How to take apart Voltcraft/Metex 3650B - Page 1
Forum section: electronics / testgear

author: chicken
author_rank: Frequent Contributor
timestamp: April 24, 2014, 03:50:54 am
content: I have an old Voltcraft/Metex M-3650B with a faded LCD that I try to take apart and see if cleaning the LCD contacts would fix it. However I can't figure out, how to remove the big selector switchAny ideas?I already tried to pull hard, harder with pliers, levering out from the front with a screwdriver..

author: georgd
author_rank: Regular Contributor
timestamp: April 24, 2014, 04:00:13 am
content: Try to lift whole board from case.Selector switch is fastened from another (recently down) side with seeger ring.Georg

author: chicken
author_rank: Frequent Contributor
timestamp: April 24, 2014, 04:14:00 am
content: I can't see a seeger ring. The board is held in place by the sandwich of front dial and wheel on the back.

author: retiredcaps
author_rank: Super Contributor
timestamp: April 24, 2014, 06:49:56 am
content: I have a similar meter. See this post for pictures and it may help you get it apart? I gently pried it up using a flat blade screwdriver.https://www.eevblog.com/forum/chat/metex-m-3610-repair-help-dmm-won't-zero-properly-in-ohms-and-dc-volts/PS. I haven't fixed this as I somehow misplaced one of the boards.

author: chicken
author_rank: Frequent Contributor
timestamp: April 24, 2014, 04:16:07 pm
content: Thanks, those pictures were helpful.Pulling hard with pliers from the front on the center of the dial worked. I had to put the case back together first to be able to firmly grab it with enough force.

author: retiredcaps
author_rank: Super Contributor
timestamp: April 24, 2014, 09:07:25 pm
content: Quote from: chicken on April 24, 2014, 04:16:07 pmLet us know if cleaning the lcd fixing your problem or not.

author: chicken
author_rank: Frequent Contributor
timestamp: April 25, 2014, 12:00:17 am
content: The digits are nice and dark again when on.Unfortunately there are still some digits partially on when they should be off (e.g. the 1 on the left), no problem when viewing straight on, but very visible at an angle.Also the bar graph at the bottom is always fully on, no matter what I do with the meter. I can't remember if that was the case before I took the meter apart (it spent the better part of a decade in a moving box).

author: retiredcaps
author_rank: Super Contributor
timestamp: April 25, 2014, 05:50:57 am
content: Some suggestions.1) Give the PCB a good IPA cleaning/scrub.2) If your 7106 is socketed like mine, remove it , and give the IC a good IPA scrub as well. Let the IPA dry thoroughly.3) I'm assuming the bar graph should be "off".4) Schematic athttp://elektrotanya.com/PREVIEWS/38933510/23432455/metex/metex_m3650b_sch.pdf_1.pngWith the schematic and 7106 datasheet, you can check voltages.

author: chicken
author_rank: Frequent Contributor
timestamp: April 25, 2014, 02:44:58 pm
content: Thanks for your suggestions.1) I will definitely attack a few spots of old flux and see if it helps.2) The 7106 is actually a Maxim MAX130CPL, still in a socket.3) My assumption as well. It is off when I remove a daughter board. It includes a Metex branded IC (M-8850, QFP64). Unfortunately I think this daughter board is missing in the schematic you linked.4) Found the document on Elektrotanya as well. Unfortunately it is only that single sheet. I suspect there's more in the document it originally came from (see point 3 above).

author: j-man
author_rank: Contributor
timestamp: January 12, 2015, 12:53:08 pm
content: I bought that multimeter around 1990 and I never learned how to read the bar... According to the manual, "the 41-points bar displays in proportion to digital value". But I can't help you right now, because I need to fix mineIt stopped working after I measured a resistor connected to a capacitor that was still charged... Big mistakeThe LCD display works but the readings are completely incorrect, changing randomly even when the probes are disconnected. Exactly like this icon... just imagine an LCD in place of the VU meterAfter I downloaded the diagram I checked the input section: diodes and resistors were OK, so I focused on the AD converter. I removed it and turned on the multimeter, but the readings were identical. But I guess it doesn't mean it's working, right? I'm quite confused...I'm gonna check if ICL7106 is socketed or not. Any suggestion will be much appreciated. Thanks in advance, and please forgive my clumsy english.

