Do I have DC on my AC line? - diyAudio
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Old 4th April 2009, 12:14 AM   #1
pjanda1 is offline pjanda1  United States
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Default Do I have DC on my AC line?

I've assumed for awhile that I've got some DC on my line. At least half of the power transformers that have passed over my bench or system have buzzed. I'm working on a new amp, and my latest PT buzzes pretty obnoxiously. When I touch one lead of my DMM to Line and one to Neutral with it set to "DC volts" I get 18.3 volts. That seems like a great deal of DC, right?

So, I built a DC blocker according to this schematic. I used a high current bridge and a pair of 6800uf 16V caps from my parts bin. (Of course, the thread does say to not just use parts from the bin . . .) Nothing changed (18.3VDC, buzzzzzzz). Similarly, would my variac block DC if I had it? I still measure 18.3VDC on the back end of it, but maybe autoformers don't block DC like transformers.

So, there are two possibilities. One is that my DC blocker doesn't work, the other is that I don't really have DC on my line, just buzz-y transformers.

I could maybe live with the buzz, but I'd like to figure it out. I just purchased this PT, a NOS Stancor. If it does indeed buzz this loud (I am using it well within it's current rating and for its intended application), I'll consider trying to return/exchange it. I know similar questions have been asked frequently, and I'm trying to dig through old posts. The likely search terms are so common that it is tough to find applicable info.

thanks for your help!

pj
www.wildburroaudio.com
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Old 4th April 2009, 12:25 AM   #2
Colt45 is offline Colt45  Serbia
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More than likely your multimeter just sucks.
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Old 4th April 2009, 01:00 AM   #3
kaos is offline kaos  United States
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Besides a sucky multimeter, you may have an asymmetrical load somewhere in the house (e.g. an electric blanket or space heater set on medium). These can distort the AC waveform to some degree and cause some transformers to buzz. It may also fool a meter into thinking there’s an offset, as the asymmetry integrates out to an offset in the meter.
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Old 4th April 2009, 01:47 AM   #4
pjanda1 is offline pjanda1  United States
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It is indeed a sucky multimeter. I don't think I've got anything problematic running in my house right now. I had a couple of lamp switches with dim and bright positions. In the former, they caused my chip amp transformer to hum somethin' fierce. I changed them to regular old on/off switches.

If only getting a better DMM would stop my transformers from buzzing . . .

pj
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Old 4th April 2009, 02:01 AM   #5
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Attach your DC meter to neutral (the big slot in the outlet) and earth (the round one below) and report back.

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Poinz
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Old 4th April 2009, 02:10 AM   #6
pjanda1 is offline pjanda1  United States
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Neutral to Earth gives me 0V DC.

pj
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Old 4th April 2009, 03:24 AM   #7
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Then, IMNSHO, you have no DC on your line.

Lots of transformers hum for other reasons, mostly poor winding, than DC on the line.  I've had a lot of trouble with it, to the extent that I bought a little venturi-type vacuum pump on ebay and built a chamber so I can vacuum impregnate with varnish.  It's a bit of a pain, but has cured it every time so far.

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Old 4th April 2009, 03:27 AM   #8
pjanda1 is offline pjanda1  United States
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Thanks Poinz. Vacuum impregnation sounds a little extreme right now. Any thoughts about my idea in my other (new) thread (cramming the endbells full of something)?

pj
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Old 4th April 2009, 04:05 AM   #9
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Cramming the endbells with shims won't help much. You have to put wood or plastic shims between the windings and the core. Some people put rubber grommets under the mounting tabs to isolate vibration from the chassis.
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Old 4th April 2009, 05:39 AM   #10
rdf is offline rdf  Canada
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Try putting a motor run cap in series with the primary.
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