Author Topic: Advantest R6581 8.5 digit DMM mini teardown/repair  (Read 201409 times)

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Online Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #875 on: October 20, 2021, 06:11:00 pm »
The voltage readings look all OK, just a slightly lower reference voltage than the nominal. This should not be a problem. The ADC input = pin 3 of R200 is also close to 0 V as it looks. So there is no large offset visible. So there must be some other odd reason for the meter to measure so much offset.

One possible fault to cause an offset may be a non working reset at the ADC integrator. So maybe check TP201 with the scope. Probably best with 1 PLC, non AZ and a short in the 10 V range.
 
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Offline leighcorrigall

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #876 on: October 20, 2021, 06:15:12 pm »
Let me remind you all that I truly appreciate your help with this Advantest R6581T problem.

I would be entirely lost without your input and I am so glad I have access to your knowledge, experience and insights.  :-+
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Offline leighcorrigall

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #877 on: October 20, 2021, 06:34:20 pm »
...

One possible fault to cause an offset may be a non-working reset at the ADC integrator. So maybe check TP201 with the scope. Probably best with 1 PLC, non-AZ and a short in the 10 V range.

I guess it is time to buy my very first oscilloscope  :-[
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Offline leighcorrigall

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #878 on: October 20, 2021, 07:12:09 pm »
Here's 03.

Have you successfully used these files?

Also, is the firmware loaded on the PLCC44 chips next to the front transformer? They seem to be.

If so, I am looking at this adapter board for my GQ-4x4 universal programmer:
https://www.gqelectronicsllc.com/store/index.php?route=product/product&path=25_64&product_id=135

I think I will buy new chips so that I have a backup before I attempt an upgrade.

This firmware upgrade will be in anticipation of when I do figure out what is actually wrong with my instrument. Firmware might be a problem, but I will exhaust all other possibilities first.
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Online Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #879 on: October 20, 2021, 07:26:45 pm »
I don't think the firnware is main cause of the problem. There may be a few smaller changes with the firmware, but the problem at -1 V looks like a serious HW problem. I don't think with could be fixed with small changes in the timing.
 

Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #880 on: October 20, 2021, 08:23:36 pm »
R6581T firmware updating - is a totally waste of time.
 
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Online MegaVolt

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #881 on: October 21, 2021, 07:46:55 am »
Have you successfully used these files?

I do not have this device :( I just collect information :)
 

Offline dietert1

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #882 on: October 21, 2021, 12:19:47 pm »
Once more i looked at the input leakage current of the R6581T with the modded input MUX.
With covers open i nulled the instrument with a low thermal short and then connected 10 MOhm with 1 uF MKS parallel to the voltage input. So 1 pA of leakage generates 10 uV offset voltage. I added a protective box over the input terminals with the resistor and the cap to suppress air drafts.
The steady state observation was 3 uV, that is 0.3 pA leakage.
After closing the instrument and waiting some hours  for the internal temperature to reach 38.5 °C i repeated the procedure and found 2 pA. As expected for semiconductors the leakage currents rise with temperature. These are the JFETs in the input MUX, the protection diodes and the JFETs in the amplfiers.
Another criterion for a "good" R6581T is low input noise. With a low thermal short, 100 PLC, Autozero and running 100x statistics i got a standard deviation between 45 and 50 nV. This is in 100 mV range.

I tried to look at the dagnostic mode measurements and check the results with another DVM. Very confusing, e.g. they display the 7.2 reference voltage as 0.706221, while the HP 3478A displays 7.0640. The diagnostic measurements may be in arbitrary units.

Regards, Dieter
 
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Offline leighcorrigall

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #883 on: October 21, 2021, 01:37:17 pm »

...

I tried to look at the diagnostic mode measurements and check the results with another DVM. Very confusing, e.g. they display the 7.2 reference voltage as 0.706221, while the HP 3478A displays 7.0640. The diagnostic measurements may be in arbitrary units.

Regards, Dieter

Interesting, Dieter. Thank you for this information.

Could these false readings from the internal check mechanism be a common issue? MickleT's "R6581(D, T) DIAGNOSTIC MODE" PDF reflects expected measurements and shares the same version ID as my own unit (i.e. "ADVANTEST,R6581T,0,A01"). I believe that that the magnitudes are not intended to be 'arbitrary'. We could assume that our anomalous readings are hardware-related. I wonder how they are collected and presented by the DMM.

