Author Topic: Faulty resistance range Velleman DMM  (Read 8130 times)

0 Members and 1 Guest are viewing this topic.

Offline Blaffetuur

  • Regular Contributor
  • *
  • Posts: 65
  • Country: be
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

  • Super Contributor
  • ***
  • Posts: 3404
  • Country: ca
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

  • Frequent Contributor
  • **
  • Posts: 510
  • Country: de
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

  • Super Contributor
  • ***
  • Posts: 3404
  • Country: ca
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

  • Regular Contributor
  • *
  • Posts: 65
  • Country: be
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

  • Frequent Contributor
  • **
  • Posts: 510
  • Country: de
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

  • Regular Contributor
  • *
  • Posts: 65
  • Country: be
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

  • Frequent Contributor
  • **
  • Posts: 730
  • Country: us
  • No user-serviceable parts inside.
    • Mr. ModemHead
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

  • Regular Contributor
  • *
  • Posts: 65
  • Country: be
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

  • Regular Contributor
  • *
  • Posts: 65
  • Country: be
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

  • Super Contributor
  • ***
  • Posts: 3404
  • Country: ca
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.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf