HP 34401A Error 612

[IO390]

I just got a 34401A. I ran the self test and it came up with one error, 612. The manual says that it's an error on the 500uA current source for the ohms range.

Usually I'm fairly happy to probe around and sort it for myself, but there's no way I can afford to replace anything major here if it breaks, so can anyone offer some guidance on what to check first?

Nothing is obviously blown up.

Thanks

[Galaxyrise]

Do you have another meter that can measure 500nA?  If so, then set the 34401A to ohms and measure the current.  Switch to the different ohms ranges and see what the test current is.

edit: seems like 612 is 500nA, not 500uA.

[Dr. Frank]

YAWN!
Sorry, I'm already 'bit extremely tired, tonight.
I feel, this or a similar failure with Ohms was reported  and solved a few weeks ago.
Sounds very familiar, also the Iconst measurement of GALAXYRISE.
Was it a defect dioode and tant cap on the +18V supplynear current source???
 
Please, search for that , for today.. I'm being back by tomorrow..after a big cap of sleep..

Frank

[IO390]

500uA? Do excuse me, I appear to have been retarded there. The range is in fact 500nA. I just tested it, the range sources 500nA and measures fine, yet the error remains. Perhaps a firmware issue or a problem elsewhere?

[Galaxyrise]

From the service manual:
  This test configures to the 10 V dc range with the internal 10 M 100:1 divider U102A connected across the input. The 500 nA ohms current source is connected to produce a nominal 5 V signal. A 20 ms ADC measurement is performed and  the result is checked against a limit of 5 V+-1 V
It's measuring its own 10M resistance.  That resistance could be off, the meter's notion of 5V could be off, the switches that set up the test could be bad, etc.

like i said : check the 10 meg divider. pin 1 to pin 12 i believe of that ceramic hybrid. is most likely it is open.
i have seen those fail two times. that hybrid is very expensive... like 250$ ..

How do the 100V and 1000V DC volts ranges perform? 

[IO390]

All the other ranges are fine.

This morning I tested the 4 wire ohms range and bingo - the upper two ranges are way out. I'm getting around .24M on a 180K resistor, the lower ranges work fine however. The schematic doesn't have any mention of the 500nA range however (that I can see anyway). I would measure that hybrid but I can't see an obvious pinout.

[HighVoltage]

A good starting point is to measure the rail of +18 and -18 Volts
I had a similar problem and it was caused by a broken LM317 and had nothing to do with what the service manual said about the error.

[VK5RC]

+1 for HighVoltage/DrFrank re voltage issues, my 34401 came up with a load of garbage in error codes and in mine it was a zener.

[IO390]

I've got +18.9 and -18.3. That's within tolerance - any other suggestions? If someone has an idea of the pinout of the ceramic hybrid then I could measure the resistors there.

[VK5RC]

The zener that died in mine is after the +/-18V rails, there are multiple + and - 15V supplies I recall labelled 15A, 15B , -15A, -15B etc. Each has its own zener and filtering cap or two.

[Dr. Frank]

Please see the schematics, p. 9-10 of service manual:
On 10M and 100M Ohm ranges, which both (?) fail, a constant current of 17.5µA is created by R201, 400k.
(I assume, you measured 240k on the 10MOhm range? But 180k correctly on the 1M range?)

All other, lower Ohm ranges, including 1MOhm, use R202, 40k for 175µA.

This 17.5µA current creates a 0.5V voltage drop over the 28.57k resistor in hybrid U102-D.
OpAmp U201-B regulates the output constant current , so that the same voltage drop of 0.5V over the 1M reference resistor in U102-D is generated.
That means, 500nA have to flow from the floating supply Ufl. ~ 13V over 1M to FET Q202

The floating supply Ufl.  should be checked in lower Ohm ranges, as the foregoing problems were caused by a defect CR203-A, as far as I remember.

I bet, this is OK, as all other lower ranges are working correctly.
R201 may be defect, and may measure around 300kOhm.
That you can perhaps check in situ with another DMM, but with de-energized 34401.

There's also service note 34401A-12, indicating a serial problem with this resistor:
http://literature.cdn.keysight.com/litweb/pdf/34401A-12.pdf

Maybe you don't find such an extreme drift... because normally, due to their technology, TF resistors always drift towards higher resistance values.. then U101-D might be fishy.

Or first check R201, top and solder side  for any dirt, which may create a parallel parasitic resistance.

Frank

[IO390]

Thanks for the suggestions everyone.

Frank, I just tried what you suggested. I measured R201 and got 400K bang on, so it can't be that. I'll take some voltage measurements and see what it says.

The unit is virtually spotless and totally clean. Not even any dust - I think it's been in a rack its entire life as it didn't have the bumpers or handle. Date of last cal is March 2000.

[Dr. Frank]

What's strange is the big difference or ratio between the measurement and real value of the 180k resistor, is it really a plain factor of 4/3? (The 34401A delivers 5 digits for that value, therefore, please post the full digits of readings in 1M and 10M range).

Then you already told us, that you measured precisely 500nA (again, to which precision, how many digits?)..

Therefore, if you also measure precisely 0.5V over the 28.57k resistor (pin 3 + 4 of U102-D), and over the 1M resistor (pin 3 + 9), then the hardware is very probably ok.


If there really is a plain  factor of 4/3, then I would strongly assume a faulty calibration constant for the value of the 500nA current source. Instead, a value of 375nA would have been stored.

If this turns out to be the case, then I wonder, how this wrong constant was created.

Maybe by wrong calibration of the 10M/100M range by connecting a wrong reference resistor... but I doubt, that the 34401A would accept that, if the calculated 500nA value would be off that far.

Maybe the EEPROM suffered a bit toggle, so that the factor changed by 4/3.. That I also doubt, because error detection and correction will be probably implemented.

In any of the latter cases, a simple re-calibration of the 10M range would solve the problem in first instance.. then it would be left to check, if the EEPROM has a problem.

Frank

[Samogon]

Or first check R201, top and solder side  for any dirt, which may create a parallel parasitic resistance.

Frank

Confirming
I had 34401A with all 612, 613, 615, 617, 618, 619, 621 errors, checking current on Lo and CR202 indicated that U201 is dead, replaced it and most of the errors have gone except 612 and 618.
Thoroughly cleaning R201 area eliminated 612,618 errors. Any residue left after rework dramatically influences on current in 500nA range.

But have another 34401A with 612,618 errors, cleaning does not eliminate them. Left it for future projects.