Author Topic: Advantest R6581 8.5 digit DMM mini teardown/repair  (Read 62066 times)

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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #100 on: January 20, 2018, 09:25:32 am »
Hi, Mr. Mickle.T,
Many thanks!

How about turn off the AZERO and apply the current and test the voltage on MUX hybrid U001 pin 3,4,5 by using 34401 or scope? 
Is there any posible that the relay replaced , who causing the zero offset ?

How to test the U503 MAX327 ?

Best Regards,
szszjdb
« Last Edit: January 20, 2018, 09:46:35 am by szszjdb »
 

Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #101 on: January 20, 2018, 10:00:38 am »
Turning AZero off and read the output of the MUX with a 2 nd DMM is possible. It is not so much about reading a current, but more like reading the voltage with a short at the input. One could also check the resistance - just in case there is still a problem for the signal from the input to come through (e.g. relay, MUX, protection ) - just make sure the current is not too high for the mux.  A current range could be still used as a second channel towards a zero. It could be also interesting to use a scope to check if there are excessive spikes or other higher frequency signals present.

Another useful test would be to measure the input bias (e.g. use a high value resistor at the input) - here it might be a good idea to do this with AZ active and enabled. As the 100 mV range might not be reliable one could also use a 2 nd meter to measure the current (e.g. change in drop over a 10 M resistor).

One should first get the 100 mV range to work properly before looking at the scaling of the low DCI ranges.  So no need to apply a test current yet.

I don't think the changes relay would have much effect. In the current ranges the voltage reading would not go through the relays. If at all dirt on the board (e.g. flux from replacing the relay might be an issue related to the relay.
 
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Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #102 on: January 20, 2018, 10:06:50 am »
As I already said, it's not possible to disable of AZERO in DCI mode.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #103 on: January 20, 2018, 01:17:03 pm »
Hi, Mr. Mickle.T and Mr. Kleinstein ,
Thanks for all!

I am back home and sit down before my 6581.
What is the current flow when in DCI mode and any draft about the circuit or the step to check the issue? 
I had re-calibrated the external zero and the offset chang a little . Then I connect 34401 to DCI input and use OHM to measure the input resistor and attached the data below.

RANGE   0.1ua           1ua                      10ua           100ua   1000ua       10ma             100ma      1000ma   
DCI      0.2191na   0.11723na      0.000132      0.00075      0.0075           0.000053       0.00045      -0.0036   
                        
34401   1005k            101.7k            10.003k          1.0003k   100.678        10.7             1.719        0.667             0.076(short 34401)
6581     overload   492.1xxxna   9.8758xx           98.377xx    986.44ua     0.986466     0.98672       0.9826   

It seems almost right except the res of 1000ma and it should be 0.1ohm ,but measured to be 0.6ohm.

Any further suggestion?

Best Regards,
szszjdb
 

Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #104 on: January 20, 2018, 02:12:43 pm »
The extra measured resistance for the 1 A range is from the fuse and maybe a little from cables and relay.
For finding a problem that still looks like one of the DCV input amplifier these resistance measurements don't help much.
The offsets in the DCI ranges support trouble measuring small voltages.

I would first test the bias current in the 100m V/ 1 V and 10 V DCV ranges.

A next point would than be looking at the input to the ADC / output of the DC amplifier with the scope. There was already an odd curve (looked like ringing for the 7 V reference)  for this case before.  One might even test this with a AC signal (like 1 kHz square wave) at the input, as the sharp edges could provoke ringing.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #105 on: January 20, 2018, 02:56:21 pm »
Hi,  Mr. Kleinstein ,
Thanks for soon reply!
I remember the DCV is workable and re-test the DCV zero, listed below.
RANGE   100mv   1000mv   10v   100v   1000v
DCV ZERO   0.00029   0.00034   0.0000003   0.000003   0.00004

The DCV zero seems normal in each range. Then I test the linearity and short-term performance from 1-110mv in the 100mv range, comparing with 34401. It is also normal.
34401   6581      34401   6581
1.0009   0.99766      9.9827   9.97756
1.9995   1.99645      19.9596   19.95501
2.9987   2.99503      29.9473   29.94221
3.9957   3.00289      39.9265   39.92107
4.9953   4.99173      49.9162   49.91031
5.9935   5.98961      59.8959   59.88951
6.9925   6.98823      69.8769   69.87031
7.9916   7.98745      79.8674   79.86071
8.9895   8.98544      89.8502   89.84275
9.9886   9.98447      99.8325   99.82467
                          109.8257   109.81778
      

So how to test the bias current in DCV ? Which is the test point?

