Author Topic: ATTN: Micsig handheld scope owners with DC offset instability issues  (Read 10041 times)

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Offline eKretz

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Hiya fellas, recently picked up a discounted handheld scope on fleabay - posted in this thread:

 https://www.eevblog.com/forum/testgear/debranded-micsig-handheld-scope-on-sale-on-ebay/

and am having some issues with the DC offset wandering around. My bench scope doesn't do anything like this - I'm wondering if this is something I should be worried about enough to request a replacement - the seller is trying to play it down.

Quick video:

http://s240.photobucket.com/user/eKretz/media/Pub/TRIM_20161231_013516.mp4.html

On the 20mV/division range, the zero floats around nearly 8mV. Makes it tough to get a cursor measurement unless the acquisition is stopped - but then I have to wonder if it stopped anywhere near where it should be. This occurs with no probe attached as well as with a signal on screen. Higher ranges jump about the same percentage of the range. The range is way higher than the claimed 2% DC accuracy. Contacted the seller and was told I was using the trigger wrong? I think they are making excuses.

Edit: I believe this issue has been resolved - there appear to be damaged or faulty LM6518 ICs. See page 2 for further detail.
« Last Edit: January 16, 2017, 06:47:19 am by eKretz »
 

Offline mzacharias

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Re: DC offset wander
« Reply #1 on: January 07, 2017, 10:26:52 pm »
And it does this with probes shorted or the AC/DC coupling switch set to "ground" ?
 

Offline eKretz

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Re: DC offset wander
« Reply #2 on: January 08, 2017, 07:54:30 am »
Less with the coupling set to ground coupled, that cuts it down quite a lot though still visibly jumping around a bit even then, but yes just as bad with a probe connected and grounded. And that is with even good quality probes, not the cheapies supplied with the scopemeter.
 

Offline mzacharias

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Re: DC offset wander
« Reply #3 on: January 08, 2017, 01:29:58 pm »
I'm not sure I would expect a handheld digital scope to maintain a flat trace with no signal. DAC noise alone probably would account for this, unless it's really bad. My experience is exclusively with analog scopes however, so others will have to advise you on this.
 

Offline eKretz

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Re: DC offset wander
« Reply #4 on: January 09, 2017, 05:26:59 am »
Well I'm not really expecting a flat trace, but this seems like an erratic thing. It will be mostly okay for a few seconds, then bam! A jump upwards or downwards for a few seconds, then bam! A jump in the opposite direction.

This is definitely worse in the first two division settings of each range before the relay can be heard switching to a higher voltage setting range. In other words, the lowest setting of 20mV/division is bad, and so is the next one, then it's a good bit better until range is increased to 1V/division - at which point the relay can be heard switching to that setting range. Then it's awful again for 1V/division and 2V/division, then a little better until the relay switches range again.

At 1V/division with a ~2.1V RMS (~6V P to P) 60Hz sine wave, the mean voltage is jumping around 400+mV erratically. My bench scope with the exact same settings is staying within 4mV, and is WAY more stable. I could totally understand a little bit more wander in a handheld scope - but this seems hugely excessive to me - especially when some ranges are way better than others and the scope is rated at 2% DC offset accuracy.
« Last Edit: January 09, 2017, 10:39:03 am by eKretz »
 

Online Howardlong

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #5 on: January 09, 2017, 12:17:40 pm »
The tablet Micsig I have doesn't seem to have this behaviour, it's rock steady with a 2.1V RMS 60Hz sine input from a signal generator, and both mean and cmean measurements hovering around 2mV.
 

Offline eKretz

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #6 on: January 09, 2017, 04:27:49 pm »
Thanks, I appreciate you checking in. I got an idea from another member to tie both scopes together with a BNC adapter so I can show that it is specifically the handheld that is having the issue. I'll make a video of that today.
 

Offline mtdoc

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #7 on: January 09, 2017, 09:00:18 pm »
I do not have that much wander on my ebay debranded Micsig handheld.

With grounded probe or no probe attached, 10x probe setting, 20mV/div,  DC coupling,  I get about 2 mV of wander at most - which is well within the 2% spec (which would be 3.2 mV at that setting).

