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 Post subject: How do describe "hum" ?
PostPosted: Sep Sun 07, 2014 2:36 am 
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Joined: Apr Mon 21, 2014 2:53 pm
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I recently bought a Zenith N731/7n07. It has a slight hum. Kind of reminds me of what you think about when you think of a classic guitar amp...

Anyhow is there an acceptable and unacceptable hum?


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 Post subject: Re: How do describe "hum" ?
PostPosted: Sep Sun 07, 2014 1:40 pm 
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Acceptable to who? I think many would say that anything you can hear--while doing your typical listening--is not acceptable. "typical listening" is different for everyone......

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 Post subject: Re: How do describe "hum" ?
PostPosted: Sep Sun 07, 2014 10:45 pm 
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A.C radios have hum, and can be reduced to low levels. If you cannot raise the volume enough to easily over come it, it may be time for some electrical work.

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 Post subject: Re: How do describe "hum" ?
PostPosted: Sep Sun 07, 2014 11:01 pm 
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Maybe I am hearing AC hum... It certainly does not wash out the audio. I only notice it dying quiet songs or breaks in audio, also notice if I an tuning and get to a unused frequency.

If I decide to recap, how do I choose which caps to replace? obviously there are many...


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 Post subject: Re: How do describe "hum" ?
PostPosted: Sep Sun 07, 2014 11:16 pm 
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For the short term, you could try bridging a like electrolytic capacitor across the existing to see if the AC hum is reduce. As for replacing capacitors, most people replace all of them (paper & electrolytic) and reuse the mica and/or ceramic. Tube radios are very forgiving since they operate at the relatively low frequency of 1 MHz so part values don't have to be precise. For example, when replacing a .05 uF paper cap with plastic, you can use .047 uF which is a commonly available value.

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 Post subject: Re: How do describe "hum" ?
PostPosted: Sep Sun 07, 2014 11:56 pm 
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Yackman stated that most people replace all the paper and electrolytic capacitors but didn't say why. The reason is that all paper capacitors are bad, including those in plastic cases (age does it; it doesn't matter even if they were never used). Many electrolytics are also bad or on the way out (age again, although ones that have been in constant use are more likely to be good than ones that have never been used). People replace them, even if they don't affect operation now, because they will in the future. It's just a way to keep from having to take the set apart over and over as they fail one by one. Also, some of them can cause serious damage if they fail.

Jim Mueller

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 Post subject: Re: How do describe "hum" ?
PostPosted: Sep Mon 08, 2014 1:46 am 
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The problem with radio hum is this. Its normal or not. If a customer had two radios, both the
same model, and one hummed. there would be no issue.

On the other hand, if a radio appeared on tech's bench, and the tag said hum, and it didn't.,
the tech would want to talk to the customer.

If I hear hum from a radio, I will know if I can fix it. If it is my radio I know if I want to.

Which is the reason why restoration hobbyists need to fix quite a few sets before they are
comfortable with hum abatement.


And then there are tape reorders. :D

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 Post subject: Re: How do describe "hum" ?
PostPosted: Sep Mon 08, 2014 1:53 am 
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What Steve said.
You often read statements like 'old radios always have hum.' I suppose that applies to gitar amps too but I don't find it to be necessarily true.
Anyway, 50+ year old filter caps typically want to be replaced.

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 Post subject: Re: How do describe "hum" ?
PostPosted: Sep Mon 08, 2014 2:52 am 
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TubeNewbie wrote:
is there an acceptable and unacceptable hum?

I would say that humming is acceptable if it can carry a tune... :wink:

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 Post subject: Re: How do describe "hum" ?
PostPosted: Sep Mon 08, 2014 6:15 am 
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Radios hum because they can't remember the words to the songs.
:lol:

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 Post subject: Re: How do describe "hum" ?
PostPosted: Sep Mon 08, 2014 7:05 am 
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Location: Woodinville, WA USA
TubeNewbie wrote:
If I decide to recap, how do I choose which caps to replace?
This article has a lot of information about recapping:

http://antiqueradio.org/recap.htm

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html


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 Post subject: Re: How do describe "hum" ?
PostPosted: Sep Thu 11, 2014 8:58 am 
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I would have to say just about all vintage electronics will exhibit some hum from the power supply or induction. If it can't be heard due to background noise, masking, or one's hearing loss, it can certainly be seen using an oscilloscope. Similar to cracks and pops from a needle on a record, it's just part of the tube equipment experience. Usually if the power supply is functioning correctly, you can turn the volume up to mask the hum.

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