I recently acquired a secondhand Fender Hotrod Telecaster. It's excellent in most aspects, but is inferior to the cheaper guitar that it replaced in one important respect: the amount of hum audible when not playing.

Last weekend, I obtained some copper tape and shielded the control cavity (why don't Fender do this?!). This has made a big difference but not wholly fixed the problem. Specifically, when the volume control is at maximum or minimum, there is now hardly any hum. But at intermediate positions of the control, there is still an annoying level of hum.

Here's some more points: -

• Less hum when touching strings or any metal part of guitar than when not touching anything
  
• All the metal parts of guitar are connected to earth/ground (checked with multimeter). There's a good connection between the metal parts of guitar and the earth/ground pin of the amp's mains plug, when the guitar is hooked up to amp.
  
• No difference between the pickups, even though one is a humbucker and one is single-coil.
  
• No difference with another guitar lead
  
• No significant hum from the same amp, with my previous guitar
  
• (to repeat)No hum with guitar volume at maximum or minimum. Only hums when at intermediate volume settings.
  

I'm wondering if I have an "earth loop", but I don't really understand what causes that.

Can anyone suggest what I should check or change next?

in what environment are you trying this in and with what other electrical appliances present? with what pick ups? internal wiring check? dirty pot? ...just thinking out loud... zen'll be along soon enough...

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ohm's where the art is

The pickups are the ones that came with the guitar as new: a Seymour Duncan humbucker at neck and a hot-ish Tele single coil at the bridge. But note that it makes minimal difference which pickup I select. I therefore guess that the hum is not being induced in the pickups.

The internal wiring in the control cavity looks neat and original and I spotted no problems. I haven't taken the scratchplate off to look at the pickups themselves.

I haven't given thought as yet to whether a pot might be dirty. They seem to operate fine, though - the volume and tone vary smoothly as the pots turn, and there is no crackle.

In my studio, the last time I was playing with this (after adding the shielding), there was just my Mesa amp turned on. At a recent couple of gigs, with loads of electric gear around, the hum was worsened, but that was before I shielded the control cavity. Before that shielding, it used to hum at all volumes and to vary a lot when I moved around the room. Now, it doesn't hum at max or min volume settings, just at intermediate settings and it doesn't change much as I move around.

I just read Wikipedia on "ground loops". I now understand what they are but I wouldn't know how to tell if I have one, or what to do about it if I have.

It's not a ground loop problem. At max or min on the volume pot the overall impedance is at it's lowest. So there is the least chance of stray electrostatic pickup, whereas around the mid setting the impedance is at it's highest so you are more likely to get this kind of hum.

It rather suggests, there is an unscreened area at the back of the guitar, so it is being picked up from proximity to you. When you touch the strings you earth yourself so now actually become an additional bit of screening!

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It wasn't me!
(Well, actually, it probably was)

spinning on the spot - revolve, move about physically, yourself (and guitar) to find a sweet (quiet) spot where it's cool to record. switch off all fluorescent lighting, electrical appliances - computer monitor? etc. bare minimum on. bare minimum cables connected. etc. remove all unnecessary connections - direct to recording device maybe. any difference? good luck.

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ohm's where the art is

Hi Pete, welcome to the SOS forums.

A couple of thoughts come to mind. When the volume pot is turned part way a percentage of the signal is fed to ground so if there is a ground loop that is where any noise would be strongest.

On most guitars the pots are grounded by linking the metal cases. However, the tele control plate is metal and with most types of pot that forms the ground link. You can test this on your own tele by using your meter to measure for continuity between the control plate and pot bodies. If you have continuity then there should not be a ground link wire between the pot bodies and the pick-up selector switch. If you have a link wire as well, then that could be your ground loop.

Secondly, while a multimeter is a useful tool you can't always rely on it when testing for continuity on guitars. Guitar signals are usually less than a volt but the meter has 9V of power. That means that a poor connection might have enough resistance to block the guitar level signal but will indicate fine with the meter.

One connection that is vulnerable is the ground link from the bridge, on your tele it is just the bare end of a wire trapped between the bridge plate and the wood of the body. The connection is only made by the pressure of the bridge plate against the wood, and because the wood is softer than the metal it compresses and reduces the quality of the connection. So, I would also consider loosening the strings, removing the screws holding the bridge in place, relocating the bare ground wire to a fresh area of wood and then replacing the bridge nice and tight to make a fresh, good, connection.

