Electronics > Metrology

Advantest R6581 8.5 digit DMM mini teardown/repair

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Here i have another R6581T inside TEC chamber result.

The R6581T is monitoring a 5x LM399 reference with PWM 10 V stage, that has been operating at constant conditions for some weeks. Each point in log averages 500 measurements of 100 PLC in 2500 seconds. Vertical scale is in uV.

Before this log the TEC chamber was off (under revision) for two days.
Near the beginning of this log chamber temperature drops to 23 +/- 0.01 °C and internal R6581T temperature drops to 38.9 +/- 0.05 °C. We can see the R6581T drift over 54 hours, about -0.9 ppm total and then settle with low noise. Since the temperature change was 38.9 - 41.3 = -2.4 K we can estimate TC = -0.9 ppm / -2.4 K = 0.37 ppm/K on this time scale. For very short time scales the result was -0.12 ppm/K (see above).

The TC derived from all observations during recent weeks until now is 0.15 ppm/K.

Regards, Dieter


--- Quote from: leighcorrigall on August 31, 2021, 03:59:51 pm ---I just got off the phone with the calibration technician. He said that he tried everything to make the R6581T measure within tolerance:

- 2-wire mode instead of 4-wire for the 10 MΩ, 100 MΩ, and 1 GΩ range (they will update the accreditation data to reflect this)
- checked both HI and LO power settings (not much difference)
- used 100 PLC for the whole procedure

Apparently, the technician had no problem with the R6581T because it is similar to the Fluke 3458 and 8508.

He mentioned that while testing, he noticed something very strange about the -1 V reading on the 10 V range. He had to calibrate the DMM three to four times to get the instrument to read correctly. This might be key to diagnosing the problem... EDIT: This sounds like ACAL R230, which is located right next to R234.

I think I must have done something wrong on my end. Maybe I overheated a component while de/soldering or left part of the board contaminated enough to have poor readings at the high-resistance range.  :(

EDIT: Change history:
1) Branadic binding post adapter with leads soldered to posts
2) R200 - easy replacement with solder sucker because the feet were straight
3) R234 - particularly difficult to remove with a soldering iron, solder sucker, and eventually wire cutters
4) K006 - followed the picture and heat shank a cover over the green wire
5) C215 - easy to overheat, but should not be a critical component

I am going to get an estimate for the cost of repair if possible. Might as well have the instrument come out completely calibrated.

--- End quote ---

I received my Advantest R6581T back from Transcat today. Sure enough, it has the problem with the high ohms range. They are not willing to diagnose or repair it.  :horse:

When I measure an Ohmite RX-1M 1 GΩ 1 % resistor, the reading is within tolerance initially.

After observing the meter readings for about 2 hours, I believe the measurements decay over time until a steady-state value is reached.

My guess is that the instrument reads correctly for a while but slowly drifts off course. The same values appear for both the front and rear terminals. I am going to take some time to measure all my resistance/voltage standards before opening it up for diagnostics.

Does anyone know how to explain this mechanism?

The high ohms performance use low currents and is sensitive to leakage and may thus depend on temperature. The higher the temperature the higher higher the leakage currents.
For resistors much above 10 M, I would not consider a normal DMM to be the right instrument. The Fluke 8858 may an exception. Otherwise it would be more like a job for an electrometer like Keithly 617.
For the very high resitors the auto zero off mode may work better than AZ on, especially with a longer cable. The cables also can make a difference - normal PVC isolation is no longer good enough, unless very short and not touching anything.

Getting trouble with -1 V in the 10 V range is a really bad sign, as this point is also used in the ACAL procedure.

Thank you, Kleinstein.

Should I try to clean the board with isopropyl alcohol, gloves, and a toothbrush or would this just make things worse?

Has anyone successfully measured high-resistances with their Advantest R6581? If others are experiencing similar issues at these ranges, then I guess the problem is inherent to the instrument.

--- Quote from: Kleinstein on October 05, 2021, 06:40:10 pm ---Getting trouble with -1 V in the 10 V range is a really bad sign, as this point is also used in the ACAL procedure.

--- End quote ---

This concerns me also... All in due time.  :-+

For now, I won't tamper with the internal calibration until I measure all my standards. At least I will be able to get some NIST traceable measurements for the trouble. With luck, some will be adequate for the record.

It is a bit hard to tell which direction the leakage is. Leakage on the board has the tendency to get better when the meter gets warm. Leakage insider the semiconductors gets worse when warmer. It is still a question in which direction the current flows  - is can be both ways. Unless there is visible dirt I would not try to clean it, at least not yet or without another hint in that direction.

A point would be to compare the resistance measurement in different ranges modes. E.g. with AZ mode on and off and maybe in a not yet fully warmed up state (or during warm up). With resonable stable resistors one could check if different ranges are consistent.
Another point to check is the input bias current, I remember that there is some problem with the MUX switching causing curent spikes in AZ mode - this is why the non AZ mode may be a option despite of the DC drift.

I faintly remember that there where reports on problems (e.g. higher ranges out of spec) with high ohms with the R6581. So high ohms may not be the strength of this meter.

In some versions one relay rund relatively hot, as current reduction when on is no working properly. This is fixable.
Is the fix for the relay power saving bug applied ?

Not using the ACAL function is more like a short time solution only.


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