Keithley 2000 transformer humming

[kloetpatra]

Hi, I recently bought a Keithley 2000. When powered I noticed that the transformer is humming very loud. I wanted to know if I can reduce the humming... maybe be adding some rubber buffer between the transformer and the case?

Or do you even think replacing the transformer by a switching power supply can be done without adding too much noise?

[Kleinstein]

Some noise from the transformer is normal, but usually not very much. The first thing would be to check the mains voltage setting. If set for to low a voltage, it can hum loud but also burn after some time.

Also a partially broken rectifier (one diode open) could cause extra hum. Another cause might be excess filter capacitance (e.g someone changing the filter caps to larger values). 

Changing to a switched mode supply is not a good idea. It will add extra noise, unless you get a really special low noise one. Especially common mode signals are a really pain to get rid of. 

Adding rubber buffers ans sometimes extra glue could help against to much hum.

[Andreas]

And you are shure that there is no dimmer active in the room?

With best regards

Andreas

[kloetpatra]

Yes the humming comes definitely from the transformer. I took everything apart and removed it from the case. Then the humming is gone. But as soon as it's screwed to the case, even with some rubber buffer (picture 1), it still humms as loud as a dimmer would do. Maybe I need to add some rubber washers too. The rectifiers and caps seem to be ok.

Now I noticed that the VFD is making more annoying crackling noise. It sounds like a constant arcing high voltage. I can't identify if it comes from the display or from those VFD driver ICs (picture 2).

[Rbastler]

Check the main filter cap after the rectifier. I bought a HP3478A that hummed more than it should (I had two at the time). After replacing the filter cap of the main power rail, the hum was much lower in amplitude.
It might be just a bad cap in your Keithley 2000 too. Just from looking at the cap, the failure isn't always obvious. In my case it kinda was, because the cap had a old style package.