rather curious.

Yeah it is U4 and the unpopulated spots near by. I'll try to find the IC that will work with it when I get home this evening.

[Rick Law]

Ahem,  Just to close off my thoughts on UT61E and DMM Check Plus...

Taking advantage of a moment of lax of self discipline, I stumbled into the voltagestandard site quite by accident, (I was controlled by unknown alien presence at the time) and some how I managed to click BUY...

I promised myself that I will not exceed my entertainment budget and I just did - The alien control was so powerful that I even clicked the dual frequency option and enclosure option.  I will punish myself later for such lack of discipline.

Now that is really really silly.  I just spend $90+ to check on my $60 dmm.  You can now begin laughing at me.

I am well prepared with proper excuses:
Excuse 1 - I have 5 dmm's, I need to know which one is more wrong
Excuse 2 - with the dual frequency, I can use my $90 checker to check if my $70 cheap USB scope drawing different square waves
Excuse 3 - My next project is to do a "Tricorder", it should be able to measure anything.  I need something to validate the tricorder readings.
Excuse 4 - I didn't do it.  I was temporarily controlled by aliens taking over my clicking finger.

Now let me go kick myself for exceeding my self-imposed entertainment budget.  But I really like the idea that I can do a sanity check.  That my measurement is by some standard with certain percentage of some standard...

[M0BSW]

 My experience with the 61E is also on resistance, I have  in a box a 5.7 ohm 1% resistor, my two Alpatek's  trusted meters read 5.7 dead on , however my 61e did read 5.7 now its drifted to 6.0 ohms , in a year. I would adjust it but have know ideal which pot to twiddle plus I'm not sure if 1.3 ohms is acceptable with the specs of the 61E.

[Rick Law]

My experience with the 61E is also on resistance, I have  in a box a 5.7 ohm 1% resistor, my two Alpatek's  trusted meters read 5.7 dead on , however my 61e did read 5.7 now its drifted to 6.0 ohms , in a year. I would adjust it but have know ideal which pot to twiddle plus I'm not sure if 1.3 ohms is acceptable with the specs of the 61E.

I found part of the problem is the probe and the socket doesn't make very good contacts.  I can twist the probe banana plug (as if it is a dial) and get +- 0.3ohm delta easily as I turn the plug like a dial.  Cleaning the contacts helps for a while.

I suspect if you clean the plugs, jiggle it, you may get 6.0 ohms back down to 5.7ohms.

[stephenlm324]

I have recently purchased a UT61E and found that the stability could be better. I found some interesting information on an external Vref which deserves credit but I don't do Chinese. The link "http://forum.eepw.com.cn/thread/239953/1" has the required info to retro fit the UT61E with an external Vref LT1790. The temp co. is typ. 5ppm. as opposed to 75ppm in the ES51922. I can confirm the accuracy of this info as I have just completed the mod along with replacing the pot VR1 with a resistor network and a 50ohm pot. The standard is 2k and drifts 1% (20ohm) with vibration alone.
The mod requires the LT1790ACS6-1.25 two 10k resistors one 10uF cap all SMD. R15 needs to be removed and replaced in position R51.
My meter is already looking much more stable, I suspect the standard Vref warms up after switch on inside the main chip, taking some time to settle.

[george graves]

^That's nice!  I think I'll wait for someone to do a English write up before I would give it a go! 

Since the 61E is a 22,000 count meter, would it be better to calibrate it at something just below 2.2 volts so that you get the full count on the meter?

[Rick Law]

My experience with the 61E is also on resistance, I have  in a box a 5.7 ohm 1% resistor, my two Alpatek's  trusted meters read 5.7 dead on , however my 61e did read 5.7 now its drifted to 6.0 ohms , in a year. I would adjust it but have know ideal which pot to twiddle plus I'm not sure if 1.3 ohms is acceptable with the specs of the 61E.

I assume you are using the lowest which is 220ohm range.  Auto-range would have selected this range for you.

Spec spec for this range is 0.5%+-10 digits.
% error: reading is 6.0ohm. 0.5% of that is 0.03ohm
digit error: At this range, each digit is 0.01ohm so digit error is 10*0.01=0.1ohm.

Total error from spec +-(0.03+0.1) = +- 0.13ohm

You are out 6.0-5.7=0.3ohm off.  Spec is 0.13ohm error for 1 year.  Your meter is out of spec.  But, I think it may be dirt building up in a year.  I cleaned my probe plugs and I took down the resistance by 0.5ohm just from the cleaning.

If you are tired of flipping through the manual or don't have the spec handy, I posted this a while back:

I hate flipping through the manual to look for the %accuracy/specs each time I change the dial on my meter.  So, I cut and pasted the UT61E's spec for all the selections into a single-page layout.  Small prints, but I can see all the %accuracy for all the ranges on this little poster on my wall.

Thought other UT61E may find this useful.  If you like it, feel free to use it.

(Since the original manual is available for the public to download, I am rather sure this consolidating into single page would fall under the "fair-user" clause of the copyright act.  Well, I think so anyway.)

https://www.eevblog.com/forum/testgear/uni-t-ut61e-multimeter-teardown-photos/?action=dlattach;attach=39571

[Rick Law]

Anyone trying to calibrate UT61E resistance measurement?

