Hints and Kinks - Finding Audio HUM induced by magnetic coupling

[K1JJ]

I was listening to Rich/ K1ETP tonight as he struggled to find some 60hz hum on his ricebox's transmit audio. Most guys could hear it. Rich switched power supplies, turned off his audio, etc. Nothing fixed it.  Then The Tron told him to lift the ricebox off his receiver. The hum went away. It was caused by magnetic coupling from a transformer in the receiver.

About a year ago, Jay/ N3WWL struggled with 60hz hum too. I think the thread was on this BB. After lots of work he found that his scope sitting near the audio gear was causing the problem. Moving it fixed the hum.

Last week it was repeorted I had 60hz hum on my audio. After hours of troubleshooting the audio gear, it turned out my scope was causing the problem too. I relocated it to the other side of the bench and the hum was gone.

Bottom line:  Take the easy way out first by eyeballing your audio gear's proximity to stuff with AC transformers. Even a switching supply can do it - my scope had one.  Try the obvious first to save time.

Tom, K1JJ

[ke7trp]

Good tip.  I have been through this myself. The Scope was coupling to the Valiant or Champion.  I could see the hum on the scope.  Moving the scope away, Solved the problem.  I also had RFI issues that would Crop up.

A Friend came over and said, "I think you should relocate all this gear in order from RX to tx and then out the building". 

The order was Rxer, TX,  Scope, amp, tuner and out the building.  This way, As you produce RF, and it gets amped,  It goes in order then out the building.  Having amps in the middle, Scope back by the RXer or watt meter on or near RXer or audio rack, Always seemed to give trouble. I have been much happier since I tore the station down and moved things around like this.

C

[K1JJ]

"I think you should relocate all this gear in order from RX to tx and then out the building". 


Good idea. I've never thought of it that way. It pays to think about the overall plan placement. I think most of us add on and evolve our stations more haphazardly....

Looking around here, I see I didn't follow that plan.  Dr. Love and Fabio are on the wrong side of the room, but there's enuff room for isolation.  If I did it again, I'd follow the rule.


T

[ke7trp]

Yep. all inline now.  Works much better. It took a few hours to fix it all but I really feel this is way to do it..  RF should flow right down the line and then exit.. Not ever cross back over for a meter, scope or amp.

C

[The Slab Bacon]

Tom,
       I had a similar problem some years back, playing with my Junkston T-bolt.

I tried 2 different exciters, a DX-60, and my HB 7 watt piss weaker. it drove me friggin nuts! ! !  I put the VFO filaments on regulated filtered DC, remade ALL of the interconnect cables and just about everything else one could think of.

The transmitters themselves wouldn't do it when working by themselves on the bench, only when hooked to the T-Bolt. I was just about ready to grab the "big f'ing hammer"! ! !

Finally, I had the headphones on, listening to my self (which drives me nuts) and noticed that the hum changed with the plate current of the finals, and / or THE POSITION OF THE OPERATING CHAIR         

I felt like I had been had! ! ! It was the magnetic field from the plate transformer of the T-Bolt being picked up by the dynamic mic that I was using at the time! ! ! ! ! 


                                                        The Slab Bacon

[N3DRB The Derb]

I've got to start thinking about tearin the station down to install the BCL transmitter. Not gonna do it until it arrives here, bu some planning and MAJOR cleaning up is gonna have to happen soon.

[WD5JKO]

Bottom line:  Take the easy way out first by eyeballing your audio gear's proximity to stuff with AC transformers.

    Tom, This is good stuff for all to remember. I once fought 120 hz hum in my Central electronics 20A. This was after a QRO conversion to 6db more power. I added an open frame smoothing choke to the power supply. This choke was near two audio transformers that were encased in a aluminum enclosure. The choke magnetic fields were spraying all over the place, and picked up by the audio transformers. The aluminum cases did nothing to stop those magnetic fields. The result was hum on the AM carrier, and a very poor carrier null when on SSB. The solution was to replace the choke with a outer metal (iron or steel) casing.

    We often have AC filaments with one side grounded to the chassis. This means that 60 hz current flows through the chassis, and therefore grounding of low level audio stages is critical, and should be single point grounds. One has to do a lot of work with this if the audio gain is high. Balancing the filament supply is just a start.

Jim
WD5JKO

[K1JJ]

Good stories, all.


"The aluminum cases did nothing to stop those magnetic fields."


Yep, I think we tend to forget that just because our rigs may be enclosed very well in layers of aluminum panels, or aluminum boxes, thus shielded for RF, it's still exposed to magnetic field coupling. That's probably why the old steel cabinets or rack enclosure standards are good.  

Even my transceiver riceboxes have steel lids and steel sides for the enclosures. All of my commercial Berhinger gear uses steel panels on the outside. It's cheaper than aluminum, but I'll bet they wud have used aluminum if they could.

It takes a different mindset to look around the shack and see how things relate to each other magnetic coupling-wise.  I just realized that my big HV supply is just below the audio input driver transformer for Fabio. No hum, but it wud be the first thing I wud look at next time.

