Author Topic: Metex m-4650cr multimeter repair  (Read 966 times)

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Offline marekpasek

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Metex m-4650cr multimeter repair
« on: November 24, 2019, 07:44:12 pm »
Hello,
For some time now I have been using an old multimeter Metex m-4650cr (year of production approximately 1992). Until today it has been working fine and performed superiorly. But today when trying to measure some 5v rails I found that the voltmeter does not work porperly (displays 4v instead of 5v and 16v instead of 20v). I checked the performance on a trustworthy supply (one which is certainly not 20% off) and indeed it appears that the measure voltage is always approximately 80% of real value. The thing is that the problem seems to occur only on the voltage ranges. The ohm range performs well (when checked with some precise 1k 0,1% resistors it displays 1,0012k after over 25 years without calibration). I do not have any ac voltage or current references to try other ranges so I can provide information only for the voltage and resistance measurements. Besides of the voltage range everything seems to work normal. What could be the cause of this behaviour and could it be fixed?

Post scriptum: I really want the multimeter to work again even if it was not rentabile. My attitude to this piece of gear is very special (you know it was my first multimeter with which I had a lot of fun and I learned all the basic principles of electronics looking at its display. :) With so many nice memories it does not seem right to throw it away when it breaks. So please refrain from advice such as buy a new one. I am planning to do so but firstly I will fix this sucker!

Schematic is attached below.

Note: Further testing done! The same deviation occurs on DC current ranges and also on diode tester! Interestingly the deviation on diode tester seems to be higher (33%), when compared to some chinese no name meter which I now only have available.
« Last Edit: November 27, 2019, 08:30:20 pm by marekpasek »
 

Offline marekpasek

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Re: Metex m-4650cr multimeter repair
« Reply #1 on: November 24, 2019, 07:47:12 pm »
Also if you had some better schematic available please provide it here. The one I found is incredible mess I think.
Thanks.
 

Offline retiredcaps

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Re: Metex m-4650cr multimeter repair
« Reply #2 on: November 24, 2019, 09:38:47 pm »
Questions/Suggestions.

1. Have you tried a brand new fresh battery?
2. When it was working previously, did you measure any high energy circuits recently?  Or have an "oops" moment?
3. Using another meter, what is the input impedance of the 4650 when it is set to 20VDC?
4. Take it apart and post some clear focused high resolution pictures of both sides of the pcb with the range switch removed.
5. What reading do you get when you short the probes on 20VDC?
 

Offline retiredcaps

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Re: Metex m-4650cr multimeter repair
« Reply #3 on: November 24, 2019, 09:53:08 pm »
6. Put 4650 into 200VDC mode. Measure your 5 and 20V source again.  What do you get for values?
7. Put 4650 into 1000VDC mode. Measure your 5 and 20V source again.  What do you get for values?
8. Put 4650 into 2VDC mode.  Measure a fresh AA or AAA cell. What does it read?  Does it measure the same using another meter?
 

Offline marekpasek

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Re: Metex m-4650cr multimeter repair
« Reply #4 on: November 24, 2019, 10:40:32 pm »
Thank you for your suggestions:
1.) Yes, I replaced the battery by a new one but the reading did not change.
2.) Few months ago I had an accident when the meter could be exposed to low energy high voltage spikes. But it seems to have some protection spark gap and automatic turn-off when high voltage appears. After this it worked normally until today.
3.) That sounds interesting. I will measure that tomorrow (I do not have another meter at home). Could you explain your thoughts when asking this?
5.) For all ranges (0,2V; 2V; 20V; 200V; 1000V) it display 0V no matter if the probes are open or shorted.
6.) It reads the same voltage on every range.
8.) I will do that tomorrow when having access to another multimeter to compare against.

Generally I think that there is some destroyed voltage divider in the circuit which feeds the input to ADC (which seems to be intact because it gives constant values). To be honest I do not understand the schematic much since the ICs are not even marked and all it is quite messy but I suppose that there is only one ADC for all functions of the multimeter (why would there be more?) and the task is to find and replace the broken resistor which messes up with the divider ratio thus feeding wrong voltage to ADC. What do you think?
Thank you for suggestions.
 

