How to take apart Voltcraft/Metex 3650B

[chicken]

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

Any ideas?

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

[georgd]

Try to lift whole board from case.
Selector switch is fastened from another (recently down) side with seeger ring.

Georg

[chicken]

I can't see a seeger ring. The board is held in place by the sandwich of front dial and wheel on the back.

[retiredcaps]

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. 

[chicken]

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.

[retiredcaps]

Thanks, those pictures were helpful.

Let us know if cleaning the lcd fixing your problem or not.

[chicken]

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).

[retiredcaps]

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.

[chicken]

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).

[j-man]

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 
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 
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.

[retiredcaps]

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/

[j-man]

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 

[retiredcaps]

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.

[retiredcaps]

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.

[retiredcaps]

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.

[retiredcaps]

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.

[j-man]

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.

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.

The component circled in red here might be a PTC?

I guess it is. 1346 ohms.

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...

[retiredcaps]

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.

[j-man]

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...

[retiredcaps]

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. 

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

[retiredcaps]

It stopped working after I measured a resistor connected to a capacitor that was still charged... Big mistake 

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.

[j-man]

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 

[j-man]

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

[retiredcaps]

Thanks for reporting back on resolution and glad to hear your meter is working again. 

[Erik_Belgium]

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.