E3631A power supply showing 70mA with nothing connected

[Danieljohnsonk]

I have 3 of these units all displaying current values even when nothing is connected to the power supply. They all completed the calibration process successfully. Can anyone point me in the right direction for repairing this issue?

[bitseeker]

Hi Daniel,

Welcome to the forums!

These power supplies fail in all kinds of ways, some straightforward (a fried resistor, capacitor, or diode) and some not so much (multiple ICs). Some problems can look similar, but be caused by very different issues. It sounds like there may be a problem with the current readback, perhaps an issue with the ADC and/or associated op amps and/or supporting passive components. Maybe the current shunt is damaged. It's a good idea to verify that the internal power rails are outputting what they should be as well as all bias voltages.

What is this calibration process you speak of and who performed it? I'm surprised any calibration lab would give a "pass" to a power supply that doesn't have proper current display. Are you referring, instead, to the built-in self test (which only indicates that the device is mostly working, not fully functional).

[crispy_tofu]

Hello!
I would also suggest adding pictures of the internals so we can also see inside. 

[tszaboo]

I would check if any of the output capacitor (or protection circuit, diodes and such) became a short. Also it might be in the measurement, some opamp might be a goner.

[lukier]

Maybe it is related to one of the service notes:

http://literature.cdn.keysight.com/litweb/pdf/E3631A-04C.pdf

I have two E3631A's that fall into this category (firmware 1.4) and if someone has the binary blob for version 1.6 that would be great

I have another, much newer unit (Agilent branded) that has AFAIR 2.3 firmware, but I'm not sure it can be used on old PCB revisions (both MCU and front panel board).

Check also this one:

http://literature.cdn.keysight.com/litweb/pdf/E3631A-09D.pdf

[bitseeker]

Thanks, lukier. I should've mentioned to check the service notes.

[Danieljohnsonk]

Thanks for all the replies.

Lukier,
The 2nd service note regarding current draw on the +6v side with nothing connected seems to cover my issue but my serial number (MY40041866) does not fall into the range listed in the service note (MY
40023861 through MY40036600). Most likely the note applies to my power supplies too.

bitseeker,
Yea, I was surprised too that he calibrated it with the self-calibration method and gave it back as passed even though the display showed 70mA on the +6v side. If you think it's an issue in the ADC circuit I'll check it out. I found a few good tear down videos for these power supplies. Do you know of a good source of info for the ADC circuit? I have the service manual with the schematic.

[bitseeker]

I don't know of a single source of info on the operation (let me know if you find good ones). There are a number of posts by free_electron in the forums here where he describes a lot of the theory of the current+time based operation of the control circuits in many HP power supplies and multimeters. That's probably the best info I've seen so far.

The various YouTube teardown and repair videos are also good resources and recommended viewing, but as I mentioned, there are more failure situations than there are videos. So, you may get lucky and have the same problem, but you may also encounter one that doesn't fit. Nevertheless, they're good to watch to add to your knowledge base and familiarity.

Tip: When looking for info here, use the Search link in the navigation menu (it's the advanced search), rather than the search box in the header (simple and less coverage).