Offline retiredcaps

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Re: Metex m-4650cr multimeter repair
« Reply #5 on: November 25, 2019, 12:00:16 am »
3.) Could you explain your thoughts when asking this?
In the manual at

http://elektron.pol.lublin.pl/users/elekp/labor_instr/METEX_M-4650CR_Manual.pdf

page 12 says input impedance is 10M ohm on all ranges for DCV.  If it isn't a stable 10M ohm reading, then that would affect the DCV readings.

It may not read 10M ohms for a number of reasons: dirt, debris, leaked battery juice, contamination, an "oops" moment, dirty range switch, carbon tracks, etc.

That is why I asked for pictures.  We can't see what you are looking at.

I can't read a schematic to save my life and still learning, but I suspect R2 through R7 is your divider network.  A real life picture of the pcb would help identify any possible input protection components that may not be in the schematic as well.
 

Offline retiredcaps

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Re: Metex m-4650cr multimeter repair
« Reply #6 on: November 25, 2019, 12:05:58 am »
When you do get another meter,

9. What is the resistance between R2 and R7?
10. What is the resistance of just R2?  Just R3, just R4, just R5, just R6 and just R7?
 

Offline imo

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Re: Metex m-4650cr multimeter repair
« Reply #7 on: November 25, 2019, 07:36:42 pm »
TR1..
 

Offline marekpasek

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Re: Metex m-4650cr multimeter repair
« Reply #8 on: November 25, 2019, 09:46:36 pm »
Hello,
I measured all the ranges and they all have 9,91M impedance (within tolerance I guess). I also shot the internals of the meter which I hereby attach.
All the diodes are good and all resistors with readable band marking are within tolerance. The yellow thing below is probably the resistor network and it seems to be intact as well (the ratio between teh resistor is ten and the case is not anyhow melted or something). The band marking value is shown in red, the measured value is in green and for those resistors which have no marking the measured value is shown in blue. Three mysterious things come up with this board though.
Most interestingly there is the 2ohm resistor in the top with seems to be blown. The cover of the resistor is burned away completely an the metal body is exposed. The band marking is unreadable save from a residue of brown band with does not tell anything. Furthermore there is a 12k resistor in pararel with the 100ohm one in the other side of the board (top right of the picture) and there seems to be another one attached in pararel with a trace (!) so it is shorted. Lastly there is some mysterious box without marking in the right bottom of the picture between a diode and the slot which connects to the main board. It is not worth to shoot close-up since it is just a gray box with two leads. It does not show any capacitance and the resistance is about 1,3k.

Note: I am using some no name chinese multimeter so the reading are not very precise (still serving well I think). The input protection is apparently only limited to a spark gab (on the top left of the  picture).

Thank you for all suggestions. I think the most important thing now is to get some table with resistor values for  this model and to identify the burned resistor which now seems to be the culprit of the problem.

Post scriptum: imo, could you be more specific? How could the T1 cause such a problem (affect the reading by a constant deviation)? Thank you.
 

Offline imo

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Re: Metex m-4650cr multimeter repair
« Reply #9 on: November 25, 2019, 10:34:27 pm »
Quote
Post scriptum: imo, could you be more specific? How could the T1 cause such a problem (affect the reading by a constant deviation)? Thank you.
TR1 in your schematics is a kind of protection, imho. It could be with a higher voltage peak it got partially damaged and it leaks, thus creating a voltage drop somewhere else.
 
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Offline marekpasek

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Re: Metex m-4650cr multimeter repair
« Reply #10 on: November 25, 2019, 10:53:31 pm »
The TR1 looks like some reverse polarity protection to me. Also it is quite close to one of the ICs. Since the multimeter works normally except of the deviation on the voltage range I suppose that all the ICs are working correctly. I cannot see the way how the voltage spike could damage the transistor without cooking the IC which is only one resistor distant from its emmiter.
Certainly it is a interesting point and I will scrutiny that after I am done with the blown resistor.
Thanks a lot.  :)
 

Offline Muttley Snickers

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Re: Metex m-4650cr multimeter repair
« Reply #11 on: November 25, 2019, 11:26:23 pm »
Someone else had a similar problem with a Metex M4650 and it was discussed in the comments section of the video linked below. The video refers to the adjustment of a Metex M3650 and this fellows prior videos refer to the meters repair. I still have a M3650CR which is a different meter again.

