Author Topic: Faulty resistance range Velleman DMM  (Read 8136 times)

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Offline Blaffetuur

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Faulty resistance range Velleman DMM
« on: June 06, 2016, 02:48:34 pm »
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
« Last Edit: June 06, 2016, 03:30:56 pm by Blaffetuur »
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #1 on: June 06, 2016, 02:57:03 pm »
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.

Quote
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.

Quote
(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

« Last Edit: June 06, 2016, 02:58:53 pm by retiredcaps »
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #2 on: June 06, 2016, 03:00:32 pm »
If the probes are shorted, do you get 0.0 ohms on the meter?
 

Offline Blaffetuur

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Re: Faulty resistance range Velleman DMM
« Reply #3 on: June 06, 2016, 03:04:35 pm »
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
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #4 on: June 06, 2016, 03:06:28 pm »
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?
 

Offline Blaffetuur

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Re: Faulty resistance range Velleman DMM
« Reply #5 on: June 06, 2016, 03:28:22 pm »
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  :P
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #6 on: June 06, 2016, 03:35:13 pm »
Couldn't find my sony camera so my phone had to do  :P
Pretty good for the phone camera.

Your range switch definitely needs a good clean.  See

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

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #7 on: June 06, 2016, 03:38:01 pm »
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.
« Last Edit: June 06, 2016, 03:58:36 pm by retiredcaps »
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #8 on: June 06, 2016, 03:44:39 pm »
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
 

Offline Blaffetuur

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Re: Faulty resistance range Velleman DMM
« Reply #9 on: June 06, 2016, 03:47:31 pm »
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  :-DMM

I measure 1k across the MOV  :o
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #10 on: June 06, 2016, 03:52:41 pm »
I measure 1k across the MOV  :o
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.
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #11 on: June 06, 2016, 03:56:20 pm »
« Last Edit: June 06, 2016, 03:59:17 pm by retiredcaps »
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #12 on: June 06, 2016, 04:03:55 pm »
I can't quite read the part number of CNR1?  Can you post it?
 

Offline Blaffetuur

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Re: Faulty resistance range Velleman DMM
« Reply #13 on: June 06, 2016, 04:10:38 pm »
I measure 1k across the MOV  :o
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 ...
 

Offline Blaffetuur

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Re: Faulty resistance range Velleman DMM
« Reply #14 on: June 06, 2016, 04:11:43 pm »
I can't quite read the part number of CNR1?  Can you post it?

VDR
07D270K
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #15 on: June 06, 2016, 04:14:03 pm »
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?
 

Offline Blaffetuur

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Re: Faulty resistance range Velleman DMM
« Reply #16 on: June 06, 2016, 04:17:44 pm »
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
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #17 on: June 06, 2016, 04:31:32 pm »
Yes, cleaned it up with some kontakt spray but didn't solve it
I use 91% IPA and scrub with a toothbrush.
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #18 on: June 06, 2016, 04:36:07 pm »
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.
 
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Offline Blaffetuur

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Re: Faulty resistance range Velleman DMM
« Reply #19 on: June 06, 2016, 05:03:03 pm »
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  :-+
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #20 on: June 06, 2016, 07:04:40 pm »
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.
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #21 on: June 06, 2016, 07:06:18 pm »
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.
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #22 on: June 06, 2016, 07:19:57 pm »
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.
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #23 on: June 06, 2016, 07:26:46 pm »
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?
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #24 on: June 06, 2016, 07:51:45 pm »
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
 

Offline Blaffetuur

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Re: Faulty resistance range Velleman DMM
« Reply #25 on: June 06, 2016, 08:13:49 pm »
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

I will check al these things wednesday or thursday, I have a pretty tough exam thursday :-\ so I will see when I have time !
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #26 on: June 06, 2016, 08:22:37 pm »
I will check al these things wednesday or thursday, I have a pretty tough exam thursday :-\ so I will see when I have time !
Okay. Good luck on your exam.  Maybe after a couple day break, the answer to this problem will become obvious to me or someone else reading this thread.  Posting your findings on Friday is good as well.
 

Offline Twoflower

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Re: Faulty resistance range Velleman DMM
« Reply #27 on: June 06, 2016, 08:47:44 pm »
It seems that there is a short between R7 and R8(?). What happens if you separate them?

