Agilent power supply; output NOT zero! (+Video) [Solved]

[The E Engineer]

Dear all,

This is my first post but i have been here as a guest for many years. Today I need your help.

I got an Agilent E3648A, used from ebay. The first channel does not go all the way to 0.00V. The minimum I can set is 0.3x volt and this voltage remains even when the output if OFF!
When I set the voltage to 8V for example, and disable the output, the power goes down to 0.3V not as instantly as the 2nd channel goes.

Any idea what the problem is?

Thank you in advance

-Update1: The behavior is like the output capacitor remains charged even when the power supply is disconnected from the mains. So the 0.3v stays and gradually goes down.

-Update2: See it in action:



-Update3: its solved, please see my reply to this post

[Magiciaen]

I know that on the E3631A, there was a firmware update to fix this very problem.

https://www.keysight.com/main/editorial.jspx?cc=GB&lc=eng&ckey=1000004506:epsg:faq&nid=-34466.963148&id=1000004506:epsg:faq

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

I wouldn't be surprised if it's the same issue on your E3648A.

[The E Engineer]

I know that on the E3631A, there was a firmware update to fix this very problem.

https://www.keysight.com/main/editorial.jspx?cc=GB&lc=eng&ckey=1000004506:epsg:faq&nid=-34466.963148&id=1000004506:epsg:faq

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

I wouldn't be surprised if it's the same issue on your E3648A.

Thank you for the reply,

That may be true, but the second channel is exactly 0.00v when the output is off or set to zero!

I've checked my firmware, its 1.7-5.0-1.0. But I could not find any info regarding the latest available firmware and whether or not these units are up-gradable.

Do you think this behavior is normal?

Any other thoughts?

[HighVoltage]

I have a 3649A and it goes to zero right away on both channels.
Something is for sure wrong with your PSU

[The E Engineer]

Guys, I'm glad to say that i found the problem and its fixed now I found a broken track on the PCB which disconnected the output GND of the defective channel from MCR264-4 thyristor (pin #1).

Here is the schematic if anyone is interested.
http://ridl.cfd.rit.edu/products/manuals/Agilent/power%20supplies/E3646AE2.pdf

Thanks everyone!

[HighVoltage]

Good that you fixed it.
Sometimes these power supplies can be a challenge to fix.

Why was the track broken?

[Magiciaen]

Good job!!

[The E Engineer]

Good that you fixed it.
Sometimes these power supplies can be a challenge to fix.

Why was the track broken?

Yes I'm lucky I didnt need to replace any of those obsolete parts.
Not sure really how this happened, but it appears to be the art of high current. I wish I took a photo of the track to ask your opinion.

I only do low voltage/current electronics so it may not be a problem for me ever, but if I had a 5A load I would do a stress test to make sure things wont go south if I ever happen to draw more amps than I usually do.

So far everything seems to be alright.

Good job!!

Thanks

[HighVoltage]

These Agilent PSUs have a "crowbar" installed.

So, be careful, to never turn it off, when you have a battery or a good charge capacitor hooked up.
You will see magic smoke and it will burn away traces.

May be that is what happened to yours.

[The E Engineer]

Thanks for the tip. I will keep that in mind

[coromonadalix]

Strange there is no revese biaised power diode on the Q503 and Q109  irf540 mosfets

Still strange the output protection diodes cr126 and cr127 in parallel ?



Glad you repaired it