UT61E drift and recalibration

[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).