Looking at the schematic, it looks like VR3 may do the job.  But VR3 has (CAP) written right next to it, so I am not sure the schematic is out of date since I know there are a few revisions of UT61E out there.  (EDIT: add this:)Further investigation points to VR3 is for capacitor measurement 220nF range.  Looks like the resistance measurement is not adjustable but controlled purely by the reference resistors.  I hope I am wrong with this because I sure would like to be able to recalibrate resistor measurement.

Has anyone tried calibrating the ohms range?  Success? failure?

Thanks
Rick

[stephenlm324]

I have been looking at calibration of the resistance range. I can confirm that VR3 is for cap.
The resistance calibration appears to be fixed, set by the voltage divider resistors (the black side on package). The divider pack would need to be removed and trimmed with external resistors, which would require a good meter. That would then mess up the AC and DC volts ranges.
Further reading to shed light on how it works can be found in the KAD7001 data sheet, similar chip with better documentation.
http://images.ihscontent.net/vipimages/VipMasterIC/IC/SAMS/SAMSD027/SAMSD027-63.pdf

[Rick Law]

I have been looking at calibration of the resistance range. I can confirm that VR4 is for cap.
The resistance calibration appears to be fixed, set by the voltage divider resistors (the black side on package). The divider pack would need to be removed and trimmed with external resistors, which would require a good meter. That would then mess up the AC and DC volts ranges.
Further reading to shed light on how it works can be found in the KAD7001 data sheet, similar chip with better documentation.
http://images.ihscontent.net/vipimages/VipMasterIC/IC/SAMS/SAMSD027/SAMSD027-63.pdf

I think you mean VR3 there.  VR4 looks like it is for AC adjustments.

[stephenlm324]

Yes, thanks, I have corrected it.

[stephenlm324]

I have drawn a blank on calibration of the resistance range. The voltage divider network could be replaced with a Caddock 1776-C4815 $23 from Digikey postage of arround $50, gasp. But I suspect that even this might not help. Page 9 of the ES51922 data sheet referring to the "divider ratio" mentions R6, so where is it? I had been under the impression that it was referring to the voltage divider resistors but there are only 5 of those. The only conclusion I can come to is that the divider is "on chip". I am open to suggestions.

[M0BSW]

My experience with the 61E is also on resistance, I have  in a box a 5.7 ohm 1% resistor, my two Alpatek's  trusted meters read 5.7 dead on , however my 61e did read 5.7 now its drifted to 6.0 ohms , in a year. I would adjust it but have know ideal which pot to twiddle plus I'm not sure if 1.3 ohms is acceptable with the specs of the 61E.

I found part of the problem is the probe and the socket doesn't make very good contacts.  I can twist the probe banana plug (as if it is a dial) and get +- 0.3ohm delta easily as I turn the plug like a dial.  Cleaning the contacts helps for a while.

I suspect if you clean the plugs, jiggle it, you may get 6.0 ohms back down to 5.7ohms.

Spot on Rick I did and it did back down to 5.7, well I never cheers Rick calm is restored.

[notsob]

try replacing the original leads with fluke or pomona ones, the result is a huge lesson in what quality costs (the fluke leads prob cost more than half of what you paid for the 61E)

[Monkeh]

I have drawn a blank on calibration of the resistance range. The voltage divider network could be replaced with a Caddock 1776-C4815 $23 from Digikey postage of arround $50, gasp. But I suspect that even this might not help. Page 9 of the ES51922 data sheet referring to the "divider ratio" mentions R6, so where is it? I had been under the impression that it was referring to the voltage divider resistors but there are only 5 of those. The only conclusion I can come to is that the divider is "on chip". I am open to suggestions.

R7, 100R, connected to pin 22 (OR1)... It's under the display.

[Rick Law]

My experience with the 61E is also on resistance, I have  in a box a 5.7 ohm 1% resistor, my two Alpatek's  trusted meters read 5.7 dead on , however my 61e did read 5.7 now its drifted to 6.0 ohms , in a year. I would adjust it but have know ideal which pot to twiddle plus I'm not sure if 1.3 ohms is acceptable with the specs of the 61E.

I found part of the problem is the probe and the socket doesn't make very good contacts.  I can twist the probe banana plug (as if it is a dial) and get +- 0.3ohm delta easily as I turn the plug like a dial.  Cleaning the contacts helps for a while.

I suspect if you clean the plugs, jiggle it, you may get 6.0 ohms back down to 5.7ohms.

Spot on Rick I did and it did back down to 5.7, well I never cheers Rick calm is restored.

Glad that helped!

[stephenlm324]

I have drawn a blank on calibration of the resistance range. The voltage divider network could be replaced with a Caddock 1776-C4815 $23 from Digikey postage of arround $50, gasp. But I suspect that even this might not help. Page 9 of the ES51922 data sheet referring to the "divider ratio" mentions R6, so where is it? I had been under the impression that it was referring to the voltage divider resistors but there are only 5 of those. The only conclusion I can come to is that the divider is "on chip". I am open to suggestions.

R7, 100R, connected to pin 22 (OR1)... It's under the display.

Already tried that, tacked a resistor across it but nothing changed.

[Rick Law]

I have drawn a blank on calibration of the resistance range. The voltage divider network could be replaced with a Caddock 1776-C4815 $23 from Digikey postage of arround $50, gasp. But I suspect that even this might not help. Page 9 of the ES51922 data sheet referring to the "divider ratio" mentions R6, so where is it? I had been under the impression that it was referring to the voltage divider resistors but there are only 5 of those. The only conclusion I can come to is that the divider is "on chip". I am open to suggestions.