Argh! too slow :-[
« Last Edit: June 06, 2016, 08:56:11 pm by Twoflower »
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #28 on: June 06, 2016, 08:58:54 pm »
It seems that there is a short between R7 and R8(?). What happens if you separate them?

Argh! too slow :-[
At least your photo shows a better angle than mine.  BTW, if even that doesn't solve the problem, it is a representation of CEM Instruments assembly quality. Or lack of it.
 

Offline Blaffetuur

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Re: Faulty resistance range Velleman DMM
« Reply #29 on: June 07, 2016, 01:17:36 pm »
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.

I alo noticed and seperated them but no improvement

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.

Here are my measurements:


RANGE       FLUKE
auto           148k1
               0,145V
               0,22mA

000,0R         38k26
               0,369V
               0,23mA

0,000k         38k26
               0,369V
               0,22mA

00,00k         18k49
               0,145V
               0,05mA

000,0k         7k51
               0,020V
               0,02mA

0,000M         5k9
               0,002V
               0,01mA

00,00M         5k73
               0,2mV
               0,01mA

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?

Here is a high quality picture :


I did remove the LCD and boy is it a mess  :o! I tought some solderpoints were bad, but this takes the cake :wtf:.


Might as well clean it up a little,€5 Velleman to the to the rescue  ::)!


Well it isn't perfect, but a lot cleaner than before, sadly the problem isn't gone
 

Offline Twoflower

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Re: Faulty resistance range Velleman DMM
« Reply #30 on: June 07, 2016, 01:34:50 pm »
What a mess! :palm:

The yellow cable looks very strange. They cut the GND plane and replaced it with the wire. It seems they had a offset voltage to compensate. Can you check if the cable at the damaged place is a) still insulated and b) still conductive from one end to the other?
 

Offline Blaffetuur

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Re: Faulty resistance range Velleman DMM
« Reply #31 on: June 07, 2016, 01:40:10 pm »
What a mess! :palm:

The yellow cable looks very strange. They cut the GND plane and replaced it with the wire. It seems they had a offset voltage to compensate. Can you check if the cable at the damaged place is a) still insulated and b) still conductive from one end to the other?

As bad as that cable looks, its still okay
 

Offline ModemHead

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Re: Faulty resistance range Velleman DMM
« Reply #32 on: June 07, 2016, 02:35:24 pm »
Wow.  I've never seen a mess that bad even inside a Hazard Fraught freebie.  :scared:

This is a complete stab in the dark, but if you look around the board you should find some transistors, usually NPN, with the B-C terminals shorted.  These are used as voltage clamps in various places by taking advantage of the reverse avalanche property of the E-B junction.  Check their B-E junctions and make sure they act like diodes.  Keep in mind you may find transistors in normal configuration, or dual-diode arrays also in SOT-23 packages, ignore them.

Don't get caught up with this awesomely-manufactured meter and forget to study for the exam.  :)
 

Offline Blaffetuur

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Re: Faulty resistance range Velleman DMM
« Reply #33 on: June 07, 2016, 04:39:13 pm »
Don't get caught up with this awesomely-manufactured meter and forget to study for the exam.  :)

I couldn't really concentrate knowing that my poor little meter is waiting for some troubleshooting  :-/O
 

Offline Blaffetuur

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Re: Faulty resistance range Velleman DMM
« Reply #34 on: June 07, 2016, 04:41:41 pm »
This is a complete stab in the dark, but if you look around the board you should find some transistors, usually NPN, with the B-C terminals shorted.  These are used as voltage clamps in various places by taking advantage of the reverse avalanche property of the E-B junction.  Check their B-E junctions and make sure they act like diodes.  Keep in mind you may find transistors in normal configuration, or dual-diode arrays also in SOT-23 packages, ignore them.

I will certainly give this a shot  :-+
 

Offline retiredcaps

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Re: Faulty resistance range Velleman DMM
« Reply #35 on: June 08, 2016, 02:24:52 am »
When you have time, I would like to get some more data points.

We know the meter doesn't show open circuit, but does show 0.0 when the probes are shorted. If you have a decade resistor box or some resistors, I would like to try one resistor for each range.

For example, measure a 100 ohm resistor in the 400 ohm range, a 1k ohm resistor in the 4k ohm range, etc.  It doesn't have to be 100 ohm, just something between 1 and 399 ohm.
 


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