60Hz and 120Hz Hum Causes

[Mike/W8BAC]

I can understand fairly easy how 60Hz hum can be injected into AM audio. An ac line coupling too close to an audio circuit. Ground loops and lack of filtering in a supply but 120Hz hum is less clear.

I assume this is introduced into audio the same way as above (maybe not ground loops) but where will I find 120Hz? Is this from power supplies using center tapped transformers? This might sound silly but I know I'm not the only one with questions on this. Thanks.

[WBear2GCR]

Rectified AC... 2x the line freq.

              _-_-bear

[N4LTA]

Full wave rectified AC is 120 Hz - 1/2 Wave is still 60 Hz - so a bias supply can inject 60 Hz.

Pat
N4LTA

[Don, W2DL]

When half-wave rectifying AC into DC only one-half of the AC sine wave is used, hence you get one DC pulse for every complete AC cycle, with the resultant possible 60 Hz hum if filtering is inadequate. However, if using a CT transformer with a full wave rectifier, for instance, each half cycle is alternately rectified by one or the other of the FW rectifiers - so both halves of the wave contribute to the DC  output. This therefore gives two DC pulses for each full AC wave cycle, or 120 pulses per cycle for 60 Hz AC. It's easier to filter this "two-pulse" DC than that from a half wave rectifier because there is no "dead" period between DC pulses when nothing is being rectified. That said, the most obvious cause of 120 Hz hum comes from inadequate filtering of the rectified DC, there is usually no other source of 120 Hz  in a rig.

73
Don, W2DL

[AB2EZ]

A half wave rectified 60Hz sine wave has frequency components at 60 Hz, 120 Hz, 180 Hz etc. The higher frequency harmonics are smaller... but still present.

Separately... modern power distribution systems may not (often do not) deliver a 60 Hz sine wave to your home. There are efficiency advantages in delivering something closer to a square wave.

As a result, the voltage delivered by the power company will produce magnetic fields that are not sine waves (e.g. when an associated current flows through a transformer... or in a line cord).

If these magnetic fields are picked up by your microphone (for example) the harmonics of the 60 Hz square-wave-like power source will produce hum in your audio at frequencies in addition to 60 Hz.

If the voltage waveform arriving at your house from the power company is symmetrical (or nearly so), it will contain only odd harmonics of 60 Hz (i.e. 180 Hz. 300 Hz, ...) If it is not symmetrical, it will also contain even harmonics (i.e. 120 Hz, 240 Hz, etc.)

Stu

[N4LTA]

We  test AC voltage delivered by the utilities fairly often.  It is rare for the harmonic distortion to be worse than 5-6 % as delivered by the utility.

In industrial environments - it is usually higher and most of it is contributed by switching power supplies and AC variable frequency drives, solid state florescent ballasts, and other solid state power devices.

In the home, computers and any switching power supply can increase harmonic distortion as well as new florescent lighting.

The new solid state switching power systems are making it very hard on the utilities. The harmonic currents do not cancel in a balanced three phase system and therefore the neutral currents become excessive and in some bad cases are higher than the phase currents.

Pat
STF

[AB2EZ]

Pat

You are much more expert than I am on this...

Here is what I am seeing today, between hot and neutral, on one of my 120 volt outlets. It appears to agree with what you are saying (some harmonic distortion... but not that much).

My recollection is that it looked much more like a square wave when I had a reason to look at it during the summer.

Best regards
Stu

[N4LTA]

Stu - Sometimes there is a lot of HD and it causes big problems. Most of it is self inflicted by the customer.

If you get an office full of computers and all of the lighting is florescent with solid state ballasts, it gets ugly fast. Much of the new HVAC equipment has solid state VFDs  running the compressors and fans.

Often the transformer neutrals and distribution panel neutrals buses overheat.

The old grid was not designed for all this new non sinusoidal load.


Pat
N4LTA

[Mike/W8BAC]

Thanks to all of you for helping me understand this. I hope Clark found some of this helpful as well as he works the gremlins out of his T-368.

