[SOLD] (USA): Uni-T UT61E DVM (damaged)

[60Hurts]

All,

Fried this DVM trying to measure the output voltage of a transformer for an e-fence.  Unit beeped and then shutdown and now the DC Volts are stuck at -0.4102 no matter the V input (not an offset, it doesn't work).   Other functions look okay but I have not tested it.  I found a repair post here on this or a similar issue for this unit over a year ago but I can't find it now and even if I could find it again the repair was beyond my abilities.  Price is only my cost to ship (preferrably CONUS) if someone is interested and wants to take a shot at fixing this unit.  Picture below is generic from a web search but mine looks the same.

PM me if interested or post any questions here. 

Regards,
David

[Nusa]

Ouch! Multimeters can't be used to measure electric fence voltages without high voltage probes that usually cost more than the meter. Simple electric fence testers with idiot lights can be had for under $20, however.

[60Hurts]

Oof, yes lesson (RTFM) learned ;-) 

[bdunham7]

I'll give it a try and I'll post everything I do here in case anyone is interested.  The only caveat is that nothing happens fast on my bench.  I'll PM you.

[60Hurts]

Great. One caveat though... out of curiosity I just tried to measure AC volts and resistance, no bueno.

[bdunham7]

Well, I think it's toast.  It arrived today and I took a minute to tear it apart and check it out.  As advertised, it shows a constanct ~-.41V on DCV, -41mV on the mV range and corresponding amounts on the current ranges.  If you rance up, the displayed voltage goes up too---410V in the 1kV range and so on.  Ohms shows OL and Hz works.  So I have a nice Hz meter.  I tried just removing Q1, Q5, Q6 and Q8, no difference.  I even removed D4 before I realized that it was part of the function switching, so I put it back.

You'll see that the schematic calls for 4 MOVs, SG1-4.  You can see from the picture that they aren't installed on this version.  I'm thinking those might have helped with the electric fence incident.  Maybe not, but I have to wonder about those CAT ratings...or maybe I don't.  The 7D7-751K are four for a buck from Mouser, so they literally saved a buck.  Or less in bulk.   

There is actually voltage on the terminals in the DCV mode.  I measured ~-2.9V from V+ to COM using a high-impedance meter, that dropped in half to ~1.4V with a 10M impedance meter.  I tested the regulator W1 and COM is +3.0V from VB-, so that seems good.  It seems obvious that there is leakage from VB- to VR1 pin 44 of U1.  I could speculate as to the internal design of U1, but it doesn't matter much--U1 is toast, or so I conclude.  Does anyone have any other ideas?

[Swainster]

Agreed - sounds like the ADC in the ASIC is dead. That's the problem with these highly integrated DMM chips - once the main chip fails then it's done. I have a UT-61D on my bench which has probably been zapped by ESD. In my case the input multiplexer/attenuator seems to have died as only AC and DC volts have stopped working, everything else, including mV is still OK. However everything is integrated into that one big unobtainium (at least outside of China) chip.

[60Hurts]

Sorry to hear the bad news. I was really hoping for an epic save.  It is going to take me a bit but I am going to walk through your diagnosis so I can learn more about electronics repair.  BTW, I think that 9V battery is fairly new so that is a small consolation. I doubt you have a big need for a dedicated Hz meter but hopefully the probes, serial cable come in handy or the unit itself can be canibalized for parts on some future project.

Thanks for your efforts and the update.

Regards,
David

[Nusa]

At the very least, you've got a couple of spare fuses that will fit many meters.

[bdunham7]

At the very least, you've got a couple of spare fuses that will fit many meters.

Unfortunately in addition to omitting the MOVs, they used the puny downsized 250V fuses as well, even though there is PCB space for the bigger ones.  I've never seen a UNI-T product before and I always wondered how bad could they be?  Another 5 bucks in parts (4-5 MOVs, 4 more transistors, real fuses) and a better job of cleaning the residue off the board and it might not be so terrible, although it still wouldn't be great.  One thing that surprised me was that it has a gazillion R & C trimmers, so apparently the calibration is all manual. 

[Nusa]

At the very least, you've got a couple of spare fuses that will fit many meters.

Unfortunately in addition to omitting the MOVs, they used the puny downsized 250V fuses as well, even though there is PCB space for the bigger ones.  I've never seen a UNI-T product before and I always wondered how bad could they be?  Another 5 bucks in parts (4-5 MOVs, 4 more transistors, real fuses) and a better job of cleaning the residue off the board and it might not be so terrible, although it still wouldn't be great.  One thing that surprised me was that it has a gazillion R & C trimmers, so apparently the calibration is all manual.

To be fair, the 250V limit is printed by the current jacks. Not that I've ever used current ranges for mains voltages. Ever.

