Agilent 6643A power supply with random output voltage when output is disabled

[NoisyBoy]

Hi community,

On my 6643A, the output is delivering a voltage of a few volts (less than 7V, it varies) when the output is disabled.  I can see this voltage goes up slowly upon power on (even with output disabled), and as I power off, this voltage would slowly drop down.  Which is consistent with a output cap charging/discharging. 

However, once I enable the output, then the output voltage and current is exactly on the spot with the programmed value, everything seems to work fine.  The trouble is with this random voltage only when the output is disabled.

For those who work with Agilent system power supplies, do you have any guidance on what to check for?

I thank you in advance for your helping hands.

[coromonadalix]

Is your sense switch  at the right position ?

Have you seen this video ?

[NoisyBoy]

Thanks for the kind reply, the sense switch is set to internal sensing.  But since the issue is there when the output is disabled, I doubt sense switch setting would make a difference.

I did see Dave's video, great idea, but I also like the simpler solution of just adding a 3D printed mounting post next to the rack mount screws. 

I have issue with the loose rear output post Dave discussed, so I will have to either resolder it, or send the whole unit back to Keysight for repair if I cannot figure out why there is a voltage with output disabled.

Odd thing is if the output is enabled, a 0V setting is indeed 0V on the output, like it should be.

[VK5RC]

Just some 'dumb' questions,
1 Do you have any idea how much impedance is behind the voltage? Could give a clue where to go looking.
I have one of the smaller units of that series, comments from my repair experience!
The schematic in the manual covers multiple units in the series - so often those components aren't there in your unit. I had to refer to the parts list and alter the schematic.
The pcb was also 'semi' populated according to the model - my units fault turned out to be a missing final filter capacitor - almost impossible to spot with so many vacancies on the pcb.
My unit could also sink quite a bit of current - controlled by a separate output ('input' transistor). There also is a crowbar circuit. SCR.
These three circuits could potentially leak some current/voltage into the final output path.
Hope these comments are helpful.

[NoisyBoy]

Hey community,

After some effort, I was able to nail down the problem with a voltage detected during output disabled mode with my 6643A, it appears one of the FET assembly has failed and leaking 0.47V on the source pin at shutdown mode (vs. 0V at all other 6 FETs).  Unfortunately, the parts is currently not in stock at Keysight, it is part number 1855-0641.  Does anyone know how to cross-reference it to a normal parts description so I can order it from Mouser or Arrow?

On the circuit diagram, I saw MTP3055.   Keysight parts described it as "Transistor-MOSFET N-channel enhancement-mode silicon TO-220AB THT".  I found some cross reference of this part to JEDEC IRF612.

I hate to order the parts and found it's the wrong parts.  Any help you can render is greatly appreciated. 

Update: Is the Vishay IRF610PBF the right part?  I just came across a component level documentation for the unit.