HP E3611A output problems

[OiD]

Hi,
I have a couple of HP 3611A power supplies at work, one of them seems to work okay but the other one is a nightmare to set the voltage. For example, I would try to set the output at 20V, adjust the voltage and watch the display slowly reach 20V... and continue getting higher and higher, adjust the voltage lower and watch the display slowly reach 20V... and continue getting lower... after about 5 or 6 iterations I might have the display showing 20V but that could be between 3-6minutes of time adjusting the voltage.

Any Ideas? I've popped it open and looks okay. Don't have a multi-meter handy to check anything though 

A quick look at the schematic and I don't know where to start checking. First I'll find a multi-miter i guess... 

[Whales]

Perhaps the pot is travelling past its normal end stop point.  The contact is now 'floating'.  Does it behave fine when turned to a few degrees shy of maximum?

[johm]

Got exactly the same problem on an E3616A, check the voltage ripple, it should be less than 200mV. In my case it reached up to 1.2V ripple and a constantly increasing or decreasing voltage. I tracked down the problem to a leaky cap on the constant voltage control circuit, the one on the feedback loop.

I suggest you check the reference and preregulator voltages and particularly their ripple.
Then perform a visual check / ESR of output caps and feedback caps.
Try loading the power supply and see if the problem is diminished.

[OiD]

Hi again, I opened it up and tested it again... now it's stable.... maybe somethings lose, they don't get treated very well... I'll check the regulators as well, got a bit of work to do now.

Whales, I mentioned 20V as an example, it happened at all voltages. Even at the end of the pot travel.

Thanks for the insight, I'll post back later if possible.