R7, 100R, connected to pin 22 (OR1)... It's under the display.

Already tried that, tacked a resistor across it but nothing changed.

Stephen,

Let me make sure I understand what you said.  Do you mean you changed the reference resistor (connected to OR1) and nothing changed?  -OR- do you mean you tacked a resistor across specific divider and nothing changed?

Thanks
Rick

[stephenlm324]

I tacked a 1k across R7 (OR1 pin) and checked the meter with 1K 10k and 100k 0.1% to find no change. I have redone this fiddle to find that the 200ohm range does change, meter reading 110ohm with 100ohm connected.
I now have 68k across R7 giving a correction I can live with. 100ohm 0.1% reading 99.92 instead of previous 99.79, which means my resistors are in spec now. Unfortunately this only effects the one range they are all out by about the same percentage.
Longer test leads would have the same effect.
I have tried a resistor across R8(VR5 pin). This appears to be the reference resistor for the 2k range as well as part of the voltage divider. This should be a value of 1.0001k for the divider making it less than ideal for use a reference resistor but would explain why the ranges are all on the low side.

[Rick Law]

….. Page 9 of the ES51922 data sheet referring to the "divider ratio" mentions R6, so where is it?...

R7, 100R, connected to pin 22 (OR1)... It's under the display.

Already tried that, tacked a resistor across it but nothing changed.

From my research:

I also gathered from other docs that VR6=OR1, but that is NOT part of the divider.  Read onward to my reply below to your other post below:

I tacked a 1k across R7 (OR1 pin) and checked the meter with 1K 10k and 100k 0.1% to find no change. I have redone this fiddle to find that the 200ohm range does change, meter reading 110ohm with 100ohm connected.
I now have 68k across R7 giving a correction I can live with. 100ohm 0.1% reading 99.92 instead of previous 99.79, which means my resistors are in spec now. Unfortunately this only effects the one range they are all out by about the same percentage.
Longer test leads would have the same effect.
I have tried a resistor across R8(VR5 pin). This appears to be the reference resistor for the 2k range as well as part of the voltage divider. This should be a value of 1.0001k for the divider making it less than ideal for use a reference resistor but would explain why the ranges are all on the low side.

Again, from my research (one may call data archaeology...  this is a lot of digging), this is what I gathered:
220ohm range uses VR6 (aka OR1) ONLY
2.2KOhm and 2.2KV range use VR1 & VR5 (for 1/10000)
22KOhm and 220V range use VR1 & VR4 (for 1/1000)
220KOhm and 22V range use VR1 & VR3 (for 1/100)
2.2MOhm and  2.2V range use VR1 & VR2 (for 1/10)
22MOhm and 220MOhm (no Volt range on these two) uses VR1 only.

VR1=R34=10MOhm
VR2=R11=1MOhm
VR3=R10=100K
VR4=R9=10K
VR5=R8=1K
VR6=OR1=R6=100Ohm

To adjust 2.2Kohm, 22Kohm, 220Kohm, and 2.2Mohm in one shot, adjusting the 10MOhm (VR1) would do it.  The 10MOhm (VR1) should be R34 from the schematic.  I can’t find R34 (which should connect to R39 and R39 in turn connects to the input jack).  I think it is on the underside (see teardown link initial post by stdnn, 2nd from last picture in the original post), but I don’t see a resistor there.

https://www.eevblog.com/forum/testgear/uni-t-ut61e-multimeter-teardown-photos/

I don't have the skill (or just plain lack of courage) to attack that problem particularly when I can't find R34.  But I think it would be helpful to your quest.  Looks like adjust VR1, you change all ranges between 22K to 220MOhm, or you can adjust VR2,VR3,and VR4 for 22K, 220K, and  2.2M individually.

If you found R34 and have a go at it, keep me posted.  I am trying to build up my courage - that I destroyed my daughter's phone when I last attack something this small was...well, something I am trying to get over.

Rick

[PedroDaGr8]

Hate to say it but R34 is staring you in the face. Based on the schematic and tracing the board it's the divider.

Sent from my EVO using Tapatalk

[Rick Law]

Hate to say it but R34 is staring you in the face. Based on the schematic and tracing the board it's the divider.

Sent from my EVO using Tapatalk

Arrgh, guess I need to "upgrade" my reading glasses yet again.  I'm looking at the tear-down photos and not the PCB.  Is it on the fuse side or the LCD side?

[PedroDaGr8]

Hate to say it but R34 is staring you in the face. Based on the schematic and tracing the board it's the divider.

Sent from my EVO using Tapatalk

Arrgh, guess I need to "upgrade" my reading glasses yet again.  I'm looking at the tear-down photos and not the PCB.  Is it on the fuse side or the LCD side?

Your glasses are fine, it's actually not labeled. I traced out the circuit using the schematic. It's the big black box on the fuse side. R39 is on the lcd side.

Sent from my EVO using Tapatalk

[stephenlm324]

I like the term archaeology.
Adjusting VR1 may give unexpected results if my latest theory is correct. VR1 is used in a different way on the resistance range to volts.
 The resistance range needs a set of reference resistors. 100R 1K 10k 100k etc. The values in the divider are 1.0001k, 10.01K, 101.01k, 1.1111M and the 10M. The 1.0001k is close enough on its own but the 10.01k has the 10M switched in parallel to give  9999.99001k. The 101.01k with the 10M in parallel is 99999.901k and the 1.1111M in parallel with 10M is 999909.9991M. It saves money and is close enough so I should be impressed.
But the 10M has a different effect on each range so if it was adjusted to 9M it would effect the 1M range by 1% but would effect the 100k range by 0.1% and the 10k range by 0.01%. Not quite what we want. It does however shed light on the chip data sheet page 9 section 1.2 which has been driving me nuts.