[modz786]

I found a ut61e on ebay very low price that Im waiting for and just for comparison when it arrives I will post a pic. Meantime what date code has yours in the bottom left of the PCB photo?
www.ebay.com/itm/232012252451

[Gyro]

Joe Smith spent a lot of time breaking and fixing his UT61E during his torture testing. Specifically, he analysed and fixed the issue of it failing the gas ignitor 'ESD' test. That's probably the closest to an electric fence hit. He did a fair number of videos on it - I suspect that he has a grudging fondness, but I'm sure he would never admit it!

Here's the one containing the ESD fix (at about 19 mins)...

[Gyro]

At the very least, you've got a couple of spare fuses that will fit many meters.

Unfortunately in addition to omitting the MOVs, they used the puny downsized 250V fuses as well, even though there is PCB space for the bigger ones.  I've never seen a UNI-T product before and I always wondered how bad could they be?  Another 5 bucks in parts (4-5 MOVs, 4 more transistors, real fuses) and a better job of cleaning the residue off the board and it might not be so terrible, although it still wouldn't be great.  One thing that surprised me was that it has a gazillion R & C trimmers, so apparently the calibration is all manual.

To be fair, the 250V limit is printed by the current jacks. Not that I've ever used current ranges for mains voltages. Ever.

Nor me! Even so, even if applied to 240V mains, those 25mm BS1363 fuses are still Fast HBC, rated at 6kA, more than the prospective fault current of virtually all UK domestic premises. They're also very cheap, especially in the UK (plug fuses) making people less likely to bodge or buy fakes. So not a terrible choice in the context of a meter of this grade even though, of course, a bigger 10kA higher voltage fuse would provide much greater protection.

[joeqsmith]

Joe Smith spent a lot of time breaking and fixing his UT61E during his torture testing. Specifically, he analysed and fixed the issue of it failing the gas ignitor 'ESD' test. That's probably the closest to an electric fence hit. He did a fair number of videos on it - I suspect that he has a grudging fondness, but I'm sure he would never admit it!

 

It's a popular meter, inexpensive and had the added bonus that the schematics were available.   Being popular, a lot of people post about sprinkling in various parts to make it more robust and other beginner hacks.  My goal of the videos was to dispel some of the hype and show some of the basics involved in hacking a meter (for educational purposes only).  This isn't a meter that I actually use for anything beyond these demonstrations.

More electrically robust (survives ESD and is on par with other higher class meters) at the cost of bandwidth
Improve temperature drift (on par with some of most stable handheld meters I have looked at)
Design and construct a custom current shunt allowing higher currents to be measured (includes large HRC fuses)
Install better banana jacks, reference trim pot   
Greatly improve the burden voltage when measuring uAs
And of course, auto back light adjustment 

I have little doubt that the meter's main IC would have survived being connected to an electric fence.  I also doubt it would survive connecting it to a MOT, NST and any other number of things.   I dare say I could damage one now just by walking into my lab and touching it (winter, dry house).

[bdunham7]

More electrically robust (survives ESD and is on par with other higher class meters) at the cost of bandwidth...

...I have little doubt that the meter's main IC would have survived being connected to an electric fence.

I skimmed through your video after Gyro posted it and I see we both immediately looked at the 2 5pF caps plus the trimmer across the 10M resistor and the obvious compromises they've made to achieve greater BW.  However I don't know the exact characteristics of a typical electric fence pulse (I do know what they feel like...) so I don't know if the damage is due to the fast edge characteristics or simply the sustained high voltages involved.  I doubt an actual fence would be as fast as your piezo ignitor.  If the electric fence output were 8kV for 250µs but not a remarkably fast edge, then perhaps that can either damage the IC through the 10M resistor or cause some part to temporarily break down and give the IC the full jolt.  So MOVs might be more helpful against electric fences. 

I obviously don't know the exact internal structure of the IC, but I'm assuming that something got toasted that gives me leakage to VB- and that is being clamped further in by some junction--perhaps a transistor pair with one damaged and the other conducting forward--to give that -0.41V.  Interestingly I also noticed that while the meter mostly functions otherwise, the 'µ' and 'm' of the µA and mA ranges don't come on.  Pin 45, right next to pin 44 VR1, is not connected on this meter but it apparently is related specifically to the function of displaying those two characters in meters that don't get that info from the manual function switches.  So I'm guessing that there is a nice crater blown in the die near where the bond wires from pin 44 & 45 attach.

[james_s]

Well, worst case you have a nice condition (I think) housing so if you can find a beat up or damaged one with a good board or some other fault you could fix it.

For testing an electric fence a grounded wire on the end of a stick is sufficient. If it makes a spark that jumps a few mm then the fence is working adequately.