T

[W1DAN]

Fabber:

Years ago I homebrewed an audio processor and had that been on top of an audio limiter that had a power transformer....hummmm. Took a while for me to find it. I just moved the processor away from the limiter.

At WODS-FM one day the chief had hum in his hone hybrid...I wired a solenoid coil to a Shure mixer and headphones and moved the coil around the room. I found  unit near the phone hybrid had a large magnetic field. Moving the box eliminated the problem.

Magnetic hum is usually known by a pure 60hz component. Ground hum usually has many harmonics and sounds more raspy. It can also have multiples of 60hz.

73,
Dan
W1DAN

[K1JJ]

Fabber:

At WODS-FM one day the chief had hum in his hone hybrid...I wired a solenoid coil to a Shure mixer and headphones and moved the coil around the room. I found  unit near the phone hybrid had a large magnetic field. Moving the box eliminated the problem.

Magnetic hum is usually known by a pure 60hz component. Ground hum usually has many harmonics and sounds more raspy. It can also have multiples of 60hz.

73,
Dan W1DAN

Those are two great ideas, Dan.  I like the magnetic sampler - and the realization that ground hum is more complex in waveform makeup.  Good to keep in mind for the "Compleate Troubleshooter" ..

[w1vtp]

The 16.7 ms time period should be the giveaway.  All full wave PS's give a 8.3 ms time period ripple (half wave is 16.7).  If you can scope it -- that might save a lot of time in the TS mode.

Al

[w1vtp]

Fabber:

<snip>

Magnetic hum is usually known by a pure 60hz component. Ground hum usually has many harmonics and sounds more raspy. It can also have multiples of 60hz.

73,
Dan
W1DAN

Yes!

[W2WDX]

Hi All,

So this thread is old, but I have a recent experience that has a certain, "Doh! I should have known" conclusion.

I was recently restoring an old Gonset Communicator 6m "Goony Box" and after restoring the entire radio decided to tune up the receiver section. If you know the Communicator it's basically a separate receiver, a separate transmitter and a common PSU mounted inside a steel cabinet with separate compartments. A nice little arrangement with cables connecting each section together. This one is the original 12v model with the screw type tuning and the Magic Eye tube.

So I pulled the receiver out of its little steel box and put it on its side behind the radio and plugged in the cables. Turned it on and .... HUUUUMM! Now mind you this was the first time I had worked on one of these, this was the first time I had turned this one on and I was tired. So I was like, "Hurumph". I had changed all the supply electrolytics and the cathode caps, but hum. No way! If I unplugged the cable that provided power to the rcvr and sent the audio to the RF deck (that's where the audio amplifier is on this radio), the hum was non-existent. So logically the hum was coming from the receiver module, of course. So I started replacing bypass caps on the receiver, found a couple of resistors that were only slightly out of value, but still had hum. I kept going through it until I had gone through every component, every tube ... everything was as it should be. I even examined the pictures and made sure every wire and component was in the exact position it should be. Still had hum. What da heck?

I was tired & frustrated at this point, real tired. I had been working on this radio since the early morning at the shop, got to the point of doing the alignment at about 7pm. I worked on the hum problem for another 6 hours after that. I had smoked an entire pack of Marlboro's, taking the schematic outside with me many times, staring at it looking for some weird quirky circuit Gonset may be using. I was saying to myself is this because I never worked on this Gonset stuff. What da heck!

Now of course I was miffed. I couldn't sort this out. So I came to this site and did a search for hum and came across this thread. I went, "Oh no ... It can't be that easy. Is it?". I went back to the radio and realized the receiver module was sitting and inch or two from the power transformer. So I slid the receiver back into its steel cage, put one mounting screw in and turned it back on. Well ... DOH!

So there ya go.

John W2WDX

Sidenote: While the circuits are very simplistic in the Gonset stuff, I have to say, not ever having worked on any of these I am impressed with the design mechanically. Simple, elegant and efficient layout. While the performance (particularly in the receiver) is marginal, the layout and construction is brilliant and fun to work on. Cute little radios.

[ke7trp]

This same thing got me this week with the GK500 on 15 meters.  The FT101 station was causing major hum in the signal. I pulled the plug out of the wall, flipped it and plugged it back in the wall, Hum gone.

C

[K5WLF]

You'd think pro-sound gear would be well-shielded and immune to that. But we had to rearrange the sidegear rack to break up some mag coupled hum while building a recording studio a few years ago. Just proves that design engineers aren't perfect and the bean-counters usually have the final design approval 

[Rob K2CU]

Two thoughts:

mu-metal shielding added to a microphonium.

a hum-bucking microphone, like the hum-bucking pickups used on electric guitars.

[K1JJ]

heheheh - I had forgotten about this thread and the good advice given here.  Yep, it's a common problem and so easily fixed.


Also, it seems hum likes to start up when after we've been away from the radio gear for awhile...  
I find that sometimes connectors need to be turned/wiggled to make better contact due to moisture after sitting - especially if you have open windows during the humid summer.


T