UT61E drift and recalibration

[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