Don, are you a teacher? If your not, you should be. Your well thought out reply made it as clear as glass. Thanks everyone.

Mike

[Don, W2DL]

Mike - I've been trying to teach physics to H.S. students for 52 years as of now, but sometimes I wonder.......

Thanks for the comment - it's more than I usually get from my students!

73
Don, W2DL

[KM1H]

In HS, I was more of a jock, and JD, than a ham nerd where I got my Novice in my sophmore year in 1955.

However my physics/chemistry instructor was the one who impressed me the most and made the subject damn interesting. Bless you wherever you may be Mr Samuel Seader! If it wasnt for your interest I have no idea what career path I would have taken.

Carl
KM1H

[w1vtp]

A half wave rectified 60Hz sine wave has frequency components at 60 Hz, 120 Hz, 180 Hz etc. The higher frequency harmonics are smaller... but still present.

<snip>
If these magnetic fields are picked up by your microphone (for example) the harmonics of the 60 Hz square-wave-like power source will produce hum in your audio at frequencies in addition to 60 Hz.

If the voltage waveform arriving at your house from the power company is symmetrical (or nearly so), it will contain only odd harmonics of 60 Hz (i.e. 180 Hz. 300 Hz, ...) If it is not symmetrical, it will also contain even harmonics (i.e. 120 Hz, 240 Hz, etc.)

Stu

Yup.  Decades ago, we were having some problems with some sensitive equipment in the lab so I did a distortion check on the voltage coming in.  13% distortion.  We used a line conditioner to remove HD and our problems want away

Just an observation - with capacitive coupling sometimes the higher components of a line voltage becomes accentuated thus the 60 Hz is greatly reduced in comparison with the higher frequency components

Al

[WQ9E]

If you want to see some really ugly stuff on the AC line, plug in a couple of the PTC heaters. 

I wasted some valuable time  a couple of years ago tracing down a sudden problem with a newly restored Gonset Communicator until I turned the heater off and all problems disappeared.  When that thing was on in its half power mode you could hear the "hum note" change in equipment power transformers and during a later experiment I found it caused my ICE whole house surge suppressor/filter to make some pretty ugly sounds.

Generally, the line waveform is pretty clean here although not as pretty as the sine wave from my HP-200 audio oscillator

[wb4iuy]

I'm beginning a journey on this very issue... I have a rice box (FT-901) with a low frequency hum. One of the guys on 3707 told me today that he looked at my audio and found 60hz component, not 120 hz, so I guess I'll look into the bias supply. I listen to the mic audio chain theough the monitor function on the rig and it sounds clean as a whistle...so I'll start with the bias supple and other half-wave rectified stuff.

Here's my question, what software is out there, where I can look at received audio via the sound card on the computer, to identify the frequency (60hz vs. 120 hz)?

Thanks,
Dave WB4IUY
www.WB4IUY.net

[N2DTS]

There is a nice program on the Elecraft web page, spectraview I think it is, used to set up the K2 filters.
It works well and is free...

Brett

[wb4iuy]

There is a nice program on the Elecraft web page, spectraview I think it is, used to set up the K2 filters.
It works well and is free...

Brett

Thanks Brett. The link on Elecraft's site is broken, but I did find the free version 5.17 on anoter site. I'm gonna tinker with this a bit, now.

Dave WB4IUY
www.WB4IUY.net

[wb4iuy]

I'm beginning a journey on this very issue... I have a rice box (FT-901) with a low frequency hum. One of the guys on 3707 told me today that he looked at my audio and found 60hz component, not 120 hz, so I guess I'll look into the bias supply.

I found the problem tonight in the 901 with the 60hz hum... I poked around the rect A, B, & C boards and found the -120vdc bias supply had about 30v of 60hz ripple. I recapped the board, no improvement. I experimented a little and found I could connect a 33uf @ 450 vdc cap from pin 6 of rect A bd (-120vdc bias supply) to ground, and would completely clean up. While I was at it, I replaced the 6146's and 12by7, neutralized the tx, wired up an old D10-4 mic, and I'm back on AM in studio A.

