Film capacitor buzzing/humming?

[motocoder]

I finally finished restoring an HP 6115A Power Supply (pics coming soon). It's pretty much fully restored with the exception of a 60 Hz buzzing/humming sound. I initially thought this was coming from the transformer, but after some probing around, it seems to be coming from a 1uF film capacitor that is attached across the secondary. I say this because I can place a plastic probe on the cap and can feel the vibrations. I don't feel anything like that when probing around the transformer.

Is that feasible? I didn't know polyester film caps could buzz like that.

[amyk]

I'd say it's normal - the electric field across the plates is enough to make them vibrate slightly and emit sound.

Incidentally I just recently saw this video where buzzing filter capacitors were mentioned.

[motocoder]

I'd say it's normal - the electric field across the plates is enough to make them vibrate slightly and emit sound.

Incidentally I just recently saw this video where buzzing filter capacitors were mentioned.

I read an app note from Nichicon that mentioned buzzing was possible with their film caps. Any recommendations for a type of cap that does not buzz? The cap is connected to 117Vrms A/C, and I guess its purpose is noise suppression.

[SeanB]

The film is piezeoelectric, and will make noise. Just indicates the film is experiencing AC voltage across it. If it is a worry to you just replace it. Use a 400V or a 630V one, physically bigger with a film that is thicker and thus less voltage stress, and with that less likely to buzz

[motocoder]

The film is piezeoelectric, and will make noise. Just indicates the film is experiencing AC voltage across it. If it is a worry to you just replace it. Use a 400V or a 630V one, physically bigger with a film that is thicker and thus less voltage stress, and with that less likely to buzz

Ok. Thanks, Sean

[dom0]

The film isn't piezoelectric. Piezoelectricity is what makes the noise in ceramic capacitors, while noise originating in film and electrolytic caps is due to electrostriction.

Film caps with higher voltage ratings will have thicker insulating films which will need a stronger force to make noise, thus they will be quieter c.p. than a cap with lower voltage rating.

[bitwelder]

Should these buzzing film capacitors left 'flapping in the breeze' or is it recommended to hold them down with celastic, etc. ?

[dom0]

That might muffle the noise a bit, but you'll have a hard time quenching off the noise itself unless you put a screw clamp on them or something like that.

Generally speaking these effects aren't harmful to the components if you don't exceed their ratings and then some. They're just annoying. And, in case of piezoelectricism, can inject unwanted noise into your signal path (microphony).

[Kleinstein]

The buzzing could also be a sign of damage, or just a resonance by coincidence. Normally film caps don't give audible noise. So it may be worth changing the cap.

Even if it is not directly across line voltage, a X rated (that is not catching fire in case of damage) capacitor is a good idea.

[motocoder]

The buzzing could also be a sign of damage, or just a resonance by coincidence. Normally film caps don't give audible noise. So it may be worth changing the cap.

Even if it is not directly across line voltage, a X rated (that is not catching fire in case of damage) capacitor is a good idea.

I believe you are correct. I swapped it out for a Nichicon film cap with the same voltage rating, and it is completely silent.

[motocoder]

Here is a before and after pic of the restored power supply. It's not one of those robrenz "just like new" restorations, but it looks quite a bit better and it's fully functional.

[robrenz]

Nice job on the restoration. How did you fix the messed up print on the front panel so well? Many may be interested.

[SeanB]

It looks like they were engraved and filled.

[robrenz]

Those nice new jacks are screaming "not original" I think you should go back to the original HP jacks and use the spare ground jack you have from your 8114A parts unit.

[motocoder]

Nice job on the restoration. How did you fix the messed up print on the front panel so well? Many may be interested.

I used the approach you showed in your video on the micro-ohm meter restoration:

https://youtu.be/tA0cDDgOtds t

For those who aren't familiar with this, here is what I did:

• I removed the front panel, popped off the HP logo, and then used the scanner on my Inkjet all-in-one printer to scan in a picture of the panel. You can set the scanner to black-and-white, and adjust the "threshold" value to get a nice usable scan:
  
  
• In Photoshop I created an image of a new panel (doesn't have to be Photoshop; Paint.net would have worked fine here). I put the scan on separate layer, and then used that to align the text and shapes on other layers. I included shapes in my image for all the cutouts in the panel, but I only used this when proofing things to help with the alignment. I printed out a number of copies of this and compared it to the front panel, tweaking things as necessary.
  
  
• I printed out the image on a sheet of Inkjet Self-Adhesive Glossy Photo Film. You can actually fit 3 copies of this image on one sheet, so I did that to minimize wasted sheets in case I messed up. You may need to re-adjust the alignments here. In my case this was not necessary.
  http://www.papilio.com/inkjet%20photo%20high%20glossy%20film%20media.html
  
• I taped the vinyl sheet down to a table using some blue painters tape. Then I applied a Papilio Polyfocus TM Matte Over Laminate Adhesive Sheet over the top of the Glossy Photo sheet, using the instructions that came in the box. The laminate protects the inkjet ink and should minimize fading due to UV exposure. I used a plastic ruler to press the laminate sheet down as I removed the backing sheet underneath (read the instructions in the box). The laminate sheets are bigger than the glossy photo sheets, so it's very easy to line things up, and the painters tape keeps everything in place. Don't worry if the laminate doesn't look perfect when applied; it actually clears up in about 24 hours.
  http://www.papilio.com/other%20poly%20focus%20matte%20over%20laminate.html
  
• I aligned and taped down the faceplate on top of the sticker+laminate. I used blue painters tape, and just taped down two corners. I transfered the whole thing to a sheet of cardboard so I wouldn't cut up my table. I then took an exacto knife and cut out the holes and then the sticker itself, using the faceplate as a template.
  
• I removed the painters tape, cleaned and lightly sanded the faceplate with some fine grit sandpaper, and then applied the sticker. I found the easiest way to do this was to use a couple of pieces of painters tape attached to the top of the sticker as handles to let me align and lower the sticker onto the faceplate. I was very careful, but even after some smoothing I had one bubble in the center of the sticker. However, this bubble vanished eventually.
  

(Note: I've attached my Photoshop file for anyone who is restoring a similar supply).

[motocoder]

Those nice new jacks are screaming "not original" I think you should go back to the original HP jacks and use the spare ground jack you have from your 8114A parts unit.

I know, and replacing those was the most labor-intensive part of the project outside of the faceplate. However, those are the jacks from my parts unit. I've asked one of the forum members here who had some spares if he has any, but I'm not sure he does. Let me know if you know another source.

[motocoder]

I just went back and added a bunch of detail about how I made the sticker. Scroll back up if you're interested. I've also included the Photoshop file in case it's of use to someone.

[pigrew]

I just ordered a cheap-ish 6115A from eBay, also with the buzzing capacitor. The unit looks nearly identical to your's. Now to order a capacitor..... I guess it should be X-rated?