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 Post subject: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 3:39 am 
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I've first noticed it about two weeks ago on my bedside alarm/radio - a gentle hum on the AM station but if you tune off-station, there's no hum. Checked on a few tube radio's and there it is. Not just on the one station but all of them - I thought it may be coming through the mains but it's there on a portable.

Thought it may have been a switch-mode power supply in a DVD player that I've just fitted but no - turned off the whole home theatre system and no change.

I can orientate the portable to null out the hum and/or the signal - if I null out the signal completely there is no hum. Turnings things off will change the direction where the maximum hum comes from, so it's obviously being radiated from somewhere, but there doesn't seem to be anywhere in the house where it's louder than anywhere else. It's still there outdoors. It's NOT there on a couple of communication receivers connected to a longwire, even on weak stations.

I forgot to mention this is 3pm, so it's not street lights.

Any help gladly received!

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 3:46 am 
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If you were to set up a spectrum analyzer in your home (office, whatever), you'd find an abundance of signals that are in some way related to the 60Hz powerline frequency.

Several things to try, but first: Take any radio that displays the issue and try it in a different location---first within the house, but then farther away.

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 4:13 am 
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Google "tunable hum" and there will be an ARF thread near the top.

The most common cause is a bad line filter cap. Isn't it always a bad cap?

Tony

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 4:53 am 
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Part of the antenna system is the AC power line. The power supply rectifiers are going in and out of conduction at the AC line frequency rate. The rectifier diodes are going between being back biased (off) and then into conduction. This can vary the received signal's amplitude at the line frequency, which in effect superimposes hum modulation on the signal.

Pete

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 5:07 am 
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Just curious but would a dimmer light switch cause that problem?


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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 5:25 am 
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But it's on a portable too...... :?:

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 5:26 am 
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It certainly could.

Dimmers are a known source of high-amplitude noise, and they radiate that noise from the power lines.

- Leigh

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 5:40 am 
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No dimmers in this house.

It's on multiple radios, some portable so not connected to the mains power.

3pm in the afternoon, no lights on in the house so not CFL's.

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 5:40 am 
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Tunable hum is not noise or interference.

Pete

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 7:27 am 
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OK, so I read all the history on the previous forums - but most of them seem to be confined to one receiver. I have this fault on many receivers and some not connected to the mains power.

So it's not filter caps, it's not suppressor caps across the mains, it's not antennas, it's not light dimmers or CFLs.

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 7:44 am 
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Try taking the portable and walking away from the house, see if/where it disappears. Similarly, closer to the source it should get worse.

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 8:28 am 
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What frequency is the "hum?"

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 9:49 am 
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Well, without digging out the frequency counter it sounds like 50Hz - normal power main frequency here.

Tomorrow I'm going to have a look at the clip on the incoming power box ground rod.

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 11:06 am 
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majoco wrote:
Tomorrow I'm going to have a look at the clip on the incoming power box ground rod.
That's a good plan, but did you try walking around with the radio?

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 1:53 pm 
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It is often caused by re-radiation of the RF signal by the power lines or wiring in your house. Because of their length, these wires are often much better antennas than the "real" antenna you are using. The power line receives the station and is re-radiated at a strength which is often more or less equal to the signal that enters the radio's antenna directly from the station's transmitter. In the process, this re-radiated signal gets phase modulated at the power line frequency. This mixes with (adds to and subtracts from) the direct signal and radio sees the result as a 60 (or 50) Hz AM modulated signal.

Dave


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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 4:36 pm 
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Dave Doughty wrote:
It is often caused by re-radiation of the RF signal by the power lines or wiring in your house. Because of their length, these wires are often much better antennas than the "real" antenna you are using. The power line receives the station and is re-radiated at a strength which is often more or less equal to the signal that enters the radio's antenna directly from the station's transmitter. In the process, this re-radiated signal gets phase modulated at the power line frequency. This mixes with (adds to and subtracts from) the direct signal and radio sees the result as a 60 (or 50) Hz AM modulated signal.

Dave


Modulation hum is especially problematic for home broadcast transmitters for that very reason. The SSTran units have an extensive amount of rectifier diode RF bypassing, and also DC power line decoupling.

From the SSTran manual: Hum Reduction

Many in-home AM transmitters are plagued by “tunable hum.” The kit designer felt most of these problems relate to poor power supply design and inadequate RF bypassing. Tunable hum is a loud hum on the receiver audio that’s only present when tuned into a station.

This kit includes an AC wall wart supply, and the diode rectifiers are RF bypassed to prevent incidental 120-Hz modulation of RF currents flowing through the power supply cables. The transmitter also features (jumper removable) RF chokes in the power and audio lines to eliminate stray RF antenna currents from flowing on these cables (if needed) to control tunable hum problems. I’m pleased to report that I never experienced this problem.


But, tunable hum can result due to the lack of RF bypassing on the rectiifier, or lack of an AC line bypass cap. It is caused by the rectifier decoupling the common mode RF carried on the power cord at the AC line frequency rate.

If there is no hum between stations, but there is hum on the carrier, it is most likely the AC on the power lines modulating any received carrier, which is normally called "tuneable hum."

Interference from dimmers, etc. will be heard between stations.

Pete

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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 5:06 pm 
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Shut the mains breaker off and if the hum is gone on the battery set isolate to a single circuit.

Carl


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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 5:16 pm 
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Burnt Fingers wrote:
Shut the mains breaker off and if the hum is gone on the battery set isolate to a single circuit.

Carl


:wink: I had a similar thought , except , that I'd turn on the portable , get it good and humming , and then shut off all the small breakers . I'd then turn them back on one at a time , and see which circuits gave the most hum . Those circuits , I would then verify what's being powered , and try to track it down that way . It sounds from the OP's description that there are more than one circuit causing the hum , and process of elimination should find them .


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 Post subject: Re: What causes modulation hum?
PostPosted: Nov Fri 30, 2012 5:44 pm 
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One other thought: any cathode to heater leakage in the RF or IF stages will also cause tunable hum modulation on a signal. There has to be a received signal (carrier) present... and if there is leakage the stage will act like a cathode modulator. This is also a very common problem.

Pete

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 Post subject: Re: What causes modulation hum?
PostPosted: Dec Sat 01, 2012 2:50 am 
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Whether we call it tuneable hum or modulation hum, there are obviously many conditions that can cause it which are unique to the situation. I had an RCA FM radio that got tunable hum on the FM band as well as AM. It was sensitive to certain 17J6 front-end tubes. After trying several NOS tubes, I finally found one that didn't cause hum. Using an oscillator tube with nearly 20 volts of AC applied to the filament was a bad idea.

Dave


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