Follow up…
I have now checked my other HP branded E3610A supply, and it shows almost
the same oscillation phenomenon. The oscillation is measured with the range
switch in 2A and 3A position with no change in amplitude at all. Adding a 100uF
(low ESR) cap in parallel with the 2.2uF film capacitor defeats the oscillation.
I’ve checked the 470uF/50V C3 output cap, and it’s fine. Even if another 470uF
cap is added directly on C3, the oscillation amplitude is only slightly reduced.
If the same 470uF cap is placed right across the output protection diode CR3,
the oscillation is almost zero. So the pcb traces from the output pads to C3
certainly come into play here!
It should be mentioned, that the 2.2uF film cap is hooked up directly on the
binding posts. No oscillation is measured if the cap is moved to the end of two
banana test leads with the length of 1 meter each. I can live with that, but I must
say that I’m rather surprised that there is some kind of stability issue with this
power supply – or have I missed something?
Here is a screenshot of the oscillation, the frequency is approximately 170 kHz.