My HP3456A Digital Voltmeter has developed a fault, manifesting as a loud "chirping" sound (reed relays going nuts)
It did this a few years ago, but only for short time, and then it stopped, so I left it be.
But now the fault has returned, and seems to have gone solid, so I had to open her up to investigate.
The sound was clearly coming from the reed relays on A20, the main inguard analog board.
So I first checked the supply voltages going to that board. All where OK, except the+5V which had massive ripple (like 4V!) .
Fortunately, the fault was the most likely culprit, and the first part I tested: A10C5 - the 4000uF 15V filter cap for the +5V inguard power supply.
It's the large blue cap, and it had gone completely open.
So I'll be now be looking at replacing all the power supply electrolytic capacitors in both my HP3456As, and my two HP3455As.
Video of teardown and repair:-