author: retiredcaps
author_rank: Super Contributor
timestamp: January 12, 2015, 06:38:30 pm
content: Quote from: j-man on January 12, 2015, 12:53:08 pmCan you post pictures of the insides of multimeter? Something like what I did in this post?https://www.eevblog.com/forum/repair/metex-m-3610-repair-help-dmm-won't-zero-properly-in-ohms-and-dc-volts/

author: j-man
author_rank: Contributor
timestamp: January 13, 2015, 12:05:36 am
content: Here are the pictures.The IC - Maxim MAX130CPL - is under the LCD display, in a socket. I checked it and I didn't notice any sign of damage or dirt, so I put it back in its place for now.There are two pictures of random measurements: with one probe shorting COM and V/?, the display moves alternatively from "overrange" and random values

author: retiredcaps
author_rank: Super Contributor
timestamp: January 13, 2015, 12:31:32 am
content: The grey spark gap between the V and COM jack should read open circuit when measured with another meter when set to resistance/ohms mode.

author: retiredcaps
author_rank: Super Contributor
timestamp: January 13, 2015, 12:32:56 am
content: Can we get a close up shot of those two blue components by the V and COM jack? I can't make out what they are because your picture is too dark.

author: retiredcaps
author_rank: Super Contributor
timestamp: January 13, 2015, 12:47:38 am
content: The component circled in red here might be a PTC? I would know for sure if I could only find my board?If it is a PTC, I think it should measure around 1k ohm.

author: retiredcaps
author_rank: Super Contributor
timestamp: January 13, 2015, 12:52:42 am
content: And finally, one other measurement that I can think of.With another multimeter, what is the input impedance of the non working Metex when its rotary switch is set to 20V DC? On most multimeters, it should be 10M ohm or 11.11M ohm depending on what input protection is implemented.

author: j-man
author_rank: Contributor
timestamp: January 13, 2015, 02:47:47 pm
content: Quote from: retiredcaps on January 13, 2015, 12:32:56 amThe varistor AG20? It does.QuoteThey are polyester capacitors, on the diagram C21 (100 nF) and C22 (10 nF). Tested and working.QuoteI guess it is. 1346 ohms.QuoteUnfortunately my little spare multimeter is limited to 2 M ohms. It reads over-range, by the way.In case you wonder, I tested the components with this useful device:http://www.ebay.com/itm/12864-LCD-Transistor-Tester-Diode-Triode-Capacita-ESR-Metro-MOS-PNP-NPN/261529581546?_trksid=p2045573.c100033.m2042&_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20131017132637%26meid%3D6fa80bdea4a44bff98932ae11ab3157f%26pid%3D100033%26prg%3D20131017132637%26rk%3D3%26rkt%3D4%26sd%3D261664214039Thanks a million for your support. I just found an ICL7106 on eBay at 3.81€ (shipping included) and I decided to bought it. We'll see...

author: retiredcaps
author_rank: Super Contributor
timestamp: January 13, 2015, 10:40:29 pm
content: Quote from: j-man on January 13, 2015, 02:47:47 pmLet us know once it arrives if that fixes the problem or not.

author: j-man
author_rank: Contributor
timestamp: January 13, 2015, 10:48:18 pm
content: Of course I will. Thanks again.By the way, did you manage to fix your 3610? I just noticed that the thread was rather old...

author: retiredcaps
author_rank: Super Contributor
timestamp: January 13, 2015, 11:03:46 pm
content: Quote from: j-man on January 13, 2015, 10:48:18 pmRight now, I can't find the top half of the pcb. I misplaced it somewhere. I was using its resistor network for some troubleshooting.If I ever find it and repair it, I will update.

author: retiredcaps
author_rank: Super Contributor
timestamp: January 13, 2015, 11:10:06 pm
content: Quote from: j-man on January 12, 2015, 12:53:08 pmBTW, what is the voltage rating of that capacitor?My guess is that most of the older multimeters can handle at least 250V RMS on the ohm range setting.

