Power Designs 2005 Power Supply

[wn1fju]

I've recently noticed that my Power Designs 2005 supply takes about one or two minutes to stabilize when I change the voltage (no load).  It acts almost like there is a capacitor charging/discharging.  For instance if I go from 1V to 10V, it will immediately jump to 9.998V and then slowly ramp up to 10V (all values approximate!).  If I change back to 1V, it will jump to 1.002V and then slowly go down to 1V.  Once I wait for it to stabilize, it is rock solid (I measured 14 uV drift over one hour).  My other precision power supply, the HP 6113A, hits the voltages instantaneously, so I suspect the 2005 may be faulty.  All the electrolytics were replaced a couple of years ago when I got the 2005.

If you have a 2005 (or similar Power Designs precision supply), does your unit act like mine? 

Otherwise, any ideas?

[vindoline]

I have a 2005A and it does not do this. I don't have any suggestions for you, sorry.

[saturation]

The parallel output cap can be failing; check the ripple is to spec; short the outputs to discharge the output cap and get output voltage down quicker; you can do this on the fly by checking the current limit, the switch does the shorting for you.  On my 4 units, the units drop to within 1mV nearly instantly, but takes a while to settle at 10+uV level.

[wn1fju]

It has the look and feel of a capacitor charging/discharging somewhere, but I can't put my finger on it. 

Shorting the output via the front-panel current set button, it always drops down to a few mV instantaneously (I would hope so given it is a physical short!). 

But when I release the button, it ramps up to the set voltage.  The ramp-time is 1) nearly instantaneous for 1V, 2) about 2 seconds for 9V, and 3) about 6 seconds for 19V.  And I'm considering it done when the 1 mV digit on my DMM stops moving (the DMM is set to 1 PLC, so it is updating very fast and is not the cause of the delay).  The ramp-up is certainly non-linear, as it gets to about 18.900V within a second before taking another 5 seconds to stabilize at 19.000V. 

Same basic behavior when down-ranging to a lower voltage - the greater the change, the longer the time.

As I said before, once it stabilizes, it stays put well within the drift spec.  And the noise spec seems fine (at least as far down as I can measure).  The 100 uF output cap is new.

[saturation]

It sounds like its working well then.  On the PD 2005 there are 2 small value filter caps between V+ and V- outputs and earth ground [see schematic], they could also be failing and cause those small variances.