Metex M-3610B multimeter drawing too much current?

[agent_power]

Hi all

I received a Metex M-3610B for free with a rattling piece inside.

The rattling piece was the COM banana plug. I resoldered it to the PCB and repaired (contact sprayed) the on/off switch so it would stay on.

I inserted a battery, but the measuring results were all wrong or random, and in diode test / continuity mode it wouldn't beep or show anything meaningful.

Even though I inserted a charged enough battery it kept displaying the battery symbol, so I attached a bench supply.

When I remove the current limiter, the current draw goes up to 200mA, which seems abnormal for a multimeter.

I also noticed that the component next to the display (see attachment, the black small square in the upper right) would get really hot.

Does anyone have schematics for this multimeter? What part is this square thing? How do I remove the PCB? it's stuck to the wheel I think but I'm afraid to pull it off with force.
I didn't see anything obvious to remove it.

Is it even worth repairing it? Wouldn't say no to a free multimeter unless I have to procure unobtainium parts.

Thanks for your help

[fzabkar]

There is an "8851" manufacture date on the PCB (week 51 of 1988). This is consistent with an "8906" date on the component (week 06 of 1989).

Therefore, the part number is probably HD-08. It looks like a hybrid IC.

The IC is listed in the following service manual.

Technical data for Metex M3610D & 3650D:
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Metex/Metex%20M3610D-3650D.pdf

The IC is shown at the RHS of page 20 and also at the LHS of page 18. It appears that it generates a negative supply (V-) from the input supply (V+). This V- supply appears to be required by IC2, a 20-pin LSI (MAX130CPL). It may be that IC2 is dragging down V-.

Here is the datasheet for the MAX130CPL:
https://datasheet.octopart.com/MAX130CPL%2B-Maxim-Integrated-datasheet-15933939.pdf

V- is connected to the negative terminal of the battery. V+ is derived from the battery's positive terminal (Vcc) via a high-side transistor switch (Q2). Therefore, ISTM that the function of HD-08 is to produce a DC bias between ACOM and IN LO for IC2.

From the datasheet ...

"Make sure that In Lo is connected to Common. Leaving both In Lo and In Hi floating with respect to Common and the power supplies will cause erratic readings since In Lo and In Hi will unpredictably float from V+ to V- unless a DC connection between either In Lo or In Hi and Common is provided."

Therefore, ISTM that HD-08 is providing the required DC connection.


Here is the tech data for another Metex DMM:

https://ia803400.us.archive.org/5/items/manual_M4650CR_SM_METEX/M4650CR_SM_METEX.pdf

The parts list includes a CBC-HD-08 hybrid IC, but I can't see it anywhere on the circuit diagram. This IC is listed in the "revised" material parts list. Therefore, I'm wondering whether the meter can operate without it. The circuit diagram shows that IN LO and COMMON are linked. The IC in this case is ICL7129CPL.

https://www.futurlec.com/Datasheet/Harris/ICL7129.pdf

[robert.rozee]

by default, the ICL7106 regulates the COM pin (32) below V+ using an onboard reference. i think that the HD-08 serves to over-ride the 7106's onboard reference, replacing it with it's own. this is a rather odd thing to do on a 3 1/2 digit LCD multimeter.

the metex M3610D is a rather strange beast, as usually an ICL7106 is sufficient to realize a 3 1/2 digit multimeter without any extra semiconductors. it seems that the extra parts in the metex are there to provide an analog bargraph, frequency, logic measurements, and serial I/O. today, this functionality (along with the ADC and reference) would all be in a single chip.

while technically interesting, i'd not be investing too much effort into making it work. it is only a 3 1/2 digit manual-ranging meter!

cheers,
rob   :-)

[fzabkar]

I would try removing HD-08 and linking pads 3 and 4. First confirm that there is a low resistance between pins 1 and 2, otherwise the current drain would be due to some other component.

[agent_power]

Hello and thanks for all your replies!

What would you consider a low enough resistance? There are 250 Ohms between pin 1 V- and pin 2 V+, but only when the device is off.

I removed the HD-08 and linked the other two pads. The current draw is still the same, but now the meter shows just 1 on all modes.

Don't see any other hot component, maybe they're under the LCD or it's the CPU. I'll try to find out

[tunk]

There's another thread about the non-B model, and for that it looks
like the rotary knob has two pieces which are pressed together:
https://www.eevblog.com/forum/repair/metex-m-3610-repair-help-dmm-won_t-zero-properly-in-ohms-and-dc-volts/
If you manage get it open and the main IC is socketed, maybe you
could remove it and see if it still draws 200mA? And check the rotary
contacts?

[agent_power]

Yep, I found out how to get it off. I grabbed the front of the knob with a pair of pliers and a paper towel in between to avoid scratches and the PCB with my other hand, then wiggled and pulled until it came out.

If I remove the IC MAX130CPL, it still draws about 100mA but I don't have a display anymore (makes sense).

If I solder HD-08 back in, the current draw is almost 150mA. Now I found that the hybrid IC next to the rotary knob (CAC-HD02 A) is overheating.

Found some schematic for it: http://www.szetszedtem.hu/000egyebek/rajzok/muszerek/metex%20m3650%20rajz.pdf but I have to study it I'm not really that experienced

[fzabkar]

CAC-HD02 appears to be an audio amp/oscillator with an input (or enable) on pin #2.

Is it possible that the meter was killed by a reversed battery polarity???

[agent_power]

The more I look at it and the more I think it suffered from some event that fried it. I also see some solder/flux residue that was added later.

No idea about its past except that it sat in an attic for decades before it was supposed to be trashed. Thought it would be an easy fix with the banana socket and on/off switch, but nope.

Can I snip CAC-HD02 out? Don't think I'll spend any money on this meter, if I can't get it to a state where it's at least a glorified battery tester I'll get rid of it and maybe someone can use the functioning parts...

[fzabkar]

If you snip out the CAC-HD02 hybrid, all you will lose will be the buzzer, AFAICT.

[agent_power]

Hi all

I removed the buzzer IC, and it looks like both the hybrid and the MAX130CPL have shorts.

To the bottom drawer it goes until the next clean up!

Thanks for your help, at least I learned something about multimeters...

[agent_power]

Hi all

I found a M-3650B for parts, time to have fun with Metex again.... maybe it will be possible to make a working one?

This one doesn't have shorts, but is out of whack or not working at all on certain functions.

For example:
- resistance measurements and continuity check look ok
- Capacitor and transistor measurements don't work at all it seems
- DC volts always shows more or less triple the value of what it should.

Would you suggest to check if the parts are compatible to the one with shorted ICs, or try to repair the M-3650B which is already halfway working?

I cleaned up the board, it looks like someone already changed some parts notably one of the hybrid ICs, the buzzer and a connector, unless the flux residue came from factory!

Thanks!