Best Regards,
szszjdb
« Last Edit: January 20, 2018, 04:14:13 pm by szszjdb »
 

Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #106 on: January 20, 2018, 04:51:37 pm »
For testing the input bias there are two options:
1) Connect a high value resistor (e.g. 10 M) to the input and use the voltage read to calculate the current. If the voltage is very low one might have to take the offset into account. It may take some time (e.g. a minute) for the voltage to settle.

2) Connect a low leakage (e.g. PS or PP) capacitor in the 1-10 nF range and record the drift of the input voltage. This also allows to measure the current at voltages different from zero, by first setting the approximate voltage. This method gets easier if one has a PC to record the data, but it is still possible to read by hand (e.g. 2-4 readings 10 or 30 seconds or so apart. With a small cap one might have to include internal capacitance in the calculated current - though usually no high accuracy is needed. Its more like the order of magnitude that counts as the current can change with temperature and humidity quite a bit. A current below about 50 pA would be good.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #107 on: January 21, 2018, 04:27:37 pm »
Hi,  Mr. Kleinstein and Mr. Mickle.T,
After add the 9.5M res between the DCV port and the reading is 0.384v for 6581's 100mv range and 33mv for 1v range,  using a YOKOGAWA 7532 handheld. So the bias current  is about 40na on 100mv range and 3.5na on 1V range. Right?

In order to find out what wrong,  I open the case and get out the analog PCB to check the detail . Really found an issue that the Q502(2sc3420) is assembled in wrong direction, please refer to the attached pciture. I was so happy when found this, but after  re-assembling , The DCI zero offset is still there ,even if ater another external zero calibration. Too upset when haveing none progress after checking for a whole day.
            100na          1ua                 10ua            100ua   1000ua   10ma          100ma   1000ma
before 0.2191na     0.11723na      0.000132    0.00075    0.0075   0.000053     0.00045   -0.0036
today  0.1737na   0.1021na         0.000055   -0.00016   -0.0005   -0.000057   -0.00045   -0.091   
short  0.3413          0.2821         0.000583   0.00477   0.0384   0.000385           0.00201   -0.0008

It's strange that when short the DCI port the DCI zero will enlarge, data as above.. 
I also noted that the relay K500 is the replaced part and when in 100na and 1000na range ,it will stay in same  situation , and change situation when set to 10ua range. 

How about the next step?  I feel very difficult now.

Best Regards,
szszjdb
« Last Edit: January 23, 2018, 05:33:01 am by szszjdb »
 

Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #108 on: January 21, 2018, 05:08:29 pm »
You should not use an external DMM to input current checking. All voltage measurements must be performed by 6581 itself (after the nulling with a shorted inputs).
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #109 on: January 21, 2018, 05:40:44 pm »
Hi,   Mr. Mickle.T,
Many thanks!
After add the 9.5M res between the DCV port and the reading is -2.6mv for 6581's 100mv range and 0.5mv for 1v range, readed form 6581. Is that right?

As your advice, Some of the possibles sources of the leakage current are U503 MAX327 switches (100 nA and 1000 nA ranges), protection transistors Q502-Q503 and MUX hybrid U001 pin 3,4,5.
I had checked the Q502/503, then to replace the MAX327 ? It is exactly the bigger zero offset in the 100na and 1000na range.   

Is there any posibility the replaced relay K500 have some problem?

How to check the next step?

Best Regards,
szszjdb
« Last Edit: January 21, 2018, 05:44:15 pm by szszjdb »
 

Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #110 on: January 21, 2018, 08:07:45 pm »
First of all, you must divide the problem into the two parts:
1) a high input leakage current on DCV ranges;
2) a high zero offset on low DCI ranges.
And you can forget about #2 without the solving of #1. The 2.6 mV voltage drop on 9.5 MOhm resistor gives about 0.27 nA input current. This is the one-two order of magnitude higher, than my results (~12 pA with AZERO=ON & PROT=ON & PLC10, ~1-5 pA with AZERO=OFF).
There are not so many parts on the input signal way. The mains are U009 buffer, D003-D004 ULL diodes, U001 MUX, Q102 and Q104 dual-FETs.
 
