Faulty resistance range Velleman DMM

[Blaffetuur]

Hi,

I have been using my Velleman DVM9912 for some years now but recently noticed that my resistance measurements are wrong. These are the things I observed:

-All functions except resistance work (checked against my dad's fluke)
-Without anything connected I measure 4k8 ish
-There is a PTC near the righthand connector on the board and when I hold it to warm it up I see the resistance measurement go up a few hunderd ohms

I don't remember what I've done wrong, one thing that could have happend is that I turned off the meter while measuring current.

Do you guys have any suggestions as to where to look ?

(And what could be the function of that PTC ? Is it a protection device or does it some compensation for the temperature measurement ?)

Here are some more pictures: http://imgur.com/a/YVxzI

[retiredcaps]

I have been using my Velleman DVM9912 for some years now but recently noticed that my resistance measurements are wrong.

Your Velleman looks like it was made by CEM Instruments.

Do you guys have any suggestions as to where to look ?

Using the Fluke, measure the resistance of the PTC.  It should be around 1k ohm.  Please post your reading.

(And what could be the function of that PTC ? Is it a protection device or does it some compensation for the temperature measurement ?)

The PTC is for input protection.  See

[retiredcaps]

If the probes are shorted, do you get 0.0 ohms on the meter?

[Blaffetuur]

If the probes are shorted, do you get 0.0 ohms on the meter?

The PTC measures 890 ohm

And with probes shorted I get 0 ohms

[retiredcaps]

The PTC measures 890 ohm

And with probes shorted I get 0 ohms

PTC reading seems okay.

Can you post clear focused pictures of the entire pcb, both sides please?

[Blaffetuur]

The PTC measures 890 ohm

And with probes shorted I get 0 ohms

PTC reading seems okay.

Can you post clear focused pictures of the entire pcb, both sides please?

Here are some more photos
http://imgur.com/a/YVxzI

Couldn't find my sony camera so my phone had to do 

[retiredcaps]

Couldn't find my sony camera so my phone had to do 

Pretty good for the phone camera.

Your range switch definitely needs a good clean.  See

http://mrmodemhead.com/blog/fluke-83-iii-rotary-switch/

[retiredcaps]

Hmm, did that flying capacitor across the main COB come from the factory?  Same for the yellow bodge wire?

CNR1 is a MOV.  Can you use the Fluke to measure its resistance?  It should measure 0L, open circuit.

edit: pictures saved for future viewers.

[retiredcaps]

It looks like version 3 of this board removes the flying capacitor.  Picture in the

https://www.eevblog.com/forum/testgear/help-required-who-is-the-oem-of-this-multimeter/msg265980/#msg265980

[Blaffetuur]

Hmm, did that flying capacitor across the main COB come from the factory?  Same for the yellow bodge wire?

CNR1 is a MOV.  Can you use the Fluke to measure its resistance?  It should measure 0L, open circuit.

This is the first time I open her up so yeah 

I measure 1k across the MOV 

[retiredcaps]

I measure 1k across the MOV 

1) Desolder the MOV and measure it out of circuit. 
2) Measure the solder points on the pcb after the MOV as been removed.
3) With MOV removed, retest the ohms measurements.

Report all of the above.

[retiredcaps]

This is the first time I open her up so yeah 

Why does that not surprise me about CEM? 

https://www.eevblog.com/forum/testgear/teardown-extech-ex570-my-first-but-also-last-extech/

http://mrmodemhead.com/blog/gallery/craftsman-82003/

Starting at 36:54 of Dave's video

[retiredcaps]

I can't quite read the part number of CNR1?  Can you post it?

[Blaffetuur]

I measure 1k across the MOV 

1) Desolder the MOV and measure it out of circuit. 
2) Measure the solder points on the pcb after the MOV as been removed.
3) With MOV removed, retest the ohms measurements.

Report all of the above.

Doesn't make a difference, desolderd MOV measures OL, pcb measures 1k and without the MOV the meter still reads 4k8 ...

[Blaffetuur]

I can't quite read the part number of CNR1?  Can you post it?

VDR
07D270K

[retiredcaps]

Doesn't make a difference, desolderd MOV measures OL, pcb measures 1k and without the MOV the meter still reads 4k8

Okay, you can put the MOV back in.  I will have a look at the MOV datasheet.

Have you cleaned the range switch yet?

[Blaffetuur]

Doesn't make a difference, desolderd MOV measures OL, pcb measures 1k and without the MOV the meter still reads 4k8

Okay, you can put the MOV back in.  I will have a look at the MOV datasheet.

Have you cleaned the range switch yet?

Yes, cleaned it up with some kontakt spray but didn't solve it

[retiredcaps]

Yes, cleaned it up with some kontakt spray but didn't solve it

I use 91% IPA and scrub with a toothbrush.

[retiredcaps]

VDR
07D270K

Hmm, this MOV is only 18V.

http://pdf1.alldatasheet.com/datasheet-pdf/view/553484/BOURNS/MOV-07D270K.html

It must be doing something else than what I thought.  Normally the MOVs are located close to the input jacks, but with the CEM spaghetti layout, I thought it was reasonable to assume this MOV was part of the input protection.

As I mentioned in the Fluke 89IV thread, I need a break.  I'm participating in 3 repair multimeter threads at the same time and it is all starting to blur together.  I'm also gone for a few hours later in the afternoon.

[Blaffetuur]

VDR
07D270K

Hmm, this MOV is only 18V.

http://pdf1.alldatasheet.com/datasheet-pdf/view/553484/BOURNS/MOV-07D270K.html

It must be doing something else than what I thought.  Normally the MOVs are located close to the input jacks, but with the CEM spaghetti layout, I thought it was reasonable to assume this MOV was part of the input protection.

As I mentioned in the Fluke 89IV thread, I need a break.  I'm participating in 3 repair multimeter threads at the same time and it is all starting to blur together.  I'm also gone for a few hours later in the afternoon.

I appreciate your help alot ! I'm the asking party so you really don't need to justify your effort 

[retiredcaps]

Hmm, it could be nothing, but R7 and R8 in your picture look like they are touching? Circled in red.

If they are touching, separate them slightly.  I don't think this will solve the ohms problem, however.

[retiredcaps]

Can you replicate the two readings in this thread

https://www.eevblog.com/forum/repair/fluke-89-iv-powers-up-measures-nothing/msg956417/#msg956417

I only need pictures #2 (1354) and #3 (1355) replicated.  Your Dad's Fluke = Fluke 77 III and your Velleman = 187.

[retiredcaps]

I forgot to say please, do the tests above with auto ranging first and then do the same going through the manual range.  I want to see what your meter output/sources through the various ranges.

[retiredcaps]

Can we also get a picture of the pcb with the LCD removed?  It looks like you need to remove the 4 screws to release the LCD.  With all the bodges, let's see if there is anything interesting there?

[retiredcaps]

Sorry for the multiple posts, but more ideas coming to me now after a little break.

Does your meter show 0L or open circuit with probes removed in following scenarios?

1) manual 400 ohm range
2) manual 4k ohm range
3) manual 40k ohm range
4) manual 400k ohm range
5) manual 4M ohm range
6) manual 40M ohm range