Gossen Metrahit Pro - strange readouts when no test lead connected - Page 1
Forum section: electronics / repair

author: mildot
author_rank: Newbie
timestamp: August 03, 2022, 11:15:56 am
content: Hi to the EEVBlog community!I spent countless hours watching YT videos and thanks to Dave even got into electronics at the age of 40+I have just bought cheaply Metrahit Pro. Now I know why it was cheap - seems it has some problem. I only hope it can be fixed.Instead of writing long text I made a video presenting what I have checked and the actual behaviour of the meter. Hope you can suggest some possible casues as I myself dont have required exp to locate and fix it myself.YT Video:https://youtu.be/7nd_9H-2SwkLooking forward!

author: armandine2
author_rank: Frequent Contributor
timestamp: August 03, 2022, 08:04:08 pm
content: my AM Xtras do this without leads - but I don't use them without leads so I've never given it a second thought.I did turn it upside down and mine do what yours does.Having said that, once you lose confidence in something - real or imagined - it is hard to get it back.

author: thm_w
author_rank: Super Contributor
timestamp: August 03, 2022, 08:47:30 pm
content: Why was it cheap, was it sold as non working?

author: coromonadalix
author_rank: Super Contributor
timestamp: August 05, 2022, 06:21:57 am
content: there was some thread about bad shielding inside the meter ?? by joeqsmith ...he did manage to built a better shield to compensate a problem ... cant recall what exactly ... do a search here ...not sure it's related BUT mmmm ?EDIT : i think there is some youtube videos too

author: armandine2
author_rank: Frequent Contributor
timestamp: August 05, 2022, 06:32:41 am
content: I seem to remember Dan Sullivan is good on Ghost Voltages

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2022, 06:43:44 am
content: A somewhat slow response in the AC ranges can be normal. The analog RMS converter chips use a rather slow filter. The AD737 and related chips have the time constant going up with low signal and thus get a very sluggish response at the low end.The effect of turning the meter upside down could also be an effect of an external electric field, if the shielding is poor (e.g. oxidized contact or poor by design).

author: wraper
author_rank: Unknown
timestamp: August 05, 2022, 06:46:42 am
content: Seems normal to me. May be some small induced voltage due to it being set to AC and it's high input resistance is part of the thing. However its specs say the AC accuracy specification is valid within a display value range of 10% to 100%. The same issue as most of bench meters. Their true RMS converter like AD637 does not work quite well in beginning of the range.

author: inse
author_rank: Super Contributor
timestamp: August 05, 2022, 06:48:44 am
content: I am specially concerned about ghost readings in the current ranges

author: wraper
author_rank: Unknown
timestamp: August 05, 2022, 06:55:22 am
content: Checking what it shows when shorting the input terminals will give more idea what it actually is. Also you could check if internal shielding has a proper connection to PCB.

author: wraper
author_rank: Unknown
timestamp: August 05, 2022, 07:01:02 am
content: BTW some meters (especially Chinese) hide this TRMS converter imperfection by showing zero instead of actual reading below certain threshold but others don't do this trickery.

author: amwales
author_rank: Regular Contributor
timestamp: August 05, 2022, 08:45:06 am
content: Nice meter, take a look at thisand congratulate yourself on getting a lovely meter, get some leads a battery and read the voltage of the battery. When I bought my first expensive meter the same thing happened to me and I freaked out too. Enjoy your meter.

author: amwales
author_rank: Regular Contributor
timestamp: August 05, 2022, 08:50:43 am
content: Sorry to double post, you may find it useful to get a second meter if you don't already have one for measuring current and voltage in a circuit at the same time. You can get a cheap one for ~£20 that will claim to do everything, Bluetooth, LED torch for when you find yourself lost in the closet searching for Narnia, but still, a second meter can be useful to have. Good luck.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 05, 2022, 03:30:49 pm
content: some "shield" talk herehttps://www.eevblog.com/forum/testgear/effect-of-the-shielding-inside-of-cheap-dmms/If it's a normal behavior, i do have some meters who does that, and they don't give a ''false zero'', i can personally live with that .... meters at Milli Volts range can / could be very sensitiveFor some bench meters who have more than 10 megs of impedance under 2 or say 20 volts, we did saw here some charging effects / behaviours

author: nightfire
author_rank: Frequent Contributor
timestamp: August 05, 2022, 06:28:51 pm
content: My Agilent U1272A also exhibits a similar behaviour, which IIRC is also documented in the manual. Far worse when test leads are connected to the meter and form an antenna...

author: mildot
author_rank: Newbie
timestamp: August 06, 2022, 11:30:39 am
content: Great thanks for all your answers. Not only I haveearnt a lot on the meters but you also calmed me down.I have spend yesterday afternoon to read and watch what younsughested above. In fact in comparison with my other materii (Brymen 869) it behaves similar. It was just me who didn't notice Brymens behavior before.I spend some time comparing the results of various measurement from both and they are close enough.Also against the oscilloscope.The main difference can be accounted for different probes used. When I use same probes results are very close.Really appreciate your support in such trivial matter. You are great Guys!