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What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?
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joeqsmith:

--- Quote from: bdunham7 on September 02, 2020, 04:04:07 pm ---I just took apart an old 8842A that apparently was connected to excessive voltage--at least 1500V---with some power to back it up.  The entire front end protection--4 MOVs and 2 fusible resistors--were incinerated.  I removed the burnt parts and subbed in 2 regular 1K resistors (to be repaired properly later, of course) and the meter works properly and is in cal.  No evidence of any external damage, the circuit board is unharmed (but sooty)  and I'm sure nobody died.  I'd say that is 'robust' although I can't quantify that.

--- End quote ---

As I have said, the first DMM I ever owned was a brand new Fluke 8000A bench meter.  Fairly costly and on par with what you may find free at Harbor Freight.   This meter has little to protect the sensitive custom ICs.  A simple mistake and you're changing $70 or so (back then) for new ICs and a handful of precision matched passive parts.   Could it be repaired? Sure, I repaired it more than once.   But the downtime and cost drove me to retire the Fluke and move to HP3468A.   An old story.   

While I would guess every meter I have evaluated could have been repaired, given enough time and money,  once the main controller is damaged I scrap them.   In very rare cases, I have gone so far as to replace ICs to recover them.   Only the 121GW prototype, Summit/TPI 194II, Yokogawa TY720, and UNI-T UT181A all fell into this category.   

There have been some really low end meters, like the UNI-T UT90A that have such poor clearances, they sustain a lot of damage but I don't test with enough energy to finish them off.   Not that I am suggesting these are robust meters but they have been repairable.   The UT90A may be the best example of that.   

Without looking at the data, I would guess somewhere around 60% of the meters were scrapped.  I am not suggesting that these products wouldn't be a fine choice for some people.  I'm not here to push any brands or sell meters.  However, personally, I don't have a use for a meter that is damaged as easily as my first Fluke.  The transients I would encounter in my early days are not representative of what I experiment with now days.    For me, the more punishment the meter can take on the bench without downtime, the better.  It's not about safety for me but uptime.   

I've had a few people comment on my degrading the MOVs over time.   After a member here also made the same suggestion, I ran a long term high voltage test on my Fluke 87V where I applied 1.6KV DC.   This meter certainly fulfills my needs for a robust meter but its lacking in other ways.   

Today, I use the 34401A if I need that performance or am running some experiment that I have automated or want to log the data.   After discovering the BM869s and the Fluke 189, if I am working on anything that I have any concerns about HP, I use these handheld meters as I have no doubt that they would survive levels that the old HP would not.



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SP
floobydust:

--- Quote from: KaneTW on September 02, 2020, 02:21:40 pm ---
--- Quote from: floobydust on September 02, 2020, 01:06:56 am ---Agilent 34461a
3. Kill me with zeros
000.0005  the leading zeros are bad practice and contain no information.

--- End quote ---
The leading zeros are likely on purpose to indicate the range.

--- End quote ---
My rant about it here.


34461a manual has Cat. II 300V rating, Input protection: 1,000 V on all ranges.
The common-mode vs differential-mode voltage limits are not spelled out, for the in-guard isolation barrier. I frequently go near or over 750-1000V, which leads me to my next disadvantage:

5. RIGHT TO REPAIR
If you damage the front-end, no schematic is available. Keithley has some kindergarten-grade block diagrams but you will never find that leaky JFET or mux.
Very costly at almost 1/2 the price of the instrument to repair it, which is the going rate. I find many bench DMM's are too fragile for real world use, they need to up the front end protection although pA measurement would suffer I guess.
Fungus:

--- Quote from: bdunham7 on September 02, 2020, 04:04:07 pm ---I just took apart an old 8842A that apparently was connected to excessive voltage--at least 1500V---with some power to back it up.  The entire front end protection--4 MOVs and 2 fusible resistors--were incinerated.  I removed the burnt parts and subbed in 2 regular 1K resistors (to be repaired properly later, of course) and the meter works properly and is in cal.  No evidence of any external damage, the circuit board is unharmed (but sooty)  and I'm sure nobody died.  I'd say that is 'robust' although I can't quantify that.

--- End quote ---

Nowhere in CAT documents does it say the the meter should survive.
Fungus:
How about this:

https://www.aliexpress.com/item/4000343031770.html
PushUp:
As a "little" interruption, I want to thank all of you, who shared their personal knowlege and experience so far, when dealing with their equipment at home or at work!!!  :clap:

In addition to this, it is always good, to discuss certain aspects in a forum - please go on with it, as this is always helpful and mostly purposeful! It is clear that experts discuss on another level than beginners and this is good, as it helps others to improve!

As a consequence of this (or nevertheless), I want to encourage all users of all level to share their thoughts, as only this creates an overall picture. You cannot post something pointless, as the question "What are - for you - the 3 biggest disadvantages of your benchtop DMM(s)?", is that I wanted to hear your personal opinion, which of course should be different to others, depending on the field you are working on and how experienced you are and your know-how in general.

It is clear, that anybody has different preferences depending on the area of responsibility, which makes it so valuable to share, so that anyone can filter for his/her own needs.


Thank you very much!!!  :-+ :-+ :-+


...don't want to disturb the flow - over and out...  :popcorn:
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