[FIXED, another one] Yes, another 34401A ohm problem - Page 1
Forum section: electronics / repair

author: 4thDoctorWhoFan
author_rank: Regular Contributor
timestamp: November 28, 2015, 08:40:45 pm
content: Hi guys/gals,Before I dig deep into the service manual, I would like to get some advice on this failure.For starters, it fails all the ohm source related self tests like 612, 613, 615 & 617.With no input, and set to 2W mode, the ohm reading just seems to float. With any sort of resistance connected, the meter will read .22 ohms. It does not matter whether I have the input shorted or measuring a 1K or 10K resistor, the meter reads .22 ohms.DC volts seems to work properly.I've read some of the threads here and already checked a few things.I get no voltage at the input jacks of the meter when measuring with another meter in any ohm range. I have also checked all the main voltages including ripple and everything seems to be ok. This includes the 7 & 10 volt references. There is also a 5 volt drop across CR203.The same result applies to the rear inputs. 4W gets pretty much the same result except instead of reading .22 ohms it reads a dead short, .000 ohms.Since this is my first 34401A, I am wondering if it's normal to hear 2 beeps when the unit is first turned on.Any help would be appreciated.Thanks in advance!

author: plesa
author_rank: Frequent Contributor
timestamp: November 28, 2015, 09:04:22 pm
content: Start with manal,where are self test described, schematic included.http://cp.literature.agilent.com/litweb/pdf/34401-90013.pdfErrors you mentioned are related to current source or Ohms measurement - schematic on page 157.

author: 4thDoctorWhoFan
author_rank: Regular Contributor
timestamp: November 29, 2015, 12:43:06 am
content: Well, I think I figured it out or I should say, I hope I figured it out with help from some previous threads.I believe the Op Amp U201 is bad. So I'm going to order some parts and hope for the best.

author: deadlylover
author_rank: Frequent Contributor
timestamp: November 29, 2015, 04:08:02 am
content: Yup just change U201 (the AD706) and you'll most likely be fine, I also had the same problem for one of my units that I bought for cheap.From memory 2 short beeps is normal on power on.

author: 4thDoctorWhoFan
author_rank: Regular Contributor
timestamp: December 06, 2015, 04:46:23 pm
content: I finally received the new part and after replacing U201 the unit is now fully functional and operating within normal parameters.Just thought I would give an update in case anyone cares.

author: 4thDoctorWhoFan
author_rank: Regular Contributor
timestamp: April 14, 2017, 11:44:05 am
content: After I fixed the 34401A mentioned in this thread, I decided to get another one off the auction site.I found one cheap and the description of the problem the seller described sounded just like the problem of my first one in this thread.So I took a chance and got it.Yes, after receiving the unit, the problem was exactly the same. So I immediately replaced U201 and again, it fixed the problem.This must have been a common issue with these units because that is two for two.Just thought I would mentioned it in case others are thinking about getting one off the auction site or if they are having similar issues.