[ke7trp]

Glad you got it fixed.  I got the T368 fixed.  I too had to add alot more capacitance to overcome the hum.   I will have to look at my power at the wall.


C

[WD5JKO]

Looks like both the T-368 and the FT-901 have design issues resulting in hum on the AM carrier. Where can we capture this info for others in the future?

Jim
WD5JKO

[KM1H]

I cant believe that the 901 left the factory with that much ripple. While my experience is with Kenwoods and their well know connector corrosion and bad solder joint issues Id have to suspect that no brand is immune to a ground loop problem.

Carl
KM1H

[ke7trp]

Yeah. The three other T3's I have heard so far all have hum.  THe owners done realize it and just tell me its my Receiver. LOL.  I could see it right away on my O scope.  I dont think enough Hams are using an O scope. I think people need to at least get a monitor scope like a Yaesu, kenwood or Heathkit.  They are SIMPLE to hook up and adjust and no matter what AM radio you have,  You can have the pleasure of seeing the carrier while tuning.  I just cant imagine running an AM radio with no Scope.

On the Yaesu,  I bet the Hum never appeared on SSB.  It was an SSB radio and it probably got by.  I think its important to go to Foxtango website and POST this in the Transiever forum.  This way, Other 901 owners will find it when they do a search or browse the forum. 

I am going to write a simple T3 survival guide.  Just like the Globe king 500 article that I posted.  It will contain exactly what I had to do and what I had to change to get the rig up and running at full power.  There is some good T3 info out there but not a ONE article that contains what I guy needs to do EXACTLY to set one up.

C

[wb4iuy]

On the Yaesu,  I bet the Hum never appeared on SSB.  It was an SSB radio and it probably got by.  I think its important to go to Foxtango website and POST this in the Transiever forum.  This way, Other 901 owners will find it when they do a search or browse the forum. 

I'll do that... I took pics of the scope during the troubleshooting process. I'd never operated the 901 in AM, but simply replacing the OEM cap did no good. On the yahoogroups FT-901 reflector, Fred N8YX said:

"Two of my '901s do it. I haven't had the third one on the bench yet, as I'm in the process of restoring the rig. All of the power-supply electrolytics on the Rect A and C boards as well as the HV filter caps have been replaced in these sets."

Dunno, but looking at the bias supply on the scope surely showed a nasty ripple. No one ever commented on it while operating SSB, and I couldn't hear it on a local rx.

Anyway, I'll post the info, in case someone else runs into this problem on the 901.

Dave WB4IUY

[wb4iuy]

I cant believe that the 901 left the factory with that much ripple. While my experience is with Kenwoods and their well know connector corrosion and bad solder joint issues Id have to suspect that no brand is immune to a ground loop problem.

Carl
KM1H

Hi Carl,

Yeah, that's kinda what I was expecting. I first shorted the mic input and tx'd manually...hum. I poked around in the audio chain with the signal tracer and couldn't find hum. I finally cut the audio chain completely loose, and still had hum. I could hear it in CW, though it didn't seem as obvious. When in AM, it's wasn't very strong with the carrier cranked up for some reason, but down at the 75ma plate current level I needed to operate at for AM, it seemed to really modulate the tx.  I replace the OEM cap, but it didn't change anything. Adding 33mf of cap to the -100vdc supply took the ripple off the scope, and the TX was clean. Weird..but it works :-)

Dave WB4IUY

[ke7trp]

Good job man.  You will find that alot of guys are very picky on AM.  Sometimes its the lack of something to talk about.  I learned its like a pool of sharks.. The second one smells Blood, They all come a running..  Its good to be informed about a Real problem though.

Just watch out for the flex radio operators. They love to stare at that Band scope and they love to police the bands.  The Flex is the official Radio of the band police

Clark