Faulty resistance range Velleman DMM - Page 1
Forum section: electronics / repair

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 06, 2016, 02:48:34 pm
content: 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 ohmsI 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

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 02:57:03 pm
content: Quote from: Blaffetuur on June 06, 2016, 02:48:34 pmYour Velleman looks like it was made by CEM Instruments.QuoteUsing the Fluke, measure the resistance of the PTC. It should be around 1k ohm. Please post your reading.QuoteThe PTC is for input protection. See

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 03:00:32 pm
content: If the probes are shorted, do you get 0.0 ohms on the meter?

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 06, 2016, 03:04:35 pm
content: Quote from: retiredcaps on June 06, 2016, 03:00:32 pmThe PTC measures 890 ohmAnd with probes shorted I get 0 ohms

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 03:06:28 pm
content: Quote from: Blaffetuur on June 06, 2016, 03:04:35 pmPTC reading seems okay.Can you post clear focused pictures of the entire pcb, both sides please?

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 06, 2016, 03:28:22 pm
content: Quote from: retiredcaps on June 06, 2016, 03:06:28 pmHere are some more photoshttp://imgur.com/a/YVxzICouldn't find my sony camera so my phone had to do

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 03:35:13 pm
content: Quote from: Blaffetuur on June 06, 2016, 03:28:22 pmPretty good for the phone camera.Your range switch definitely needs a good clean. Seehttp://mrmodemhead.com/blog/fluke-83-iii-rotary-switch/

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 03:38:01 pm
content: 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.

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 03:44:39 pm
content: It looks like version 3 of this board removes the flying capacitor. Picture in thehttps://www.eevblog.com/forum/testgear/help-required-who-is-the-oem-of-this-multimeter/msg265980/#msg265980

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 06, 2016, 03:47:31 pm
content: Quote from: retiredcaps on June 06, 2016, 03:38:01 pmThis is the first time I open her up so yeahI measure 1k across the MOV

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 03:52:41 pm
content: Quote from: Blaffetuur on June 06, 2016, 03:47:31 pm1) 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.

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 03:56:20 pm
content: Quote from: Blaffetuur on June 06, 2016, 03:47:31 pmWhy 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

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 04:03:55 pm
content: I can't quite read the part number of CNR1? Can you post it?

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 06, 2016, 04:10:38 pm
content: Quote from: retiredcaps on June 06, 2016, 03:52:41 pmDoesn't make a difference, desolderd MOV measures OL, pcb measures 1k and without the MOV the meter still reads 4k8 ...

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 06, 2016, 04:11:43 pm
content: Quote from: retiredcaps on June 06, 2016, 04:03:55 pmVDR07D270K

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 04:14:03 pm
content: Quote from: Blaffetuur on June 06, 2016, 04:10:38 pmOkay, you can put the MOV back in. I will have a look at the MOV datasheet.Have you cleaned the range switch yet?

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 06, 2016, 04:17:44 pm
content: Quote from: retiredcaps on June 06, 2016, 04:14:03 pmYes, cleaned it up with some kontakt spray but didn't solve it

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 04:31:32 pm
content: Quote from: Blaffetuur on June 06, 2016, 04:17:44 pmI use 91% IPA and scrub with a toothbrush.

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 04:36:07 pm
content: Quote from: Blaffetuur on June 06, 2016, 04:11:43 pmHmm, this MOV is only 18V.http://pdf1.alldatasheet.com/datasheet-pdf/view/553484/BOURNS/MOV-07D270K.htmlIt 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.

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 06, 2016, 05:03:03 pm
content: Quote from: retiredcaps on June 06, 2016, 04:36:07 pmI appreciate your help alot ! I'm the asking party so you really don't need to justify your effort

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 07:04:40 pm
content: 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.

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 07:06:18 pm
content: Can you replicate the two readings in this threadhttps://www.eevblog.com/forum/repair/fluke-89-iv-powers-up-measures-nothing/msg956417/#msg956417I only need pictures #2 (1354) and #3 (1355) replicated. Your Dad's Fluke = Fluke 77 III and your Velleman = 187.

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 07:19:57 pm
content: 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.

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 07:26:46 pm
content: 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?

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 07:51:45 pm
content: 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 range2) manual 4k ohm range3) manual 40k ohm range4) manual 400k ohm range5) manual 4M ohm range6) manual 40M ohm range

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 06, 2016, 08:13:49 pm
content: Quote from: retiredcaps on June 06, 2016, 07:51:45 pmI will check al these things wednesday or thursday, I have a pretty tough exam thursdayso I will see when I have time !

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 08:22:37 pm
content: Quote from: Blaffetuur on June 06, 2016, 08:13:49 pmOkay. 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.

author: Twoflower
author_rank: Frequent Contributor
timestamp: June 06, 2016, 08:47:44 pm
content: It seems that there is a short between R7 and R8(?). What happens if you separate them?Argh! too slow

author: retiredcaps
author_rank: Super Contributor
timestamp: June 06, 2016, 08:58:54 pm
content: Quote from: Twoflower on June 06, 2016, 08:47:44 pmAt 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.

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 07, 2016, 01:17:36 pm
content: Quote from: retiredcaps on June 06, 2016, 07:04:40 pmI alo noticed and seperated them but no improvementQuote from: retiredcaps on June 06, 2016, 07:06:18 pmHere are my measurements:RANGE FLUKEauto 148k10,145V0,22mA000,0R 38k260,369V0,23mA0,000k 38k260,369V0,22mA00,00k 18k490,145V0,05mA000,0k 7k510,020V0,02mA0,000M 5k90,002V0,01mA00,00M 5k730,2mV0,01mAQuote from: retiredcaps on June 06, 2016, 07:26:46 pmHere is a high quality picture :I did remove the LCD and boy is it a mess! I tought some solderpoints were bad, but this takes the cake.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

author: Twoflower
author_rank: Frequent Contributor
timestamp: June 07, 2016, 01:34:50 pm
content: What a mess!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?

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 07, 2016, 01:40:10 pm
content: Quote from: Twoflower on June 07, 2016, 01:34:50 pmAs bad as that cable looks, its still okay

author: ModemHead
author_rank: Frequent Contributor
timestamp: June 07, 2016, 02:35:24 pm
content: Wow. I've never seen a mess that bad even inside a Hazard Fraught freebie.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.

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 07, 2016, 04:39:13 pm
content: Quote from: ModemHead on June 07, 2016, 02:35:24 pmI couldn't really concentrate knowing that my poor little meter is waiting for some troubleshooting

author: Blaffetuur
author_rank: Regular Contributor
timestamp: June 07, 2016, 04:41:41 pm
content: Quote from: ModemHead on June 07, 2016, 02:35:24 pmI will certainly give this a shot

author: retiredcaps
author_rank: Super Contributor
timestamp: June 08, 2016, 02:24:52 am
content: 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.