Got me a Agilent 34401A (repair needed - SOLVED)

[zed65]

So, I bought a 34401A on e-bay that was sold with fault (needs repair or for parts).

On selftest these error codes is prsent: 613, 615, 617 and 619.

The unit powers up an seems to work on all ranges except for OHM.
It only reads OVLD on lowest ohm (sometimes not there either)it never reads OVLD on higher scale just fluctuates wildly.
The continuity beeps constant when i try that.

I've done some basic measurements, voltage rails seem fine.
U551 / LM317 sits at 18.76v
U552 / LM337 sits at -18.27v
Voltage across CR203 / Zener4v7 is 4.56v
Voltage across CR201 / Zener4v7 is 4.22v
Voltage at pin 3 on U201 +7vref is 6.93v

One odd thing is the Q202  J-FET P-Chan, I measure at Drain 13.14v and the Gate is 17.98v but the Source sits at 0v
Isn't Q202 supposed to supply the OHMS current?

What should I do next? 

[tautech]

Threads that might be of help 

https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/
https://www.eevblog.com/forum/repair/agilent-34410a-repair-attempt-of-2-dead-meters/

[HighVoltage]

I stored away this great pictures from Dr.Frank and it has helped me before to isolate a problem.
34401_Ohm current source_Dr-Frank

[Dr. Frank]

One odd thing is the Q202  J-FET P-Chan, I measure at Drain 13.14v and the Gate is 17.98v but the Source sits at 0v
Isn't Q202 supposed to supply the OHMS current?

What should I do next?

Please, open the instrument, place a short on the input jacks and select 2W Ohm, 10k range.

Then take that circuit diagram and measure all the given test points. The hand written values are the "good" ones.

Your problem seems to be be similar,  and as far as I remember, U201 was defect, so a replacement cured the issue completely.

Frank

[zed65]

I did some measuring today, the ones that stands out is 12,13,14 as I can see.
What do you guys think?



As this fault is "common" I made a empty sheet and a picture where the testpoint are.
Please correct me if i'm wrong on the testpoints or something else .

[Dr. Frank]

Most probably, U201-B is defect, i.e. the input transistors are broken.
These are bipolar super beta input stages, having only about 50 pA bias current.

I assume, that the + input, pin5,  drives excess current (~35µA) into the node between Q201, pin1 and the resistor array, 28k75,  pin4, as this voltage 2) should be about 9.08V, instead of 10.8V.
This also excludes the possibility, that the failure is somewhere in the output stage.

As + input 2) is lower than - input 3), the OpAmps output 14) should have zero volt output.

That also indicates, that this OpAmp is defect.
I think, that is exactly the same problem as recently.

Replacement of this OpAmp AD706 should fix the problem.

Frank

[zed65]

I have now replaced the U201 (AD706J) and now all seems fine! Pass test OK

Thank you all!!