[Rick Law]

I like the term archaeology.
Adjusting VR1 may give unexpected results if my latest theory is correct. VR1 is used in a different way on the resistance range to volts.
 The resistance range needs a set of reference resistors. 100R 1K 10k 100k etc. The values in the divider are 1.0001k, 10.01K, 101.01k, 1.1111M and the 10M. The 1.0001k is close enough on its own but the 10.01k has the 10M switched in parallel to give  9999.99001k. The 101.01k with the 10M in parallel is 99999.901k and the 1.1111M in parallel with 10M is 999909.9991M. It saves money and is close enough so I should be impressed.
But the 10M has a different effect on each range so if it was adjusted to 9M it would effect the 1M range by 1% but would effect the 100k range by 0.1% and the 10k range by 0.01%. Not quite what we want. It does however shed light on the chip data sheet page 9 section 1.2 which has been driving me nuts.

I like the term archaeology.
Adjusting VR1 may give unexpected results if my latest theory is correct. VR1 is used in a different way on the resistance range to volts.
 The resistance range needs a set of reference resistors. 100R 1K 10k 100k etc. The values in the divider are 1.0001k, 10.01K, 101.01k, 1.1111M and the 10M. The 1.0001k is close enough on its own but the 10.01k has the 10M switched in parallel to give  9999.99001k. The 101.01k with the 10M in parallel is 99999.901k and the 1.1111M in parallel with 10M is 999909.9991M. It saves money and is close enough so I should be impressed.
But the 10M has a different effect on each range so if it was adjusted to 9M it would effect the 1M range by 1% but would effect the 100k range by 0.1% and the 10k range by 0.01%. Not quite what we want. It does however shed light on the chip data sheet page 9 section 1.2 which has been driving me nuts.

Yeah, VR1 is used with VR2-5 for four different volt ranges which I wrote earlier.  So, any change to VR1-VR5 will affect at least Volts and Ohms.  (Don't see anything else, but there could be).
2.2KOhm and 2.2KV range use VR1 & VR5 (for 1/10000)
22KOhm and 220V range use VR1 & VR4 (for 1/1000)
220KOhm and 22V range use VR1 & VR3 (for 1/100)
2.2MOhm and  2.2V range use VR1 & VR2 (for 1/10)

To do the job right, one probably has to make adjustments to all VR2 to VR5 individually, and then recalibrate it for either Volts or Ohms - in other words, if one is exact and the other is not good enough, a choice has to be made.

What would be fun to do is for each from VR1 to VR6, to parallel each with one that is perhaps 10x the value, and serial a 0.1x VR value trimpot.  With all 6 adjustable, one can play around till kingdom come and see how well the UT61E can perform.

Not to be too self-centered...  While I am not planning to such major surgury to my UT61E, the info I dug out are still turn out to be useful to me.  The DMM Plus resistor measurements can be used to suggest how much the other voltage range are off - nowI know which resistance range and voltage range use the same divider resistors.  In my case, the four DMM Plus Resistors are off by 0.11%, 0.19%, 0.09%, and 0.095%.  So, the corresponding voltage range (using the same set of VR's) would likely be off the same amount.

[Nuno_pt]

Hi,
Has anyone play with the CW1,2,3 pots?

[stephenlm324]

Hi,
Has anyone play with the CW1,2,3 pots?

These trimmers are for AC voltage, to improve linearity at high frequency, so I'm not going to touch them.

[Nuno_pt]

Hi stephenlm324,

I see that you did Chinese external Vref mod and replace the VR1 2k trimpot with two resistors and an 50Ohm trimpot, as this mod improve the calibration and stability?

Did you use an 50Ohm precision pot and what values do you use on the resistors ?

Did anyone gather any new info about calibrating the resistance side, maybe replacing R34 and the others with more precise ones?
     

I just saw this mod on the mjlorton forum for adding a bunch of new functions using the yellow and blue buttons, see the link ( http://mjlorton.com/forum/index.php?topic=285.0 )
and the YouTube video, would this be worth the trouble?

[cuban8]

My UT61E (~3 years old) came with an "out-of-the-box" modification: An LM385 (TO92) and a series resistor 
No idea why, maybe the internal reference was faulty and somebody decided to"salvage" the meter by slapping another reference on...
Looking at the specs for the LM385 this is not really an upgrade though.

[Wytnucls]

Gosh, what a mess. Where did you buy that lemon?

[cuban8]

German distributor, wasn't even from ebay.
Performance-wise the LM385 is probably not too far away from the reference in the Cyrustek chip (Cyrustek lists 75ppm/°C typ, LM385 is 150ppm/°C max. for the standard grade, so typical drift, while not stated, is likely similar). Measured against my 10V reference, it has drifted ca. 5mV since I got it, which is not too bad.
I'm not impressed with the soldering though.

