Author Topic: How to take apart Voltcraft/Metex 3650B  (Read 15046 times)

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Offline chicken

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How to take apart Voltcraft/Metex 3650B
« on: April 24, 2014, 03:50:54 am »
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 switch |O

Any ideas?

I already tried to pull hard, harder with pliers, levering out from the front with a screwdriver..

 

Online georgd

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #1 on: April 24, 2014, 04:00:13 am »
Try to lift whole board from case.
Selector switch is fastened from another (recently down) side with seeger ring.

Georg
 

Offline chicken

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #2 on: April 24, 2014, 04:14:00 am »
I can't see a seeger ring. The board is held in place by the sandwich of front dial and wheel on the back.
 

Offline retiredcaps

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #3 on: April 24, 2014, 06:49:56 am »
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.  :palm:
 

Offline chicken

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #4 on: April 24, 2014, 04:16:07 pm »
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.
 

Offline retiredcaps

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #5 on: April 24, 2014, 09:07:25 pm »
Thanks, those pictures were helpful.
Let us know if cleaning the lcd fixing your problem or not.
 

Offline chicken

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #6 on: April 25, 2014, 12:00:17 am »
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).
 

Offline retiredcaps

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #7 on: April 25, 2014, 05:50:57 am »
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 at

http://elektrotanya.com/PREVIEWS/38933510/23432455/metex/metex_m3650b_sch.pdf_1.png

With the schematic and 7106 datasheet, you can check voltages.
« Last Edit: April 25, 2014, 06:13:40 am by retiredcaps »
 

Offline chicken

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #8 on: April 25, 2014, 02:44:58 pm »
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).
 

Offline j-man

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #9 on: January 12, 2015, 12:53:08 pm »
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 mine :-//
It stopped working after I measured a resistor connected to a capacitor that was still charged... Big mistake  :palm:
The 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 meter  :bullshit:
After 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.
 

Offline retiredcaps

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #10 on: January 12, 2015, 06:38:30 pm »
Any suggestion will be much appreciated.
Can 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/
 

Offline j-man

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #11 on: January 13, 2015, 12:05:36 am »
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  :-//
 

Offline retiredcaps

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #12 on: January 13, 2015, 12:31:32 am »
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.
 

Offline retiredcaps

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #13 on: January 13, 2015, 12:32:56 am »
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.
 

Offline retiredcaps

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #14 on: January 13, 2015, 12:47:38 am »
The component circled in red here might be a PTC?  I would know for sure if I could only find my board?  :palm:

If it is a PTC, I think it should measure around 1k ohm.
 

Offline retiredcaps

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #15 on: January 13, 2015, 12:52:42 am »
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.
 

Offline j-man

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #16 on: January 13, 2015, 02:47:47 pm »
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.
The varistor AG20? It does.
Quote
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.
They are polyester capacitors, on the diagram C21 (100 nF) and C22 (10 nF). Tested and working.
Quote
The component circled in red here might be a PTC?
I guess it is. 1346 ohms.
Quote
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.
Unfortunately 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%3D261664214039

Thanks 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...
« Last Edit: January 13, 2015, 04:52:00 pm by j-man »
 

Offline retiredcaps

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #17 on: January 13, 2015, 10:40:29 pm »
I just found an ICL7106 on eBay at 3.81€ (shipping included) and I decided to bought it. We'll see...
Let us know once it arrives if that fixes the problem or not.
 

Offline j-man

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #18 on: January 13, 2015, 10:48:18 pm »
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...
 

Offline retiredcaps

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #19 on: January 13, 2015, 11:03:46 pm »
By the way, did you manage to fix your 3610?
Right now, I can't find the top half of the pcb. I misplaced it somewhere. I was using its resistor network for some troubleshooting.  :palm:

If I ever find it and repair it, I will update.
 

Offline retiredcaps

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #20 on: January 13, 2015, 11:10:06 pm »
It stopped working after I measured a resistor connected to a capacitor that was still charged... Big mistake  :palm:
BTW, 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.
 

Offline j-man

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #21 on: January 14, 2015, 09:17:42 am »
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  :-\
 

Offline j-man

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #22 on: January 21, 2015, 07:45:05 pm »
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 :D :D :D
 

Offline retiredcaps

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Re: How to take apart Voltcraft/Metex 3650B
« Reply #23 on: January 21, 2015, 08:02:57 pm »
Thanks for reporting back on resolution and glad to hear your meter is working again.  :-DMM
 


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