Hi,
I have a rather strange issue with my Fluke 81 function generator whereas, after keeping it turned off for a long period of time (e.g. months, sometimes even years), when first turning it back on, a tantalum cap develops issues (usually, it burns).
This happened 3 times so far and, up to now, each time it was a different cap (as in, in different location, on a different board and on a different power rail).
This time, it happened with a cap on the +15V rail where, after a few minutes of running fine, it would start drawing too much current and get really hot.
At that point in time, the display on the FG would turn off, but the fan would stay on.
In this instance, I managed to turn off the FG on time, before the cap had a chance to burn.
I then took the specific board where the cap was located (i.e. the current generator board) out of the FG and powered on just the +15V rail on that board from a bench power supply.
Sure enough, after about 30 seconds or so, the power supply went in constant current mode (current limit was set to 1A, I think) and C36 got really hot.
So, I removed that capacitor and replaced it with a similarly specced electrolytic cap and then installed the board back in the FG.
Now, everything seems to be working fine and the +15V rail is spot on and solid (monitoring with a scope).
Ok, so up to now, this would indicate an issue with the tantalum cap.
However, the interesting part is that, with the cap now removed from the board, I powered just the cap from my bench power supply, up to 20VDC.
And, the cap is not getting hot and seems to be fine (current reading on the power supply at 20VDC was 2mA - not sure if that is indicative of a leakage problem on the cap).
I also checked the cap with my LCR meter and it appears to be good.
So, right now, I am at a loss on what is causing this strange behavior, since the power rails seem to be good and without ripple (I understand tantalums don't like ripple).
Also, checking the larger electrolytic caps for the main power rails, they look fine from a capacitance and ESR perspective (so, again, unlikely to have ripple on the power rails).
Anyone any ideas what might be causing this and/or where to look next?
Thanks!