[Nuno_pt]

It was better to remove that mode and do de Chinese external Verf mod witch is 5ppm/C°.
Stephenlm324 have done that mod and also replace the VR1 pot with two resistors and an 50Ohm pot, and claims better stability and accuracy, see on page 2 post

I've a friend that works here in a factory that makes all kind of resistors, including precision and I'll ask him if he can bring me some precision resistors better than 0.01% 10ppm/C°, and I want to see if I can get my hands on some silver mica caps with ~1% 50ppm/C° to measure on all the DMM ranges and try to adjust capacitance with the VR3 pot (Since from what I can see it's the only thing that will affect capacitance, I'm I wrong on this?)
 

[cuban8]

Yeah, doing the LT1790 mod would definitely improve things. I'll try to get hold of one of those.

[iloveelectronics]

That Chinese mod looks very interesting and simple to implement, I think I will give it a go myself! Ordering the parts now...

[Nuno_pt]

Franky,

The Chinese mod (Vref) is no big deal, like change the VR1 pot with 2 resistors and a 50 Ohm pot, for better stability and accuracy on the Vdc.

I'm thinking to make the mod to add more functions, list on the mjlorton forum.

Add:

Backlight for 60s
Auto power after 15min function (ON/OFF)
RS232 port (ON/OFF)
MAX/MIN / MAXMIN

See the link ( http://mjlorton.com/forum/index.php?topic=285.0 )

Franky u got PM.

[cuban8]

Looking at the schematic, there is no need for using two resistors when changing the VR1 pot; VR1 is used as a rheostat (variable resistor) in series with R16, not as a voltage divider. Therefore, you can reduce VR1 and increase R16 (or add another series resistor).

[Nuno_pt]

cuban8,

Nice catch, do that or only replace the 2K pot with an more stable one, but one with 5% tolerance and 10ppm/C° will cost about the same as the DMM.
So increasing / adding / replacing R16 for another more precise and lowering / replacing VR1, should give more precise and stable readings in Vdc.

[cuban8]

Actually, tolerance is basically irrelevant for both VR1 and R16 - you're trimming this resistance anyway. Tempco is more important, but since VR1 and R16 form a voltage divider with R15, this resistor also needs to be changed for a better one if you really want to improve things.

[Nuno_pt]

Ok, since we are one this, the schematic that I've list VR1 = 1K and R 15 / 16 = 9K09, we already know that VR1 is 2K, so the best is to take out the resistances and measure them and then replace both of them, for new values if you change the VR1 value, I think that Vdc will've better stability and accuracy with this mod.

Now I'm looking at the capacitance, for what I can see on the ES51922 data, R1k (pin 49) and R9k (pin 48) require an precise 1k and 9k resistors, they have 909k (R17) and 9k09 (R19) on the same schematic, and R18 (1k) is this in series / parallel with the other two, for better accuracy should this three be remove and place some precise ones with low % and low ppm/C°?

If you change that would it be better to change also VR3 (10k pot), R33 (100k) and R36 (200k x 4)?

Does VR3 adjust all ranges of capacitance or just one?

EDIT:For adding Capacitance below.

From what I can see on the ES51922 data, page 11 parag. 1.5:

C1 is adjust by itself for 22.000nF range (this should be an precise cap, % and ppm/C°), for more precise readings and stability.
VR3 adjust only 220.00nF = That is C2
10k adjust 2.2000nF = That is C3 (On my schematic there is one 100k resistor between C2 and C3, connected to pin 9 = BUFFH)

The ranges from 22.000µF to 220.00mF are all adjust by the ratio of the pins R9k/R1k, (R17,18,19 network) the more precise this network more precise the ridings on this ranges.

Is my reading correct on this?

[stephenlm324]

Although fitting the LT1790 improved the temp stability it did not cure the problem. I have stuck a thermocouple on the PCB under the main chip and tested with a 2V reference over 18 to 28 degrees C. Without the LT1790 the display count changes by 10, doing a good impression of a thermometer. After the LT1790 was fitted this dropped to a count of 6 over the same temp change.
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. This compensation could be used without the LT1790 but the series resistor would need to be 780k ohm (calculated) to get more compensation, probably cheaper as well. This will of course mess up the calibration but no problem if the horrible 2k pot is still in.
Temp stability of the resistors in the potential divider should not be an issue, so long as they are all the same coefficient they will go up and down together with temp, and the voltage should stay the same.
Big problem is that 2k multiturn, the data sheet for this type of pot states 1% error with vibration which is 20ohm. I replaced with a 50ohm and a couple of resistors in series (there wasn't one standard value so used two). Vibration susceptibility now 0.5ohm and much easier to adjust. To figure out what resistor/s are required desolder the pot, measure the resistance with another meter and subtract 25ohm so that it's in the middle of the travel on 50ohm replacement.
The good news is that the resistance range does not drift significantly with temp trimming R34 will not help.

Thanks to Nuno_pt for spotting the mistake, now corrected.

[Nuno_pt]

Setephenlm384,

Thanks for the follow up on the update about the temp stability, seem that we are starting to get somewhere even without external Vref, the big must in here is always replace the 2k trimpot for one more stable / lower value, to get the precision and stability.
Like you point out if we can get it stable at 0 / 1 count at the 18° to 28°C  temp range that will prove that this meter could sold from factory without being more expensive, just by trimming better the components.

Could you leave the meter on in that temp range for about 24h or more and see what is the drift after that time with external Vref and without only with the resistors arrangement, one thing that I read about the references it's that you've to burn it for about 300 hours or more to make them age, then redo the calibration.