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Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #111 on: January 21, 2018, 08:42:26 pm »
0,27 nA is already much better, but still not good. It might be a good idea to get readings for both the AZ case and the non AZ case, as the AZ mode can add to input bias. Normally one should get a value well below 50 pA, better below 10-20 pA.

Normally there should not be a difference in input current between the 100 mV and 1 V range. The difference could be from the DCV offset problem. One could get away from the DC offset by using the capacitor way to measure the input bias. This method is kind of not sensitive to an offset.
Sometimes the input bias current at a different voltage tells something about the source of trouble.

I don't think the replaced relay is causing the trouble.

I also don't think the max327 is the problem - at least not for the DCV problem. It might have something to do with a problem in the current ranges - but this would be only after the DCV range is really working well.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #112 on: January 22, 2018, 02:53:40 am »
Hi, Mr. Mickle.T and Mr. Kleinstein ,
Many Thanks!

I used to take that the DCV is normally working, as the DCV zero and fullscale seems ok  for the moment. Thanks for point out that issue.I will re-test the bias tonight by two method of the res and cap.

Is the signal flow like that DCV port -> U009 ->D003-D004->U001 MUX-> Q102 and Q104 ? If it is, the U009 is in  front of all and should be damaged by the surge voltage before. The only way is to replace the U009 and re-check the bias?  The others behind U009 should have slightly influence to the bias?

Best Regards,
szszjdb
« Last Edit: January 22, 2018, 07:35:07 am by szszjdb »
 

Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #113 on: January 22, 2018, 07:40:16 am »
U009 isn't in front of all, but can be damaged by the surge voltage too.
You can temporarily disconnect the overvoltage protection components, by desoldering R061, D003 and D004.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #114 on: January 22, 2018, 02:06:09 pm »
Hi, Mr. Mickle.T and Mr. Kleinstein ,
I had test the bias by the res method and get the result of 0.4mv with AZ ON and 0.2mv without AZ  in 100mv-10v range. So the bias is 42pa when AZ on and 21pa when off. It is nearlly 4 times larger than your number with AZ ON and 20 times without AZ.  Both in 10plc condition.  I am using a shorter cable than yestoday, so the reading  become smaller.

The cap method is not so stable when I connect the 22nf  cap, and the time to charge the cap vary hugely , so I dropped the data.

I am going to disconnect the parts you mentioned.

Updated , the reading is still 0.4mv and 0.2mv with or without AZ when disconnecting the R061 and D003/004.
What can I do in the next step?

Many Thanks and Best Regards,
szszjdb
« Last Edit: January 25, 2018, 01:43:08 am by szszjdb »
 

Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #115 on: January 22, 2018, 02:17:38 pm »
21-42 pA are sufficiently small values. The problem is somewhere else, but not in DCV signal path.
 
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Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #116 on: January 22, 2018, 03:53:51 pm »
The capacitor method should also work,  but it needs a good quality cap, like PP or PS. With a large cap there might be dielectric absorption and it thus needs some soaking the cap before getting useful data. The bias can also change with input voltage - especially the AZ case can be quite different with a voltage applied.

With the input bias so low, there is no need to disconnect the protection.
The bias can be different between different instruments - it can also change with contamination (e.g. flux), humidity and temperature. Also light can cause some bias, especially with some SMT parts. One could test U009 offset, with a short at the input and measure the output of that OP with a 2nd meter. Too much (e.g. > 5-10 mV)  of an offset could cause a higher bias.

When doing sensitive measurements it is a good idea to keep the mobile phone away from the meter / circuit - it can cause an upset, especially in some older models.

If everything is OK so far it may be a good idea to do something like noise test with an input shorter in the 100 mV and 10 V range at one speed like 10 PLC.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #117 on: January 22, 2018, 06:07:38 pm »
Hi, Mr. Mickle.T and Mr. Kleinstein ,
Thanks a lot!
Yes I found the reading is still 0.4mv and 0.2mv with or without AZ when disconnecting the R061 and D003/004, then I recover it.