A picture of the controls and wiring might help us spot anything else obvious if those don't help.

Andy

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When the going gets weird, the Weird turn Pro.

Use a croc-croc lead to temporarily link bridge to earth, jack body say.
This will prove if the bridge is truly bonded or whether you are in fact adding extra screening when you touch the strings as Will said.

Since it is "1/2 pot", electrostatic pickup you sure as eggs have something,somewhere not bonded.

Dave.

I did as suggested, connecting one end of a croc lead to the bridge (and then various other metal bits of guitar) and the other to various earthed/grounded things (jack body, jack socket surround, various bits of amp that I know are earthed). It made no difference.

The only thing that makes a difference is if I earth myself via any earthed surface, while wearing the guitar; this greatly reduces the hum experienced at middle-volume on the guitar. Touching metal bits of the guitar works. Touching metal bits of the amp works just as well. It doesn't work if I am not wearing the guitar (e.g. if guitar is on a stand and I reach over to touch it).

I wondered if putting an earthed sheet of metal foil behind the guitar (simulating me) would work. It doesn't.

Unfortunately, I no longer have the guitar that I previously used, to compare to the current one, to check it really was near-hum-free, like I remember. I have just tried a spare Squier Affinity (i.e. very cheap) Strat that a relative has leant me as a backup guitar. It hums even more than the Hotrod Tele, so that does suggest that it's in fact not specific to the Tele.

But the fact is that I had no hum problems with the previous guitar and there is no hum at max volume setting (on the guitar) with the new Tele. So "no hum" is clearly possible.

Perhaps it's some other element of the set-up. I've tried other leads, which made no difference. I don't have access to another amp to try. I can try some more guitars, on next Band Night.

Oh, here's the picture of the control wiring that was requested .

The thin white wire is added by me yesterday. It goes to some new shielding of the neck pickup cavity. Didn't make any difference to the mid-volume hum, which I think it some other problem entirely. But didn't make anything worse either.

Quote peteplayspop:

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The only thing that makes a difference is if I earth myself via any earthed surface, while wearing the guitar; this greatly reduces the hum experienced at middle-volume on the guitar. Touching metal bits of the guitar works. Touching metal bits of the amp works just as well. It doesn't work if I am not wearing the guitar (e.g. if guitar is on a stand and I reach over to touch it).

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pete,

i seriously wouldn't do this again. if you have a floating ground anywhere, this could complete the circuit and you'll be conducting current through your arm. especially don't use your left arm - it's the heart side.

there are many more qualified bench-tech types here than i (i'm an equations guy when it comes to electricity, though i do solder on occasion ;-) who can chime in on this. that said, i've had my share of truly bizarre grounds loops in home studios in older homes w/ dodgy wiring, and will never forget the time i touched the metal window screen while also touching my aluminum 19" gear rack. after the shock wore off (literally), i got out my voltmeter and measured a potential difference of over 20 volts (IIRC, it was fifteen years ago).

floating grounds have killed more guitarists than anything other than heroin and crazy fans/groupies. look up the history o/t pretenders and also renaissance (the band, not the historical era) for prominent examples. remember that there's a whole chapter in the "Big Book of Famous Last Words" titled "I Wonder What Happens if I Touch This" ;-)

cheers,

d

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"Patsy had the drug tolerance of Keith Richards and the moral rectitude of Brian Jones." - Dr. Walter Bishop, "Fringe"

Actually, I am not suicidal.

When I noticed the hum a few weeks ago, it was one of my first thoughts that something wasn't earthed and that it might be dangerous. So I went around with my multimeter. I have confirmed that there is a connection (tiny fraction of an ohm) between various parts of the plugged-in guitar and the earthed bits of the amp, including the earth pin of its mains plug and the metal controls on its panel.

I then wondered if the less than brilliant electrician who did the studio electrics had failed to hook up the earth in the whole room, so I checked connection between earth in that circuit and earth in another circuit in the house.

Unless the whole house has a floating earth connection (!), I think it's OK...