As for the resistance I don't see nothing to improve the ranges that uses R34 as an divider.

I'm looking now at the capacitance side, like I describe in the early post, after looking an little better, R19 is not present, only R17 (9k09) and R18 (1k).

On the 1M and the thermistor in series shouldn't be R16 instead of R1.

[Nuno_pt]

Another Chinese Vref mod see here ( http://www.crystalradio.cn/thread-398876-1-1.html ).

Would be nice if someone made the translation.

[markman]

Translated:

http://translate.google.com/translate?sl=auto&tl=en&js=n&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.crystalradio.cn%2Fthread-398876-1-1.html

I think the LT1790 might still be a better match with specs maybe slightly worse but current consumption also less.   

So...  If one substitutes all the key resistors and adds a reference, then this might become a stable meter over time??   Maybe while we're add it, we add some circuitry so it can do low-voltage current measurements?  Is there anything significantly better for $60 + $25 in resistors + $10 in reference parts?

Does anyone have a schematic for this meter?

[Nuno_pt]

Hi markman,

The schematic that I've is in attach, is the one that is on this forum, but be careful that this DMM has suffer many revisions.

R16 is 7.55k will on schematic is 9k09, the other is R19 is not there and so one.

About the low current measurement is an problem in all multimeters, some more than others, and is problem from the shunt resistor, so there is nothing that we can do about.

Only attenuate by Dave's µCurrent.

[stephenlm324]

I have finished fiddling my temp compensation. The meter now reads correct from 18 to 33 deg C briefly displaying +1 count.
I suspect it is the main chip that is temp sensitive, certainly the on chip Vref is off so it's a good bet that the rest of the chip is off.
Anyone considering adding the external Vref might consider using LT1790BCS6-1.25 which has a typical temp coefficient of 12ppm they are £2.70 GBP from Farnell, half what I paid for the top spec and I still had to add the compensation.
I have seen no sign of any long term drift. All of the heating and cooling I have done would accelerate this but it always goes back once the temperature settles. Any reports of long term instability should be put down to vibration and temp changes as these are having a much bigger effect.

[Nuno_pt]

Stephenlm384,

Can you post some pictures and the schematic of your mod, so the mod will be better understand by all, (1 picture worth 1000 words).

Glad that you got it stable at ~1 count, that is pretty impressive, now we've a stable DMM from 18 to 33°C.

Now that we've the stability problem solve, maybe we can address the calibration on the capacitance side, if that will be worthy of the time, what you think?


I've just found this from another Chinese mod, they are calibrating the resistance side, with trimmers.

See link ( http://bbs1.38hot.net/thread-6945-1-1.html )

[Lightages]

I am curious. What are the costs for the external ref mod and the other parts? It seems to me that this is turning a relatively inexpensive meter into something more expensive and then also needs to be recalibrated after the mods to be of any use. Is it worth it? If it is just for the fun of it then yes of course it is worth it but as far as being of real value?

[stephenlm324]

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.

[don.r]

Would these thermistors serve? http://www.ebay.ca/itm/10-pcs-Thermistor-Temperature-Sensor-NTC-MF58-3950-B-100K-ohm-5-DIY-New-/121252422483?pt=LH_DefaultDomain_0&hash=item1c3b352353&_uhb=1

They have a Beta of 3950K vs 3977K for the Vishays you got. They are also 5%.

[Nuno_pt]

It should be ok.

See also the research that was made regarding Ohm and Cap.

[don.r]

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.

Would 50R be sufficient for VR1? I was thinking of using a 200R to get an order of magnitude reduction in vibration deviation but still allow for a wider adjustment over time.

[Nuno_pt]

See the comments from stephenlm324 on the previous pages, he switch with one of 50Ohm, and a network of two resistors.

The more course the pot (Ohm) more drift it will have, most have 1%, it's more drift on 200Ohm that on 100 or 50Ohm.

[don.r]

As I understand it, its not the drift on VR1 that is the issue but more the vibration sensitivity which seems pretty constant across all these pots at around 1%. So instead of 20R of change we get 2R for a 200R pot or 0.5R for a 50R pot. My question to Stephen, I guess, is why 50R and not another value?

[mcinque]

Maybe the complete schematics could be useful to find what exactly what that VR does. Hope it helps.
Found on http://www.frankshospitalworkshop.com/electronics/diy-voltage_reference.html

[don.r]

I am curious. What are the costs for the external ref mod and the other parts? It seems to me that this is turning a relatively inexpensive meter into something more expensive and then also needs to be recalibrated after the mods to be of any use. Is it worth it? If it is just for the fun of it then yes of course it is worth it but as far as being of real value?

As no one has replied to this yet and I am in the process of applying several of the mods here is the cost.

LT1790ACS-1.25 - $5 (I got two for that price but that is the going rate)
caps/res - 10c total
NTC thermistor - 20c (10 for $2)
200ohm (or 50ohm) genuine Bourns pot - $2. (Chinese knockoff - 50c)

So all together less than $10... probably a lot less if you check your junk box or shop around.

[Lightages]

Hmmmm, so now the "bargain" UT61E goes from being a $60 meter to being a $70 meter, and still no proof of long term drift benefit, has very poor input protection, and needs to be re-calibrated. Don't forget the time to do the mod in the equation. I am not so sure I would recommend this meter anymore. I hope those who have taken the time to do this actually benefit from it. If you want the benefit to last a while, I would look at adding the input protection too. It won't necessarily make it safer for you but might prevent the meter from dying prematurely.