I use a short jumper to short the 5 port(HI,LOW,HISEN,LOSEN, GUARD) for at least 5 min long and the DCV zero are as below. The data is worse when az off and the AZ OFF MAX is obtained  when power on and the reading is running up . Sometime it will drift to around 20uv, but when I turn on AZ, it will return to arround -1uv.
                     100mv      1v           10v
AZ ON              -1uv        -1uv       -1.1uv
AZ OFF max      19uv        20uv      25uv
AZ OFF normal    3.5uv     3.4uv       4.0uv

The setup of the noise test is also as above, and  run the statistics test internal from 100mv to 10v range.
   100mv azon   100mv azoff   1v azon   1v azoff   10v azon   10v azoff         ,N=100, 10plc ,all in mv     
MAX   -0.00117   0.00275   -0.00093   0.00317   -0.0000008   0.0000034
MIN   -0.00164   0.00198   -0.00188   0.00257   -0.0000029   -0.000001
AVG   -0.00145   0.00236   -0.00137   0.00289   -0.0000018   0.0000009
VPP    0.00047   0.00077    0.00095     0.0006     0.0000021   0.0000044
dev    9.63E-08   1.86E-07   2.09E-07   1.35E-07   3.93E-07     1.14E-06

Any further check should I make? 

Best Regards,
szszjdb
« Last Edit: January 23, 2018, 01:43:58 am by szszjdb »
 

Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #118 on: January 22, 2018, 07:09:53 pm »
I get 10V PLC10 noise std. dev. value 2.8E-07 V. Very close to what you received.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #119 on: January 23, 2018, 02:22:51 am »
Hi, Mr. Mickle.T and Mr. Kleinstein ,
Thanks a lot!

It 's can be seen from my data that the Vpp when AZ off is almost twice the number when AZ on. I should note that the DCV zero will be a large number like 50uv when in cold state and will slowly come down to 2-3uv after warming up,all with AZ off.

Is that means the DCV is probably normal working?  How I can check in the next step?

Further more, if the DCV is fine, let's look at the DCI zero data I recorded before. I also contacted a friend who give me his data,also listed below.

            100na          1ua                 10ua            100ua   1000ua   10ma          100ma   1000ma
before 0.2191na     0.11723na      0.000132    0.00075    0.0075   0.000053       0.00045   -0.0036
today  0.1737na   0.1021na         0.000055   -0.00016   -0.0005   -0.000057      -0.00045   -0.091   
short  0.3413          0.2821         0.000583   0.00477   0.0384     0.000385        0.00201   -0.0008
friend -0.007x         -0.01x          +0.0000x   +0.000x  +0.00x   +0.0000xx       +0.000xx  -0.000x

1. My data is closed to the friend's except the lower range  100na  and 1ua. Is that means the DCI issue is only  in that 2 range, and the MAX327 or some other parts might cause it? Further information shows that the relay k500 had been replaced before due to DCI measure failure, and it is in charge of the lower 2 range switching. Can we suppose that the K500 relay is abnormal and can not change range before and the meter is input a huge inrush current ,which leading to the potential damage or more leakage of the MXA327 or some other parts?
 
2. Why the DCI zero data change so obviously in every range when I short the DCI port, comparing to the open the port?

Best Regards,
szszjdb
« Last Edit: January 23, 2018, 07:19:52 am by szszjdb »
 

Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #120 on: January 23, 2018, 09:14:39 am »
This is my data. No thoughts yet.
Surely it's time to draw a schematics diagram.
 
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Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #121 on: January 23, 2018, 12:54:33 pm »
The DCV part looks good so far.

The DCI offset is still too high. Has there been an new offset zero after the repair of the DCV input section ?

A schematics (even if only crude) of the DCI section would really help to locate possible faults. Without an schematics it is hard to tell how the MAX327 is used and where the relays are used.

The current section should have an OP to drive the protection element, just like in the DCV front end. This could be one of the first parts to check. Due to the much larger diodes, an offset here would be more critical.