author: j-man
author_rank: Contributor
timestamp: January 14, 2015, 09:17:42 am
content: No, it was much less than 250 volts. I was working on an old Denon PMA510 hi-fi amp, which has a +/- 51v power supply. But I made a mess with the ground connections...Both multimeters (one for measuring voltages and one for ohms) were connected to the same external power supply, instead of the internal battery. The "voltmeter" ground probe was also connected to the amp chassis. When I measured the resistor with the Metex it read over-range, but the buzzer also made a weird tone... It's just my guessing, but I suspect that something coming through the ground loop did harm to my DMM. A quite empiric diagnosis, I reckon

author: j-man
author_rank: Contributor
timestamp: January 21, 2015, 07:45:05 pm
content: Great news. Today I found in the mail the ICL7106 I bought: I replaced the old Maxim IC and my Metex is back in business, working like a charm

author: retiredcaps
author_rank: Super Contributor
timestamp: January 21, 2015, 08:02:57 pm
content: Thanks for reporting back on resolution and glad to hear your meter is working again.

author: Erik_Belgium
author_rank: Newbie
timestamp: December 02, 2022, 01:09:06 pm
content: Hi guys,This is my first post since I can't find an answer online about my METEX M-3650B which is causing me a headache due to an always on "low battery" warning.I've been looking and searching the web, read the ICL7106 datasheet and looked into the ICL8069.I know it's an old meter but I've had it since college and I would like to repair it.Took it all apart (which was an adventure on it's own ), checked the pcb and did test the battery with the it's own leads and it shows full ( added the 3,06V and 6,46V measured on the battery connections )I don't have a scope to check, only another multimeter.Where does the meter check it's low battery value? I can't locate the ICL8069 zener on my PCB.It's the same version as shown in this topic from "Chicken".The meter does seem to function normally except for the low battery indication.thanks for any suggestions.( I finally joined the EEvblog forum after watching Dave's channel and lurking on this forum for years. )Erik.

author: mqsaharan
author_rank: Regular Contributor
timestamp: December 02, 2022, 06:31:56 pm
content: Hi Erik,Hopefully, someone who has this meter or has the better understanding of its schematic will chime in. My reply may not be particularly helpful.ICL 8069, if it is present in the meter is the voltage reference IC. It is not used for low battery warning. As per schematic in reply #7, it is ZD1 and its output to ICL7106 is marked as R.The following are my guesses which may be completely wrong.The *whole display is not directly driven by ICL7106 but in conjunction with RA1-RA4, which is the METEX IC with a clock crystal on a daughter board in the picture in reply #11.I guess the IC4 in the schematic is generating the low battery signal but I am not sure. My guess is based on the assumption that this IC is feeding signal to the display chip (RA1-RA4).Edit: * correction. Also, I am not adding further explanation here in order to bump it and in the hope that someone might give better idea to solve your problem with this meter.

author: mqsaharan
author_rank: Regular Contributor
timestamp: December 03, 2022, 03:46:56 am
content: A bit more explanation in the hope it might be useful for you to solve your problem. After looking at the schematic again, I'm sure the digits and negative sign " - 1 8 8 8 " are being driven by ICL7106 because it contains both A/D and display driver. But the rest of the segments including bar graph, low battery icon and any other features on the display are being driven by the microprocessor/CPU/Metex IC.The low battery signal on ICL7106 based meters is external to this chip because it does not have any circuitry onboard for this function. It is usually implemented by using the external circuitry and the Test signal from pin 37 of the chip. On the schematic in reply #7, it goes straight to IC4. This is all a guess because this signal can also be used as a power supply for some other IC in the meter. The schematic is incomplete and I don't have this meter so it is difficult to suggest something with confidence.

author: j-man
author_rank: Contributor
timestamp: March 02, 2024, 11:01:18 am
content: Hi. I hope you already solved your issue: in case you don't, let me know if I can help you measuring and checking my own Metex (that is powered by an external 9v power supply, BTW)