[Monkeh]

$5 of GDTs fixes the input protection (really, the fuses are fine. Really. You're not measuring 800VDC current with this meter. If you are, just think again.).

I think it's still a pretty capable meter for the price, but like any tool it has its limits and they must be known.

[don.r]

I'll be adding a couple of MOVs when I do the VRef mod. I don't normally measure anything over 150V so a 300V MOV should do it. Long term drift will only be known over the course of time maybe a year after the mod is done. It only takes a few minutes to do the work as all the pads are there. I will say that it really is only a 2200 count meter without the mods and you could pick up good 6000 count for the same money. I have another mod in mind however that is only capable with this meter. I'll let you know what it is when I have finished it.

[sorin]

I will say that it really is only a 2200 count meter without the mods and you could pick up good 6000 count for the same money.

Which model do you refer?

$5 of GDTs fixes the input protection

What you mean with "GDTs" ?

[don.r]

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).

[Nuno_pt]

Hi all,

I think that was some tutorial about changing the PTC that he brings with GDT or MOV, but can't find it.

The long term stability can only be view after an year after the Vref mod, against a calibrated source, DMMCheck Plus after the annual calibration, or something better.

Trimming the resistance side it's much more trouble since it controls V + Ohm, you've to mess with all the resistors in that chain + R34.
The 100Ohm rage it's adjust by itself, just put a lower value.

Still studying the capacitance side.

You can spend maybe another $40 in mods, the time doesn't count it's just for fun, so I think that for $80, 4.5 digits with 22.000 counts but more precise and stable, and maybe better on Ohm and pF, it's hard to beat for the price.

You can always by a HP 3478A, or others for around $150, but not portable, maybe not much space on the lab, etc...

[don.r]

The PTC doesn't need to be changed unless you are going above 600V which I believe is the minimum breakdown for them (WMZ11 75HV). There are mounting holes for the GDT/SG/MOVs already next to the PTC (as you can see in the teardown) but I'm not sure how the added components, especially the MOV, will affect capacitance. I'm not terribly fussed as I have a dedicated cap meter already.

I think a lot of the drift was down to VR1 constantly moving out of cal from vibration, a 20ohm change at 1%. I'll look at things longer term once I get the 200ohm Bourns in there.

[sorin]

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).

Thanks, don.r!
I think that MOV-s are inadequate for UT61E, because you lost the hight impedance input on mV range.
You can find GDT down to 70-100V.

[don.r]

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).

Thanks, don.r!
I think that MOV-s are inadequate for UT61E, because you lost the hight impedance input on mV range.
You can find GDT down to 70-100V.

Is that due to the capacitance of the MOV? I'll dig a little deeper into the GDT.

[sorin]

Its about the leakage of the MOV.

[Nuno_pt]

Hi,

Are the components 0805 or 1210 imperial?

Don't have anything to measure and I'm not use to work with smd.

[don.r]

Yes.

0805 = 2012 = 2.0mm X 1.2mm
1210 = 3225 = 3.2mm X 2.5mm

[Nuno_pt]

Hi,

Ok guess, R = 1210 and C = 0805, right?

[don.r]

R = 0603 I believe. That is what I measured on the board.

[Nuno_pt]

I need the cap for C50, and resistors for R52, R53, can't find them here local.

[iloveelectronics]

I need the cap for C50, and resistors for R52, R53, can't find them here local.

I have some of those I can send you if you don't mind the small postage fee and the wait.

[Nuno_pt]

Franky,

Thanks for the offer, I just need to know the sizes, I'll be ordering a batch of components from Farnell and from Mouser, for other projects, so I'll include this on the batch. 

[iloveelectronics]

I need the cap for C50, and resistors for R52, R53, can't find them here local.

I have some of those I can send you if you don't mind the small postage fee and the wait.

don.r is correct. The cap is 0805 and the resistors are 0603.

[Nuno_pt]

Hi ali,

Just received my UT61e today from Franky, it brings one fuse of 10A and another of 1A (bs1362), still brings ply 2 PTC's, and still miss the Ext Vref from factory.

 I'll only upload 3 pictures so you can see this board.

[Wytnucls]

and in the midst of calibrating, i find that the UT61E seems to take some time to settle to a stable reading in AC, does that happen to all of you 61E users? i have other meters and it seems this settling time is too long for UT61E.

It is normal. It is a quirk of the AD737. The true RMS to DC converter has a long settling time for small input levels. The settling time is also influenced by the value of the external averaging capacitor (33uF on the 61E).

[Wytnucls]

Well I think 1 volt or 200mV is the maximum input for the converter, so depending on the scaling, 5s for 5Vrms may be ok. Mine is settling after about 4s. The 71D in 8s (AD636 CAV 10uF) and the Gossen 26S in about 5s also.
No much to worry about.

[hgg]

Have you noticed if the UT61E has a drift "preference"?
(Showing a greater than or a smaller than the actual measured voltage)

Thanks.

[Nisei]

Big problem is that 2k multiturn, the data sheet for this type of pot states 1% error with vibration which is 20ohm. I replaced with a 50ohm and a couple of resistors in series (there wasn't one standard value so used two). Vibration susceptibility now 0.5ohm and much easier to adjust.

How many counts does that leave you to correct up and down? If it's still a lot then perhaps an even a lower Ohm trimmer would be possible?