One more test would be, if it makes a difference if the voltage input is shorted or a 9 V signal is applied during a current measurement. It should not have much of an effect, but it would be a test to check the off state of the MUX used for DCV.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #122 on: January 23, 2018, 04:08:20 pm »
Hi, Mr. Mickle.T and Mr. Kleinstein ,
Thanks a lot!

Yes it is  time to analize upon the circuit. I will try to get the draft.

Acturely I had repaired nothing but find the way to perform the external calibration. The DCV and DCI zero is improved after that, especially  for the lower range. For example ,the DCI 0.1ma is used to be as 0.07177ma and now is nearlly same with my 34401.

For the further test, I connect the DCV port to 6144 DC source and it is 0.32na when apply 0v, and 0.34na when add +10v, and 0.29na when add -10v. It is 0.29na when left all port open or short the DCV port, and 0.35na when short the DCI port.  Both are in 100na range.  I must point out that the reading changed when open all the port from 0.1737na ,the data recorded yestoday, to 0.29na, and this might be caused by the drift.
Re-test the DCI zero and attached FYI.
           100na      1000na      10ua    100ua    1000ua    10ma       100ma       1000ma               
open   0.2752   0.2034   0.000141   -0.00011   0.0069   -0.000052   -0.00032   -0.0124
short   0.3511   0.2941   0.000651   0.00454   0.0405   0.000285      0.00112   -0.0047
delta   0.0759   0.0907   0.00051   0.00465   0.0336   0.000337      0.00144   0.0077
ppm         759         90.7         51       46.5           33.6        33.7         14.4              7.7
delta uv   75.9         9.07         5.1       4.65       3.36        3.37         1.44             0.77

Pls look at the delta uv on shunt , the trend is very clear and the number is become smaller with the DCI range increasing. That is the correct data compared to yestoday's, which might be some mistake.  That might imply something?

I had found a occurrence that the DCV zero will be a large number like -50uv when in cold state and will slowly come down to -3uv after warming up for 2 hours and stay there,all with AZ off. After I turn on the AZ, the reading become smaller like around -1uv and then turn off AZ ,it will stay there around -1uv , more like that the AZ on fixed the zero offset from the 1st time it was turned on in this power on cycle.  I don't know if it will cause the DCI zero offset issue.  Does it fine in the DCV zero offset ?

Best Regards,
szszjdb
« Last Edit: January 24, 2018, 06:06:51 am by szszjdb »
 

Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #123 on: January 23, 2018, 04:46:08 pm »
The effect of AZ on the offset in non AZ mode is normal. The AZ mode makes the DMM alternate between measuring zero and the input. The result is the difference. In non AZ mode it still uses a zero measurement - likely the last available.

The DCI mode likely measures the voltage at both sides of the shunt and this way needs the AZ mode to work properly. This would at least explain why one can not turn of AZ in DCI mode.

As far As I understood it so far there was a change in the input protection part, which reduced the input bias.

There is some effect of the input voltage (DCV) on the DCI reading in the lowest range - which is kind of indication leakage at the MUX. I don't know if the observed size is normal, it is only 30 pA, but it looks a little high to me.  It's about the order of magnitude as the input bias - so maybe that is the leakage level of the MUX.  One might reduce the effect by having a short at the DCV input when doing a very low level current measurement. When left open the DCV input can (and will ) drift due to the bias (that can be a little different when not active) - this would than cause drift or random movements for the DCI readings. If I understand the table from Mickle correct there are separate points to measure the voltage at the shunt chain. Thus it should be the lowest 2 ranges that can be a little sensitive to leakage in the MUX.

One possible source of offset errors in the DCI readings might be due to an internal bias of the MUX+input amplifier when measuring the voltages at the shunt. The values still look a little odd - but there can be some numerical compensation already. So the readings would not directly reflect the ADC readings.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #124 on: January 24, 2018, 07:25:07 am »
Hi, Mr. Mickle.T and Mr. Kleinstein ,

I have checked the shunt with 34401 and found no isssue with them. The lo side of 0.1R is connected to GND. I hope the mux have none issue as there can not find the replacement.
Any furthter test should I make before de-assembly the PCB?

Best Regards,
szszjdb

 
« Last Edit: January 24, 2018, 07:42:57 am by szszjdb »
 


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