Brief 60 Hz hum from PET 2001-8N

[retrobits]

Hi all,

Today I turned on my PET 2001-8N, and about one minute later, there was a 60 Hz hum of moderate volume that lasted about 2 seconds. About a minute later, it happened again. The PET continued working.

I popped the hood on the PET, and there was only one thing mildly suspicious. It looked like the voltage regulators on the mainboard might have a little residue around them. I wasn't sure if this was new or not, but I hadn't noticed it before, and I had given the whole PET a fairly recent isopropyl cleaning. The big filter capacitor looked OK, as did the transformer.

Any ideas? Power line (mains) hums don't sit well with me.

- Earl

 

[A4000Bear]

It's most likely to be a loose power transformer or the laminated iron strips that make up the transformer core have become loose and are vibrating against each other. They are normally stuck together by the varnish used from the moisture proofing treatment when the transformer was made. Age can sometimes degrade the varnish.

Try tightening up the bolts holding the transformer in place. There are some other suggestions here: https://forum.allaboutcircuits.com/threads/transformer-humming-vibrating.36298/

 

[KC9UDX]

Yep, I've had this same thing on other equipment. Totally sporadic and intermittent loud hum due to loose laminations.

So long as you have the proper fuse installed, there's not much worth worrying about.