... or did I miss something? I'd sure want to know if that wasn't a sufficient check.

re: houses' grounding. it can happen that either the whole house isn't grounded or that parts aren't. one thing that builders have done in past year is to use the plumbing to connect the electrical system to earth. as the house settles, the connecting wires can pull loose. likewise, if a part of the plumbing is replaced w/ PVC pipes, truly bizarre problems can result (like losing ground when a toilet is flushed as the water that was conducting 'around' the plastic bit is displaced by air).

good to hear you're not suicidal ;-)

cheers,

d

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"Patsy had the drug tolerance of Keith Richards and the moral rectitude of Brian Jones." - Dr. Walter Bishop, "Fringe"

OK, thanks for the pic.

I've enlarged it and had a close look. It's difficult to be certain because not all of the connections are clearly visible, but that switch doesn't look right to me. I can trace the hot and ground wires from the bridge pick-up just fine, but I'm struggling with the neck pick-up.

What I would do would be to desolder all of the connections, clean the pot and switch contacts with a solder pump/solder wick, clean them with some wet & dry paper, trim and re-tin all the leads, and then rewire according to this schematic, courtesy of Seymour Duncan, as it is the correct wiring for the guitar and components. Just add your extra screen from the neck pick-up cavity to the same place as the bridge ground wire. The way your tone cap is wired is electrically identical to the way it is shown on the schematic, so no need to worry and no need to change it.

And if it's any consolation, I've often found that with wiring problems it's often a lot quicker and easier to rewire from scratch like that than to try to track down the problem. Make a fresh start and assume that the existing wiring is wrong

Andy

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When the going gets weird, the Weird turn Pro.

You might not have a death wish but you might, in the absence of another guitar with which to compare, have higher than realistic expectations? I have done some tests on sons’ Squier Tele.
Resistances at jack plug:
V pot min 14R
Vpot max switch “down” 6.3k, mid 3,3k, up 6.55k
Vpot ½ 133k 111k 112k
I then ran the guitar into the high Z input of my ZED10 HP filter out (always in) . Guitar on stand 1.2mtrs from an HT-20 on “standby”. Set levels such that stroking the strings gave about -22rms in Sam8se (24 bits, 44.1kHz mono). Noise levels..
Pot min -75dBFS, pot max, -65, -73, -68 (sw down, mid, up)
Pot ½ way, worse case switch down -59dBFS. Picking up the guitar and touching the bridge improved things by about 5-6dB.
A Rightmark plot of worsecase showed 50Hz at -60dB 100Hz, -96* 150Hz, -74, 350Hz -80. Note it is odd harmonics that prevail, typical of traff radiation I belive?
*NB. This is not calibrated. Nothing in my system is THAT good! E.g. Baseline noise is given as -114dB!
Dave.

Post deleted by James Perrett

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Hindi songs

Quote umar78:

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while a multimeter is a useful tool you can't always rely on it when testing for continuity on guitars. Guitar signals are usually less than a volt but the meter has 9V of power.

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Not quite sure of the point you are trying to make there?

My Fluke puts out 112mV on resistance test and 3.6V on diode test. We are checking earth bonds on guitar screening not lightning bonding on Zeplins, a few tens of Ohms in the earth path will matter little.
And volts is,er, volts! Power is watts.

Dave.

Just a thought - I wasn't clear whether you had shielded both pickup cavities as well as the control cavity? Might make a difference. I shielded my Mex standard Tele following the instructions here: http://www.guitarnuts.com/wiring/shielding/tele.php and it certainly cut down hugely on hum.

Peter

Thanks for the thought.

Initially, I just did the control cavity and that did significantly reduce hum, particularly at max volume.

Since then, I have also shielded both pickup cavities (the neck one with the enclosed humbucking pickup was probably pointless, now I think about it) and the back of the scratchguard. This didn't make any further difference.

The little "tunnel" through which the wires run to the jack socket is about the only bit that now isn't shielded and the wires to this don't have an integral screen either. I wonder if I should replace with shielded wire?

Quote ef37a:

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Quote umar78:

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while a multimeter is a useful tool you can't always rely on it when testing for continuity on guitars. Guitar signals are usually less than a volt but the meter has 9V of power.

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Not quite sure of the point you are trying to make there?

My Fluke puts out 112mV on resistance test and 3.6V on diode test. We are checking earth bonds on guitar screening not lightning bonding on Zeplins, a few tens of Ohms in the earth path will matter little.
And volts is,er, volts! Power is watts.

Dave.

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Fair comment Dave. I'm a guitar guy not an electronics one, which is why I look to yourself and Will for advice and info. And guilty as charged, sloppy terminology, consider my wrist slapped.

