AGILENT E3631A Repair Help with Display HELP!!

[cjpilot]

Hello All,

New the the forum just today.  I have been reading alot of the posts here and have learned a bunch.  I have a Three E3631A power supplies that have failed.  They are doing almost the exact same thing with some Small differences.  When I turn the Output ON and try to adjust the Voltage on 6V or 25V or -25V the display will NOT change However the voltage OUTPUT does go up and down (on the other it does the same BUT the display has eroneous numbers displayed, AND the Third one will change just a FEW volts on the display).  When I enter into the Limiting section the Voltage as well as the Current is adjustable just fine.  I have after studying all I can, Verified all supply/bias voltages per the Manual/Schematic (and believe me with 3 different Ground points that wasnt very easy...).  I also know it is Not the front display PCB or the Bottom PCB as I had another working unit to do some swapping around with and verified I do have a Top PCB problem?  Any help would be so very much appreciated.

[free_electron]

Then you haven't read enough on the forum. This has been posted a number of times. The a/d converter is broken. This is a common problem for these supplies.
You cannot measure in the a/d converter as it is a charge balancer. Everything happens in current domain.

The easiest way to repair is to swap the four ic's that make up the converter. There is an ad706 an ad711 , and two others. Look at the schematic of the a/d comverter. It is kade from an opamp a comparator and there is a vref generator making +10 and -10 volt with two opamps. Those are the bad boys.
Also ,look around the 4051 . The measuring signals as well as the vref is multiplexed. That multiplexr need replacing. Also measure the series resistors coming from vref. If they are the brownish colored ones with yellow lettering : i have seen those go bad...

Once that stuff is swapped you will need to calibrate the supply. Activate the self calibration the password is 003631.
You will need a voltmeter, a currentmeter and a load resistor.

[cjpilot]

THANK you so very much.  I now have 3 More of these that just went down.  I am employed at a college and there are students using these but I dont think they are damaging them? They are about 10 years old.  Is there a possibility the students are doing something to have this many go out in a matter of a week??  Again thank you for the info as you are helping me to save the school money in repair of these units and we can use the money somewhere else now. If there is anything I might tell them to keep this from happening that would be great.. Thanks again so very much for pointing me in the proper Repair direction.  I did read most all of the other posts on this model I just dont remember seeing any that were like mine where you had the voltage coming Out but it wouldnt read on the display OR a bunch of scrambled digits that wouldnt change BUT when in the Limit mode Voltage & Current would change on the display.  I am sorry if I missed it.

[cjpilot]

free_electron I have another question for you if you dont mind helping me once more.  On these E3631A power supplies that have all failed (8 total now) there seems to be the Exact same problem with them all.  R4 and R5 (30K?) resistors are open.  After replacing them I expected them to fail again due to a shorted component.  The 3 units I have replaced them in have been running perfectly fine after replacing the Resistors,  cleaning out all the dust, cleaning the EPROM IC & Socket etc, and alignment for about a week now in the Labs.  So my question finally is if you have seen just the Resistors fail and that take care of the problem OR will I probably see these fail again soon unless all four of the IC's you mentioned are replaced OR possibly the students are hooking something up Incorrectly to blow these particular parts in turn causing the +10V and -10V unavailable for the U5 IC?  Thank you again so much for all your help and pointing me in  the right direction to get these all back up and running.  If anyone else has any ideas on this please chime in... Thanks

[saturation]

Side questions folks, other threads on eevblog allude that the E3631a OEM is Array in Taiwan.  Is there anything printed on the PCB to suggest its not HP or Agilent in-house design?

See thread:

https://www.eevblog.com/forum/testgear/6-5-digit-datalogging-multimeter-opinions/msg306578/#msg306578

[free_electron]

Those 30K are the brown resistors with yellow text.. they go bad very often.

Nothing wrong with the design. It is a bad resistor.

[echen1024]

How much would you charge for one  of the broken ones...

[cjpilot]

Sorry I couldnt sell them as they belong to a College.  I have repaired all but two of them so far.  These last two have a few more problems. 

[rolycat]

Side questions folks, other threads on eevblog allude that the E3631a OEM is Array in Taiwan.  Is there anything printed on the PCB to suggest its not HP or Agilent in-house design?

No - as free_electron has said, it is definitely an HP design. Nothing on the PCBs mentions Array in any shape or form.

If you are still not convinced, here is the silkscreen from the top board:



and the bottom: