Agilent E3646A - odd voltage fluctuations on channel 1

[spiff72]

Hey all,

Wondering if anyone might have some troubleshooting tips for this bench PS. It was a recent Ebay acquisition that seems fine at first, but I am now seeing some strange fluctuations in the output voltage of channel 1. Channel 2 seems fine so far.

The linked video below shows a recording I took during one of these odd occurrences.  There is no load on the supply and I think I had the output voltage set at 20.00v. One thing I am trying to understand is whether the SET voltage is dropping or just the output. There also appears to be a fair bit of noise on the scope measurement I was taking, but I dont know if that is  on the output or if it is noise I am picking up with some font jumper wires.

There is a bit of additional info in the description of the video, too.

Thanks!

[spiff72]

One additional video showing it connected to a digital meter instead of the scope.  The meter is a lot slower to respond to the voltage changes, but the values just show it isn't only the display of the PSU that is showing the droop/recovery.

[TheDefpom]

The channels have a feedback loop, what you see on the display is a read back of what the output actually is, not what it is set to, so from that you know that the unit is getting the correct feedback, so therefore it must be an output control issue.

The output is fed via a mosfet, which is controlled by a couple of op-amps and also ties to the supervisor IC (at least for channel 2), which is used to shut off the output at power up to give time for things to stabilise, my initial suspicion would be the supervisor IC going leaky and starting to turn off the mosfet, you can test voltages at the mosfet feed circuitry to confirm (it is passed through a diode as is the control loop so you can see which signal path is pulling it down, the control loop or the supervisor IC).

Channel 1 has an op amp connected in a similar way to the supervisor IC of Channel 2, test each channel independently as they are slightly different, but linked in some ways.

I cover this circuitry a bit in the repair videos I did on a unit, which explains how that section works. I have a few videos on the unit so you may be interested in them.

I discuss the circuit design at 35 minutes on the Part 2 video.






Full playlist for all E3646A videos I have done:
https://www.youtube.com/playlist?list=PL2z3V9RkHQE2Twk6VoEBURx88TDXPKVSP

[spiff72]

Thanks so much for the reply and guidance!

I will check out those videos later tonight.  I reached out to the seller after seeing this, and they are offering either a partial refund or offering to accept a return.  I think I may do the partial refund at this point, since I think this is a very nice supply for the price I paid (after this partial refund).

Thanks!