My UEi DMM gone mad

[ht2ht]

Hi,

My old UEi (formerly its name is LR germany MT701) has just gone mad. Before that, I used it without any problem.
Just few days before, I turned it on and only Vdc worked. It means when I turn the knob to Vdv, the screen displays value and I can even measure Vdc properly. But other measurements disappear.

I have no idea why that could occur.

Then I decided to check the MM myself. I opened the MM, checked the fuse and found out the DMM-B-44/100 fast blow Bussman copper fuse did not work. I changed the fuse but the MM did not work: same thing happened.

I carefully check the MM's circuit by eyes and nose: nothing's strange.

Anyone here have idea what happened to my DMM and how to check, and fix it? I really need your help!
Thank you in advance!

HT

[Sabertooth]

Dirty contacts on the switch???

[Bored@Work]

Battery low?

[ht2ht]

- The battery is good. I even measured it with a Fluke borrowed from school Lab. If the battery is low, it would have fade when displaying numbers, but all functions should work then. But here, only Vdc can be turned on, others disappear.
- Seem the contacts are dirty but I doubt that as I was using it just about a month ago. Not used it recently because I was busy with other project. However, I will open and check them.

I even detached two fuses (11A and 440mA) from the MM and used it to measure Vdc. 

There may be some technical problems, but I am a hobby newbie then I don't know how to detect any of them. 

[saturation]

It may pay then to look at the switch; if you're thinking of a new DMM and may damage the UEi because you're not familiar with disassembly, you've nothing to lose by trying.

- The battery is good. I even measured it with a Fluke borrowed from school Lab. If the battery is low, it would have fade when displaying numbers, but all functions should work then. But here, only Vdc can be turned on, others disappear.
- Seem the contacts are dirty but I doubt that as I was using it just about a month ago. Not used it recently because I was busy with other project. However, I will open and check them.

I even detached two fuses (11A and 440mA) from the MM and used it to measure Vdc. 

There may be some technical problems, but I am a hobby newbie then I don't know how to detect any of them. 

[ht2ht]

Switches are shining and very good condition.

Any further suggestion or advice, PLEASE!
Thanks

[Kiriakos-GR]

I opened the MM, checked the fuse and found out the DMM-B-44/100 fast blow Bussman copper fuse did not work. I changed the fuse but the MM did not work: same thing happened.


Find the person who had use your DMM with out your knowledge , and ask him what he did with the DMM !!

[ht2ht]

Ah, I finally found the fault.

First of all, I were looking around for anyone uses the UEi DMM like me. I gave up after tens of search result pages from Google.   
However, I read one paper (http://electronickits.com/kit/complete/meas/m-2665k.pdf) that introduce how to assembly the DM kit, in which a shield spring is soldered onto the board. I doubt that the shield spring plays some role in DMM.
Look back my UEi, I found 1 spring in the front side (side with LCD) while the back side there is no spring. On back side, however, I saw an SP1 sign next to the open round-shape area which I inferred would be reserved for another spring. There should be two springs in my UEi.
Then I opened the Dave's #99: 100$ MM shoot-out (http://www.eevblog.com/2010/07/11/eevblog-99-100-multimeter-shootout-extech-amprobe-bk-precision-ideal-uei-uni-t-part-1of2/), quickly scrolled to the "take it apart" with UEi DMM. Got it right there! In the video, there are two springs, one in front, another in the back. Then, one spring might be lost in my UEi.

I used a spring taken from a pen to the SP1 place on the board. And surprise, the DMM works again.   
Very strange that after I tried to take the spring off the board, my UEi continues working.  I will check if that little spring is truly the cue for my UEi DMM problem?

I still have not known the purpose of those shield spring(s) in DMMs yet. Will study later.

Any comment?
Thanks
HT

[saturation]

Sounds like grounding plane issues.

Kudos to your persistence and problem solving skills.  Putting together all the resources you mentioned took some skill and suggests to me you will do very well in electronics.