Products > Test Equipment
[SOLVED] 34401a - Cal Error 707
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sdg:
Not really brilliant trace routing, there...
Dr. Frank:

--- Quote from: zucca on May 15, 2014, 04:10:29 am ---No!


passed  :-+

Now I will contact the lab and see if they agree this unit is now back in business.

Bastard 24K Resistors! I allow myself to declare that not only the front/rear switch but also those 24K sense resistors are a weak point of our beloved 34401a.



--- End quote ---

Hi, congrats that YOU found the trouble!
You should tell the lab guys, that the instrument IS fine again, although THEY pointed in the totally wrong direction.
Hopefully, they are better in calibration.

Before you send the instrument, make sure, that the switch still is ok, by measuring a short in 2W OHM mode on the front and on the rear and actuating the front / rear switch several times.

The short should give the same readings after switching.
And normally, if it's a massive Cu  short, should be below 10mOhm.
But that zero value may be affected by the failed calibration..

Frank
Zucca:
The lab contacted me again:


--- Quote --- The 34401A passes all tests accept for the 100 Ohm 2 wire gain test.  I attempted to adjust it in several times, however it will not adjust into specification.  Most likely is a bad resistor somewhere on the main board.  All other tests pass and are within OEM specification.  We can issue a limited calibration on this unit.  Please advise.
--- End quote ---

 ???

See also the attached service note from Agilent.

Now the 34401a will be sent back to me, I plan to run a test over time by remote controlling the unit (why I didn't it before?  |O). I will then see how and if it drifts, it looks like the R202 is the suspected one.

Keep you posted.
Dr. Frank:
Did you check the zero ohm reading in 2W mode, its stability and if the switch is oK?

As R202 is used in ALL lower Ohm ranges, it probably is ok.
Otherwise, all other ranges would also drift.
And also, your unit seems to be much younger than 1994, when this error occurred.

Oh yes, and if the instrument is still in the lab, they could try to calibrate the 100 Ohm range using the rear input jacks.

If that is stable, it's definitely the switch.

Then they could return it to you, and the calibration will be ok, as you clean the switch.

Frank
Zucca:
I tested the zero ohm reading and the switch, the stability I watched the display 1 min and it was ok. I did not set up a proper test with a graph of values over time.
Agree on the R202, I think it is used for range up to 1MOhm. Above that the R201 will kick in.

I will also cross check the serial number with the service note.


--- Quote from: Dr. Frank on May 27, 2014, 05:17:01 pm ---Oh yes, and if the instrument is still in the lab, they could try to calibrate the 100 Ohm range using the rear input jacks.
...

--- End quote ---

Nice idea, I feel like stupid now: why I did not have thought about that immediately? Oh yes maybe I was assuming there are 2 different sets of calibrations parameters depending on the switch position.

I proposed your idea to the lab let´s see what happen.

Thanks Frank, I need to buy you a beer.

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