If I first ground the input coupling (which drops the wander to about 0.1 mV) then go back to DC coupling, the wander is only about 1 mV.
« Last Edit: January 09, 2017, 09:07:31 pm by mtdoc »
 

Offline eKretz

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #8 on: January 09, 2017, 10:42:33 pm »
Thank you for the input mtdoc! I had a feeling this was quite abnormal. Just watching a trace on the display - it can be seen jumping around quite erratically. Sometimes I also see it make a huge jump on screen but it's between measurement updates (measurements only seem to update once a second or so) so it doesn't register.

I made a video today of the handheld and my bench scope side by side during which I run the handheld through calibration and then feed the same signal into both scopes; then within less than a minute the mean voltage is jumping around over 200mV on the handheld. On the bench scope it's staying steady within 6mV. I'll post that tonight.

If they don't decide to replace this one after this video I guess I'll have to instigate something with eBay.
« Last Edit: January 09, 2017, 10:44:07 pm by eKretz »
 

Online nctnico

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #9 on: January 10, 2017, 12:37:33 am »
If you bought it through Ebay then open a case immediately! Only then can you elevate a case to a claim/refund if necessary. Don't ever wait for a seller to replace a product because the moment you find out the seller isn't going to replace or refund the period you can file a claim is passed.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline eKretz

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #10 on: January 10, 2017, 01:47:59 am »
Good advice actually, I just did that. I'm not particularly happy with the first response, already making excuses. I have had this unit for barely a week and they are already claiming I have had it over a month.

It's a shame really, I would really quite like it if it worked correctly.
 

Offline eKretz

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #11 on: January 10, 2017, 07:51:25 pm »
Okay, here's the video of both scopes fed the same signal at the same time. All settings are either the same or as close as they can be (timebases aren't exactly the same, but as close as I could get). I let both scopes warm up for an hour before doing the calibration on the handheld. Within a minute after the calibration the mean has wandered above and below zero to around a 200mV range at the 1V/division setting with ~6V peak to peak 60Hz sine wave. Both scopes DC coupled and at their full bandwidths, no averaging.

https://youtu.be/Vs3TblDIWMs
 

Online nctnico

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #12 on: January 10, 2017, 08:02:43 pm »
Why does it say 'scope is adjusting...' in the screen? Do you have some automatic signal/trigger setup enabled?
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline eKretz

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #13 on: January 10, 2017, 08:09:20 pm »
That's the self calibration. The procedure is to remove the probe(s) then perform the calibration. In the video I start calibration, then it self adjusts/calibrates, then I plug the BNC back in with the same signal going to both scopes. So this jumping around DC offset instability is immediately after the calibration. It only gets worse with more time.
 

Online nctnico

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #14 on: January 10, 2017, 08:29:09 pm »
Does the signal also wander around without the probe connected (nothing connected to the oscilloscope's BNC input)? That is not clear from the video. You need to make a clear distinction whether the problem is in the oscilloscope or the probe/cable.
« Last Edit: January 10, 2017, 08:31:07 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline eKretz

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #15 on: January 10, 2017, 08:36:10 pm »
Yes it does the exact same thing with no probe attached and I've already sent them a video of that as well. Here is a short vid of the scope with no probes attached - this is in the 20mV/div range though.

http://s240.photobucket.com/user/eKretz/media/Pub/TRIM_20161231_013516.mp4.html
 

Online nctnico

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #16 on: January 10, 2017, 08:48:50 pm »
In that case it is broken and should be replaced.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline mtdoc

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #17 on: January 10, 2017, 10:08:15 pm »
Within a minute after the calibration the mean has wandered above and below zero to around a 200mV range at the 1V/division setting with ~6V peak to peak 60Hz sine wave.