Out of curiosity, what did your unit state for the internal zero checks (x1Zero_1, x1Zero_2, X10Zero, and X100Zero)?
« Last Edit: October 21, 2021, 01:40:36 pm by leighcorrigall »
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Offline serg-el

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #884 on: October 21, 2021, 01:53:52 pm »
https://cloud.mail.ru/public/xwNi/b5vD62nsc
Internal check data of my multimeter.  For comparison.
 
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Offline leighcorrigall

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #885 on: October 21, 2021, 02:11:45 pm »
https://cloud.mail.ru/public/xwNi/b5vD62nsc
Internal check data of my multimeter.  For comparison.

Thank you, serg-el.  :-+

Is your R6581 unit have original components or have you modified the analogue board?
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Offline serg-el

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #886 on: October 21, 2021, 02:28:04 pm »
Only relay K006 connection and replacement of capacitors C215 and C212.

https://cloud.mail.ru/public/qCSD/2tCLvdhJ5
Yes.  This multimeter has had a difficult life.  I had to align the case, and wash, and clean the buttons ...
« Last Edit: October 21, 2021, 02:32:25 pm by serg-el »
 
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Offline leighcorrigall

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #887 on: October 21, 2021, 04:06:46 pm »
...

This multimeter has had a difficult life.  I had to align the case, and wash, and clean the buttons ...

My unit required little to no maintenance. It is much cleaner than your unit too. The T version might have lower-quality components that are more prone to failure. Going through your pictures I notice a few variations (besides the obvious AC board). Thank you for these.
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Offline branadic

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #888 on: October 21, 2021, 05:37:21 pm »
@ leighcorrigall

By the looks of it you have some serious problem, not only that there is something screwed up with the voltages, also internal temperature is pretty unusal and high, unless your unit is close to the vulcano on La Palma.

-branadic-
Computers exist to solve problems that we wouldn't have without them. AI exists to answer questions, we wouldn't ask without it.
 
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Offline dietert1

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #889 on: October 21, 2021, 06:54:53 pm »
The higher temperature may also explain the higher input leakage. Maybe the fan isn't working properly.

The diagnostic mode has a "Reset" item. I used it trying to terminate and leave diagnostic mode for normal operation, but nothing happened. The next time i was using diagnostic mode, all the internal voltage readings came on scale.  Mmh. That unit is firmware A01. Maybe somebody knows the details.

Regards, Dieter

Edit: Or the fan is working and the internal temperature is normal, but the LM35 temperature sensor reading is wrong by the same offset that was present before. 30 °C extra is a 300 mV offset. At a normal operating temperature of 30 °C when covers are open the LM35 output should be 0.30 V.
« Last Edit: October 21, 2021, 07:18:19 pm by dietert1 »
 

Offline leighcorrigall

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #890 on: October 21, 2021, 08:44:32 pm »
...

With covers open I nulled the instrument with a low thermal short and then connected 10 MOhm with 1 uF MKS parallel to the voltage input. So 1 pA of leakage generates 10 uV offset voltage. I added a protective box over the input terminals with the resistor and the cap to suppress air drafts.
The steady-state observation was 3 uV, that is 0.3 pA leakage.

...


I repeated a similar experiment using a Keithley 8610 CAL SHORT and a 10.00211 MΩ Caddock resistor (ML212 10M 0.1%) combined with a "DIEL 100 2.5" capacitor that I removed from a Keithley 616 electrometer.

On the 10 V range, the value is < 0.00044 V (100 NPLC) with a ground shield cover over the binding posts after nulling.
On the 1000 mV range, the value is < 0.425 mV (100 NPLC) with a ground shield cover over the binding posts after nulling.

EDIT:
For confirmation, another 10.0108 MΩ Caddock resistor (ML212 10M 0.1%)  was used in combination with another "DIEL 100" film capacitor to measure the leakage. A picture has been attached of the resistor-capacitor combination.

10 V range: < 0.00044 V
1000 mV range: < 0.423 mV
« Last Edit: October 21, 2021, 09:53:11 pm by leighcorrigall »
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Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #891 on: October 21, 2021, 08:55:14 pm »
There are absolutely no problems with the input current at zero voltage at the input. But far from zero, it becomes catastrophic due to charge injection during multiplexing.
 

