DER EE DE5000 repair and curious behaviour

[ermionesrl]

Hello everyone.

I bought a DER EE DE5000 new, and arrived with a strange fault that puzzle me and I would like to understand, more than repairing the instrument (I already ordered a different brand LCR, a Techmize TH2822D), so I am open to do tests.

After zero calibration it doesn't show zero, but about 0.04 ohm at 100Hz, 0.043 ohm at 1 and 10kHz, 0.047 at 100k. This happens with TL-21 and TL-22, and also placing a short bar between + and - spring contacts.
It seems the instrument calibrate as negative resistance, as measuring a known 1 ohm resistor, reads as 0.957 ohm.
If I calibrate zero by placing a 0.1 ohm resistor instead of a short, it shows zero on the display and then resistor readings are correct if I keep the 0.1 ohm in series with the DUT, but probably this will mess with capacitor readings, ESR, and so on, so this is not a solution.

DCR readings seems to be ok.

I already changed all the 100 ohm 1% resistors (R47, R48, R30, R33) without any result. I checked the VL-VRL -500mV test point and it was correct (it was -0.5005 and I corrected to -0.5000 but without any result).

Everything inside seems fine, no traces of corrosion, solder residuals, etc... The resistors connected to inputs were covered with transparent lacquer but even removing it didn't change anything.

Any ideas?

[coromonadalix]

you do the de5000 calibration to the letter, i mean there is a open count down and a short count down,  and still give an error ?

unless something has damaged the lcr chip ?  there was some functional schematics  here or on the web  with their datasheets

[ermionesrl]

Hello!

Yep, calibration to the letter and just after calibration, without moving anything, you expect a reading of zero, instead it reads 0.043.

I saw the schematics and datasheet of the ES51920 chip, that's why I changed the 100 ohm resistors that are relative to current source and input... but after that I am short of ideas.
The meter is brand new, I unpacked it, placed battery, and performed first open and short calibration, and it shown the error immediately.

[mawyatt]

Why didn't you return/exchange the meter?

Best

[ermionesrl]

Yes, I got refunded almost fully from the seller who was very honest (he let me the meter as the return procedure with courier would have costed too much), but I also paid customs fees to import it from Japan. Those aren't refunded and are about to one third of the total.

In any case, try to solve non obvious faults is always a good learning experience, even if the residual value of the instrument is zero.

I am curious: why the MCU is not able to zero completely the meter, yet the procedure didn't result "fail"? Something is overflowing? My first reasoning was "maybe the resistors drifted", but they weren't and even after changing them with 0.1% parts the calibration finish exactly with the same number.

[indman]

Any ideas?

Hi!
Show us a photo of the device board in good resolution.
It will also be useful if you make a short video of how you perform calibration on the TL-21 and TL-22.

[ermionesrl]

Any ideas?

Hi!
Show us a photo of the device board in good resolution.
It will also be useful if you make a short video of how you perform calibration on the TL-21 and TL-22.

Here is a video I made to show the problem to the seller. I didn't calibrate with TL22 but it does exactly the same, also with a shorting wire between + and -.
http://www.ermione.com/de5000.mp4

and here is the photo of the mainboard (green resistors are 0.1%)

[indman]

Here is a video I made to show the problem to the seller.

Why do I see 3 stripes on the display — are they cracks or film deformation? Why is the image on the display so poor, and why are there some extraneous smudges?
Has the device been subjected to mechanical impact?

[ermionesrl]

Here is a video I made to show the problem to the seller.

Why do I see 3 stripes on the display — are they cracks or film deformation? Why is the image on the display so poor, and why are there some extraneous smudges?
Has the device been subjected to mechanical impact?

Everything is fine, there is a protection film on the display that was placed to prepare the instrument for being shipped back.