With the same signal, my debranded Micsig portable shows no visually apparent movement - and the measured mean wanders by only 2-3 mV. This is true without a prior self calibrate. Interestingly, if I do a self calibrate immediately prior, the measured mean wanders by a bit more,  about 30mv - but this still not visually apparent at that scale.  The mean does sit at about -200mV if DC coupled, but that's no bother.

eKretz - it would probably help your case if you submitted a less blurry video. It's hard to make out the readings on the scopes. You should probably also pan back at some point to show your cabling set up.
« Last Edit: January 10, 2017, 10:14:06 pm by mtdoc »
 

Offline eKretz

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #18 on: January 11, 2017, 12:17:00 am »
I had a hard time getting the camera to focus any better - I don't have anything but my cell phone to take video. I can read the on-screen numbers when I view it anyway. I don't have any good means of uploading either. My home internet is awful, I have to make a special trip to upload video - and although I understand my videos aren't professional quality (more like kid-level) they should be plenty good enough to see what's going on. I also was trying my darnedest to keep the videos as short and small as possible so I had some chance at uploading them without spending hours borrowing someone else's internet connection. I am disabled so not able to afford better internet ATM - that also makes it hard for me to get out of the house on a lot days.

If my scope sat steadily, I wouldn't give a darn about a slight offset within spec, but the jumping all around is a different story. And after calibration mine is at its best - if I made a longer video it would get up closer to 400mV difference from the high to low mean voltage.
« Last Edit: January 11, 2017, 12:25:13 am by eKretz »
 

Offline eKretz

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Re: HELP!! ANY handheld scope owners! DC offset wander question
« Reply #19 on: January 11, 2017, 02:44:35 am »
Well, looks like there are trying to screw me over. Here's the reply I just got:

Quote
Hi dear, our engineer just checked your video and issue, at the same setting to test this scope, the result is clear. It is normal thing to this oscilloscope, please notice the Tek TDS754 A price is almost 10 times expensive than this oscilloscope, people can understand this reason dear. You may also contact other buyers who brought this scope, get a video see if his scope is the same situation.
 If you want the same results as Tek TDS754 A, we highly recommend our MS500 Series handheld oscilloscope, we promise this oscilloscope must be can satisified you.

Can some of you other guys post a video of yours showing they aren't doing this? Unbelievable.
 

Offline eKretz

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Re: HELP!! ANY Micsig scope owners! Micsig trying to screw me! Need your help!
« Reply #20 on: January 11, 2017, 07:32:50 am »
I guess if you want a REAL scope don't buy Micsig, they pretty much said it themselves. I can't believe they are asking for videos of other peoples' scopes as proof. Considering they listed this scope as "fully functional" except for having disabled the multimeter function I am very disappointed.

You guys that don't have this issue on your Micsig please help me out! I need videos to refute their BS.
 

Online TheSteve

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Re: HELP!! ANY Micsig scope owners! Micsig trying to screw me! Need your help!
« Reply #21 on: January 11, 2017, 07:47:52 am »
I suspect you'll need to file an ebay/paypal dispute with them. Before you do I would contact them at least once more and give them the option to replace it or refund your money as it is defective. Let them know you will file a dispute if they do not resolve the issue as it is defective.
VE7FM
 

Offline Micsig_support

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Re: HELP!! ANY Micsig scope owners! Micsig trying to screw me! Need your help!
« Reply #22 on: January 11, 2017, 10:34:02 am »
Dear  eKretz,

We are sorry to hear this. But this is not the Micsig products, can you contact the seller to solve your problems?  And we also suggest you to contact them to return your products.

Micsig always try to provide the best products and services to our customers. if you have any question with Micisg products, and you can email to america@micsig.com or sales@micsig.com, we will reply you very soon.

Micsig technical support
JL
 

Offline BravoV

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Re: HELP!! ANY Micsig scope owners! Micsig trying to screw me! Need your help!
« Reply #23 on: January 11, 2017, 10:41:31 am »
Hmm .. I thought all this time its a Migsig brand ?  :o

eKretz, whats the brand anyway ?

Offline eKretz

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Re: HELP!! ANY Micsig scope owners! Micsig trying to screw me! Need your help!
« Reply #24 on: January 11, 2017, 11:07:16 am »
It's "debranded" (name label removed) but of course it's sold by the Micsig factory eBay seller and when I initially complained about the problem they referred me to a Micsig email address - from which I got the above quoted message. So I don't know - you tell me what brand it is. It's just more dishonest/weaselly behavior as far I'm concerned.

Now since I just added the name Micsig they are all of a sudden much more conciliatory. Except now they are asking me to pay to ship the thing back on my dime rather than replacing it or issuing a prepaid label for the shipping of the defective item - which ain't gonna happen.
« Last Edit: January 11, 2017, 11:08:52 am by eKretz »
 


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