Offline dietert1

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #892 on: October 21, 2021, 10:02:52 pm »
Sorry, but these are switching errors of the input MUX caused by wrong control signals. For example when switching from LO input to negative DCV input after Autozero. Since the Guard signal comes from the slow amplifier output, it remains at Gnd level for up to 200 usec after switching and current flows into the gate of the DCV input JFET. This forward diode current flows from the Guard buffer to the instrument input. Only when the amplifier has settled, the DCV input JFET gets gate voltage near zero as it is meant to be.
The mod i proposed above produces correct control signals, at least for DCV with Autozero. The Guard pin of the input MUX needs to go negative and/or settle to the correct voltage during the break-before-make time interval. We cannot use the MUX output to determine that voltage level as it is undetermined during break-before-make. Each input except LO needs to have a buffer similar to the AD549 of the DCV input. Then we can select the MUX control signal Pin 27 from those buffered signals and the switching errors will be  gone.

Anyway, it is a good idea to check the basic input leakage. As you wrote, there will be no or little switching errors near zero input voltage.

Regards, Dieter

Edit: Also the JFET switches of the input MUX don't work well when both input and output are high impedance. There is that 330 pF cap C008 to help a little. I will probably try and replace the MUX hybrid by 2x MAX328 low leakage mosfet mux chips plus some glue logic. The comparators used for level shifting can go, as well as the heat generated by the 14x 100 K resistors inside the hybrid.
« Last Edit: October 22, 2021, 07:55:20 am by dietert1 »
 
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Online Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #893 on: October 22, 2021, 07:45:03 am »
There are 3 parts to the input curent at higher votlages. One is the JFET gate current from the slow guard amplfier. Another one is charge pumping from a missing precharge phase: the capacitance of the amplifer input an output side of the mux are charges by the input.  With some 10 pF and 2.5Hz switching (10 PLC AZ) this would act like 40 GOhms to ground.
A third part is charge injection from the gate capacitance. This may vary quite a bit with 2 JFETs in series in the mux. Depending on where the tresholds are the charge flows more to the amplfier or input.

The correction of the control signal would only fix one of the points.

Given the problems with switching and as it is possible to adjust the amplifers temperature drift, it would make sense to use the high ohms ranges mainly in the non AZ mode. As the switching problem also applies to the ACAL procedure, chances are the higher ohms and low current ranges may show some systematic error.
 
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Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #894 on: October 22, 2021, 08:03:06 am »
I think the main problem of higher ohms ranges in R6581T is rather simple: ACAL 100M & 1000M is performing with AZERO ON, but all normal measurements - with forced AZERO OFF.

P.S. Thus, I see two solutions:
1) eliminate the above 3 sources of increased input current.
2) increase the delay in the firmware for performing AZERO during ACAL of 10M-1000M ranges.
« Last Edit: October 22, 2021, 08:08:53 am by Mickle T. »
 

Offline dietert1

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #895 on: October 22, 2021, 08:10:09 am »
Kleinstein, your comment is a little misleading as you forgot to mention that leakage caused by the wrong MUX control signal is up to 10 000x times more than the other two problems you mentioned. Other people get confused if you use the term charge injection when the main, large issue is forward diode current into a JFET gate.

Regards, Dieter
 

Online Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #896 on: October 22, 2021, 08:29:48 am »
I aggree that the wrong control voltage for the MUX is the larger problem. The current can be huge, if the input is low impedance. This however only explains the current for the negative side, below some -3 V. The positive side current is from the other 2 parts. This is still within specs (at least for 10 PLC), but still not great.



 

Offline leighcorrigall

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #897 on: October 22, 2021, 05:31:09 pm »
There are absolutely no problems with the input current at zero voltage at the input. But far from zero, it becomes catastrophic due to charge injection during multiplexing.

You inspired me to run an applied voltage sweep on the Advantest 10 V range. Attached are the results. I waited for the Keithley 238 (SV, MI) current measurements to reach an approximate steady-state before measuring the voltage from the Advantest R6581T. The Advantest was set to 100 NPLC (DFILTER: n = 25 mean), AZERO = ON, PROT = ON. A CRADDOCK ML212 10 MΩ 0.1% resistor (with a DIET capacitor in parallel as shown above) was used as indicated by your illustration.

EDIT: There was a mistake in the applied voltage. The new attachments correct this.
« Last Edit: October 22, 2021, 05:48:11 pm by leighcorrigall »
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Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #898 on: October 22, 2021, 05:49:11 pm »
Input current, calculated from R6581T voltage readings:
 
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Offline leighcorrigall

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #899 on: October 22, 2021, 06:19:25 pm »
Input current, calculated from R6581T voltage readings:

Interesting. It is a bit erratic, isn't it?

I decided to substitute calibration data for the Keithley 238 voltage source so that more realistic voltages were used for the x-axis.
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