[DrMag]

Bringing up an old thread--I'm very interested in doing the Vref mod on my UT61E. Unfortunately, I think I have a later version of the meter, and there's no U4 at all, that I can find. 

Does anyone with a newer UT61E know about this? Is this mod still easily done (without jumpering directly to the processor, at least)? There are spots on either side of the board labeled as VREF-, and I'll do some sleuthing to see if these are connected to the right pin, but before I get crazy I wanted to check and see if anyone has made this modification recently.

[Fixup]

Hi,

I also was planning to do some modifications to the UT61E. I opened it up and it is a bit different from the pictures found on the forums.
I am planning to at least change the potentiometer and possibly also do the reference modification. In my case I am more interested in long time stability than in temperature stability. Temp stability could be fixed with the thermistor.
Temp stability could possibly be measured with a known voltage in some different temperatures. After that I would use LTspice or some other simulation program to calculate the needed thermistor with some serial resistance to adjust the slope.

Please tell us how you modification went 
 

 

[MosherIV]

Hi

I am new to the forum here.

Been thinking about getting the UT61e, so have been looking at the tear downs, the schematic, etc.
(I have a Tenma - re-badged UT70b)

With regards to doing the U4 Vref mod on the later revision boards, where the U4 pads have gone.
Try looking at the post (same thread) "My UT61E (~3 years old) came with an "out-of-the-box" modification: An LM385 (TO92) and a series resistor" (2nd page)
The mod looks simple to do : disconnect R15 from the chip (this provides the Vref from the chip).
Copy where the resistor and LM385 goes in the photo.
You can get LM385 with temp co of 20ppm/C which is better than the 75ppm/C of the Cyrustek chip.

I would also definitely replace VR1 which calibrates DC Volts and resistance, 100ppm/C is easily available.
Bournes or other reputable brand from known good source.
Better than 100ppm/C is going to cost serious money (£15 upwards) and have wrong footprint.

I am curious to know what is in the black package - is it a precision laser trimmed resistor pack?
(That is the only way to get 0.1% claimed accuracy. You see them in Flukes. Not sure how they do it for the price.
Maybe because they use cheap ones, that is why we get localised temperature drift)
Personally, I would not do the thermistor mod for temp drift. Need to work out the root cause of the temp drift instead of a work around.
As I said, I wonder if it is the resistor pack they have used.

Also, definitely fit the MOVs (labeled SG1, 2, 3) - if nothing else will protect the meter from ESD.

[Arhammon]

I tested (In 22-33C range) 3 multimeters: 2 Victor with LM385 and UT61E with internal reference. It is same tempco.
Video (in russian):
https://youtu.be/DhKY4iilgC8

[MosherIV]

Hi

Just got my UT61E. Wooopie 

Measures very accurate at the moment, last digit on UT61E is only 1 count out worst case compare with a TTI1905a against a AD594JN
Will have to test again in 1 year to see if there is a bad drift.

Temp stability.
I notice there are 2 versions of the board
The original board has a black plastic cover over the precision resistor divider network.
The RevC board (the one I have) has a metal cover over the plastic covered precision resistor divider network.
I am waiting for a sunny day so that my conservatory will reach 40C so that I can test for temp stability.
I suspect UniT knew they had a problem and added the metal cover to stabilize the temp coefficient.
(I suspect the reason why Fluke use a precision resistor divider network on ceramic is because the ceramic is highly temperature stable)

[MosherIV]

So it was sunny enough yesterday that I put my UT61E in a box in the conservatory for an hour. I also put the other UniT meter (Tenma 72-7755 otherwise known as UT70B) and Keysight U1232a in the box as well.
I did a quick check with each meter before putting them in the box.
Sadly, this was in the evening and the conservatory was cooling down.
The box only reached 30C, not the near 40C I was hoping for

So I did not bother moving the TTI1905a and AD594JN voltage standard .
I did a quick check by bringing the meters in and just quickly measuring the voltage standard.

Both the UniT meters showed a drift with temperature.
The Keysight read the same as before.

So, sadly the UniT meters do suffer from temperature drift 
Both UniT slowly drifted back to the readings before I put them in the box, so they do recover.

Just looking at a tear down of the Keysight U1232a, I do not see the ceramic precision resistor divider array, so I wonder how they achieved the temperature stability.

I might try wrapping the UT61E precision resistor divider array with insulating polystyrene (my one came wrapped in the stuff) and see if that makes a difference, I doubt it since it is the substrate that need to remain stable.

[Fungus]

Both the UniT meters showed a drift with temperature.

How much? Details, man, details....

[MosherIV]

How much? Details, man, details....

Sorry, because there was not enough of a temperature differential and it was not conducted scientifically enough, I did not write down the details.

The readings were out by a max of 5 counts on the LS digit.
It is sunny again today, maybe I will try again tonight or over the weekend

[joeqsmith]

I've thought about running a test like this as well.   I checked a UT210E's TC when someone posted about being concerned with drift.   If you can run it and get some numbers, please post them.   

[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  )

[joeqsmith]

DT830B 2.5V=2.50V 5V=5.00V 10V=10.00V
The DT830 only chnages by 2 counts worst case. Amazing when you consider how cheap it is 

(Sorry the patio door is very dirty  )

Thanks for running this again.  10mV / count would have guessed it would be solid.  Even your 61E does not look too bad. 

[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