Fluke 343A Voltage Calibrator repair oscillating output

[sahko123]

Ive gotten a 343a recently and when i got it initially it was supposed to be unstable but actually ran quite steady for quite a while until recently when it started to oscillate very very badly. It at about 500-600mHz (not MHz) with swings of about 8-12vdc for a 10V setting. This oscillation carries on through most if not all ranges. I suspect something dodgy has happened to the chopper amp and im thinking of following the xdevs upgrade path shown off here (https://xdevs.com/doc/Fluke/343A/) and replacing the main amp with something more stable and less finicky.

Also i need to find a way to stop it building up as much common mode voltage (order of kv) without having to have the output ground earthed at all times. I get annoying zaps from it otherwise.

Does anybody have any advice on what exactly the issue could be for both the dick-swinging voltage and the high voltage buildup.? cause i want to get it working before modernising anything.

[TizianoHV]

Stupid question, did you checked the caps?

He ->() has repaired a few of these and bad caps were often responsible.

Try reversing the mains plug and see if you still get shocked.

[sahko123]

I tested a good few of the capacitors on the power supply and they were all good so can be put at the end of the queue for the full recap. This video pointed me in EXACTLY the right direction C42 being a 5uF 25Vdc now is a 250pF capacitor.

after replacing two caps and now the thing is rock solid.

Why would swapping mains wiring do anything? I'm on 3 pin uk plugs so the exterior is earthed I tend to get zapped by the outputs if they arent earthed

[TizianoHV]

Nice to hear!

Here in Italy mains plug aren't polarized so you can "swamp" phase and neutral. There cold be a leaky interference capacitor causing leak from mains phase to output. By swamping phases the leaky cap would see only a neutral to ground (few volts) instead of phase to ground voltage (220?).

*I asked you to reverse the plug because, at first, I thought you were italian (similar flag...) .