 

Offline marekpasek

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Re: Metex m-4650cr multimeter repair
« Reply #12 on: November 26, 2019, 10:44:06 pm »
That´s a different case. The guy in the comment had problem with just one range and the deviation was negligible (actually it was not a repair but calibration of trimmer after twenty years). I have the same deviation on all ranges and it is very significant (28%). :(

Thank you though. As I said I appreciate every advice and watching the video series might be helpful. :)
 

Offline shakalnokturn

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Re: Metex m-4650cr multimeter repair
« Reply #13 on: November 27, 2019, 05:48:28 pm »
No help on your voltage reading problem just yet, need more good photos or a readable schematic.
Just a comment on the parallel resistors: The low valued ones are the current measurement shunts, paralleling a higher value is a factory method to trim the shunt resistance.
The cemented 2 \$\Omega\$ resistor will have crumbled the cement layer during a intense current abuse.
 
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Offline marekpasek

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Re: Metex m-4650cr multimeter repair
« Reply #14 on: November 27, 2019, 08:42:14 pm »
Yes, the 100 ohm resistor is probably the low current shunt. The burned resistor is not cement, it is just a metallic one with higher power rate. The high current shunt which might have undergone intense current stress is in the top of the picture and does not seems to be anyhow affected. I am currently requesting a service manual for this meter here (https://www.eevblog.com/forum/chat/metex-request/). Even though I tried to assing some funny sounding name to the thread to bring attention, nobody have answered there so far. The schematic I found is truely pointless I know. :(
 

Offline shakalnokturn

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Re: Metex m-4650cr multimeter repair
« Reply #15 on: November 29, 2019, 08:24:58 am »
The Greek looking current shunt (10m \$\Omega\$) for 20A, 100 \$\Omega\$ for the 2mA range, of course from this you could have guessed that the 200mA range resistor should be 1 \$\Omega\$ even with the poor schematic.

I didn't comment on that value thinking it was just measurement error due to your cheap DMM used for troubleshooting.

The resistor looks wirewound, by "cement" I was referring to the grey heat resistant coating over the wire.

You'll want to change it anyway to have correct readings (parallel resistor value may need readjusting), it shouldn't affect your voltage readings however.

Have you touched any of the trimpots?
 

Offline shakalnokturn

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Re: Metex m-4650cr multimeter repair
« Reply #16 on: November 29, 2019, 12:56:00 pm »
Here's how I'd try going about it:

On the main IC, the one that's most of the multimeter, locate the Vref pin, measure voltage there. Does the value make sense? There are a handful of figures that are very often seen for ADC Vref.

Locate Vin (measurement) pin, input a voltage to get a round figure (I.e. read 1V on 2V range). Check voltage on Vin pin, how does it compare to Vref?

If the relationship between Vin/Vref makes no sense suspect the ASIC itself. If it does make sense, work back towards the input socket trying to find where the few mV have been lost.

It may be worth the bother of reflowing SMD solders especially the ASIC before searching.
« Last Edit: November 30, 2019, 01:25:18 am by shakalnokturn »
 
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Offline marekpasek

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Re: Metex m-4650cr multimeter repair
« Reply #17 on: November 29, 2019, 11:14:19 pm »
That seems to be it!

It took me a while but I found the voltage reference on the schematic. It is made of discrete components which are marked as R26-28, D8, VR1. D8 is LS-5010HN which is a 1.2V reference. So a measured the voltage across the zener and it is 1,53V! The ratio between 1,2 and 1,53 is 0,784 which is exactly the ratio between the measured voltage and the anticipated actual voltage. This also explains why only the resistance ranges are fine since the reading of resistance is determined by the ratio of test current and measured voltage which both are dependent on the voltage reference (the reading should therefore be independent on the reference voltage, is it not?). On the other hand current and voltage ranges just compare the measured voltage to the reference voltage therefore the reading changes with reference voltage (does it not?).

So that is fantastic. Now there are the questions. Why did the reference voltage change? What could suddenly damage the zener? Is it the zener itself what is damaged or could there be some other causes? (These questions are supposed to be rhetorical to direct the discussion).

So please post your thoughts and conjectures.

Datasheet of LS5010 is attached bellow.
 


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