repairing DMM's

[Mar]

he every one ..
is there any post in here talk about repairing Digital multimeter ??
i have 4 units .. every one miss something to work good .. i hope that some ppl can share there knowledge about this ...

[ejeffrey]

Please provide useful information.  What are the models of DMMs, what is wrong with them, and what will you use them for.

[Mar]

Please provide useful information.  What are the models of DMMs, what is wrong with them, and what will you use them for.

ofcource i will .. but wonder if here should i write about this ? .. any way ..

first  : uni-t ut70b with no signs of live .. but the light work .. no numbers on screen ..

second is UT70A ..  always beebs on continuity test or when testing resistance   ..wrong numbers in other tests  .

third another UT70A ... no sign of life at all ..

last one is Mastech 5 in 1 ... this unit is work but problem on the screen .. the ( dot and upper right symbols are clear on the screen but every thing else is very hard to see at all angels  ... wonder if there is something control the contrast in it ??

i check all the fuses .. try to check transistors and diodes also ....
maybe one of the IC need change .. TL062 or 10583 or something else .. ....
especially for the last one with very low contrast in screen ..

[retiredcaps]

Personally, I would be happy to help if I can.

Right now, I'm trying to learn more about how multimeters work so repairing them is a good test of my understanding.

I will make a few suggestions so that everyone's time and efforts are best utilized.

1) Pick one multimeter to work one and make a new post.  This avoids confusion because I don't have the multimeters in front of me and I can't keep track of what was done to what unit.  For example, let's pick the third  UT70A ... no sign of life at all

2) In that new post, provide clear focused pictures and/or service manuals/schematics so we have all the necessary information and don't have to go searching for them.

3) Tell us what country you live in by filling in your profile.

4) Tell us what tests work (i.e. DCV, ACV, etc) and what doesn't work so we can narrow down the problem.

While ModemHead hasn't documented any Uni-T repairs, his blog contains excellent information on other multimeter repairs.  You can learn a lot and get ideas from

http://mrmodemhead.com/

[PA0PBZ]

About the last one with the faint display: take it apart, take the display off and clean the pcb, the display and the zebra strips with alcohol.

[ejeffrey]

I will make a few suggestions so that everyone's time and efforts are best utilized.
<excellent suggestions removed>

One more thing: explain any potentially relevant history to the device.  Examples of useful information are: "worked fine until it was dropped" or "I found electrolyte in the battery compartment."  Even something like "I got this for free because it was broken.  I have replaced the battery and the amps input fuse, but it still doesn't work" is helpful information.

For everyone's edification, if you haven't read Eric Raymond's "How to ask smart questions", go read that now:  http://www.catb.org/esr/faqs/smart-questions.html  It is geared towards computer/software questions, but the principle is the same.  Learning how to ask smart questions gets you more and better answers.

[Mar]

thanks every one

..its not worth to ask if its not the place where should i ask .. thats why i ask in the Beginners section  if there is something about this issue  posted anywhere here before .. to start in the right place ..then we can talk about how the question could be ...

again as no one answer my main question ((is there any post in here talk about repairing Digital multimeter ??))

i will post here a details about the MASTECH ( MS8209 ) DMM problem  to start with as i already take pictures  for it .

i have it ( and others ) as trade so i dont have any idea about the past of those units and whats the cause  of the problem
the first pic show the numbers but its hard to c them
the second one show the screen in 90 degree angle

the DOT and test sympol appear on the screen only for C . RH% . db . Lux . x10Lux ... but in tests like V .A . Ohm nothing appear at 90 deg angle ( picture number 3 show screen for measuring Voltage ) continuity test also work fine

What i did ... ?
i test all transistors on the board  and clean the screen attachment to the PCB ..every thing is normal .. but the problem still ..

[Mar]

this are tow uni-t ..units
the first is ut70A take look at the first tow pictures ... no measuring at all .. false numbers .. always beeping in continuity test

the second is uni-t UT70B
nothing work .. just the back light
the third picture

i didnt do anything in those units .. ..... yet

[Psi]

A problem i've seen multiple times on cheap meters is blown signal diodes. Both short and open.

It usually results in weird current range measurements but can cause continuity problems as well.

They're often under-spec'ed on the cheap meters, if you ignore the time limit on the current range they over-temp and blow.
ie, most meters say something like '10A for 20 seconds'

Shorts are easy to check for, as you can do it in circuit, but if they fail open you have to unsolder them first.

[Mar]

A problem i've seen multiple times on cheap meters is blown signal diodes. Both short and open.

It usually results in weird current range measurements but can cause continuity problems as well.

They're often under-spec'ed on the cheap meters, if you ignore the time limit on the current range they over-temp and blow.
ie, most meters say something like '10A for 20 seconds'

Shorts are easy to check for, as you can do it in circuit, but if they fail open you have to unsolder them first.

me too had success with repairing cheap one also ... but with ( blown ) open SMD resistor !!!  .... i believe that  ( for those who give numbers on screen ) that there is chance for repairing .. as usually the ADC is still work but take float or wrong inputs from somewhere ..... faulty comparator maybe !!..
as i have no replacement or another same units its hard to say which element give wrong values 

this pic from the UT70B under screen holder .....  i have the same IC but not SMD ..
 this IC powered directly from the battery poles so wonder if its the battery or high input voltage or maybe because of inverting the polarity hope that replacing it will solve the problem ..