Please help to fix my Agilent 34401A

[h4z4rd]

Hi guys!

I've an Agilent 34401A DMM I bought it on Ebay.
The seller didn't mention that it has a problem but he sold it AS-IS and it also was really cheap.

About the problem:
When I turn it on it shows an error message. The error #604 (A/D slope convergence failed).
When it shows this error message I can't measure AC because it's all over the place.
When it does not drop the error message it works good but most of the time it doesn't.
I checked the solder joints and all looks good.

I'd like to get some help to fix my DMM. What could be the problem? What should I check first?

Thank you!

[MarkPalmer]

Here is its service manual. 

http://literature.cdn.keysight.com/litweb/pdf/34401-90013.pdf

Quoted from manual:  A/D slope convergence failed:  The dc section is configured to the measure zero state (MZ) in the 10 V range. This test checks whether the ADC integrator produces nominally the same number of positive and
negative slope decisions (±10%) during a 20 ms interval. 

This problem most likely refers to the A/D converter IC itself, or possibly whatever is supplying power to it. 

-Mark-

[Zapro]

My 34401a had this fault. After replacing almost all op-amps in the input, i found out that some zener regulators for U201 was faulty. And U102 too (and that cost me an arm and a leg) but replacing the chip and zeners fixed it.

[vinovino]

What chip did you replace?
U201 or U102?