Allen Bradley Control Logix PS problem

[david90]

I bought a used 1756-PA72 (control logix PS) and it makes crackling noises when I turn it on. The noise gets worst if the load increases. The noise also gets louder than usual for about 2 sec when I turn off the PS.

The noise sound like a wire rubbing against a computer fan while it's running. Initially I thought that the PS has a small fan inside, but I couldn't find any when I opened it.

What could be making this noise??? Could a bad cap makes this noise?

 

[KJ6EAD]

A. Any thermally intermittent connection. B. Yes.

 

[david90]

I have new information. I took apart the heatsink that was covering up the unit and was able to take a closer look. It looks like the sound comes from a transformer. I've seen transformers make humming but never crackling sound.

What's causing this problem??

 

[KJ6EAD]

Thermal expansion on startup, loose laminations, a shorted turn that is arcing to another turn or the core or a cracked solder joint arcing (on pcb mount transformer). You may be able to localize the problem by pushing on different areas of the transformer with an insulated plastic tool while it's running, also known as poking it with a stick. Reflow the solder or replace the transformer.

 

[david90]

It does sound like arcing. The weird thing is that when I flip off the power switch, the noise would intensify and increases in frequency for about 2 sec and then goes quiet. I wonder what's causing that to happen.

 

[KJ6EAD]

The discharge of a large capacitor through the fault.