The point I was making? Many times I've had a guitar on the bench with a screening/grounding problem. Glaringly obvious when plugged in, but no obvious cause. All the right components, all correctly wired, checked and double checked, and the multimeter showing good continuity. I would patiently trace the wiring and draw it out as a schematic, then trace the connections with the meter. All with no joy. Then as a last resort I would desolder all the connections, remove the old solder with solder mop and solder sucker, clean the solder tags/pot bodies with a file/wet & dry to get back to good metal, and rewire with new hook-up wire. And problem solved. Identical wiring, same components, tests exactly the same with the meter, but problem gone.

And it was the same whether I used my old analogue meter, the shop's analogue meter, my digital meter or the shop's digital meter. However, all were/are budget meters not a decent tool like your Fluke.

So, while I may well be wrong in my reasoning about how it happens, I have regularly experienced guitar screening/grounding problems where the meter indicates continuity but is solved by reworking the connections. I'd love to understand exactly what is going on, maybe I should invest in a Fluke one day and see if it really is an issue with the budget meters hobbyists use or something else entirely.

Andy

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When the going gets weird, the Weird turn Pro.

No worries Andy!

I doubt that my now quite old, Fluke 83 is any better than a budget jobbie at conn' testing.

The task of fault tracing/fixing is NOT an exact science..It SHOULD be! In theory we should be able to look at a set of voltage or resistance readings and say "There's the problem". Often it does not work that way, sometimes you just have to change components, active or passive on "specc" and you never know what the exact cause of the problem was.
(Sometimes tho' you can really dumbfound a customer..."Just turned the telly on and it has no sound or picture". "Is there a faint glow from the screen and a slight hum from the speaker?" "Yes". "You have just returned from holiday sir?" "How the ****did you know that?!"
Took for EVER to find THAT one! Rechargable CS battery keeps the sound and brightness RC levels backed up, till the battery dies a bit and cannot hold charge over 1-2weeks!

Funny old gave this tricity!

Dave.

Quote ef37a:

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Funny old gave this tricity!

Dave.

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So true, but worth it for all the mayhem it enables

And the more you spend time studying it, the more you understand. Last week I was looking at some old FX schematics for the first time in a couple of years and suddenly realised that I actually understood what most of the components were doing!! It was a real buzz (if you pardon the pun) to spot voltage dividers and bias resistors and realise that I knew what they were.

Andy

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When the going gets weird, the Weird turn Pro.

Quote zenguitar:

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Quote ef37a:

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Funny old gave this tricity!

Dave.

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So true, but worth it for all the mayhem it enables

And the more you spend time studying it, the more you understand. Last week I was looking at some old FX schematics for the first time in a couple of years and suddenly realised that I actually understood what most of the components were doing!! It was a real buzz (if you pardon the pun) to spot voltage dividers and bias resistors and realise that I knew what they were.

Andy

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For those of us who's minds just don't work that way, please, please pass it on to the next generation


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Strictly project and just for fun

Sadly Frisonic I fear I and a few, very few enthusiasts are the last of a line.

Whilst the young people of today have phenominal skill at the OPERATION the latest technology(I, on the other hand am the opposite. I shall never text, twitter or use Facebook!) they have no interest it seems in learning the basics of electrical theory and making simple devices.

I can only assume that schools do not teach any electronics, and why should they? They have enough pressure on them with bloody league tables not to mention grave financial problems. H&S overkill also has a lot to answer for as well I think. I understand chemistry lessons are pretty emasculated compared to my day?

Lasty. when I was told I had the job at the amp firm after a month of their interviews (and the ad' ran for 2 months before that)they said I was the only suitable applicant, nobody else came close. Now I NOW I am not nearly that clever and I did not tell porkies so the only reason must be that the other applicants were very, very poor indeed!

Dave.

Trouble is it's worse that that Dave. There is almost no incentive to get involved in making or repairing stuff.

When I was a teenager I built myself a regulated bench PSU entirely with discrete components. The regulation wasn't fantastic, somewhere about 1% I'd say, but I was very proud of the overload trip I'd designed, and the fact I had decade switches for the O/P voltage. It only delivered a couple of amps, was the size of a house brick - and just as heavy.

At that time, it wasn't cheap but was a very real economic prospect, the nearest commercial units would have left me without food or beer for months! Fast-forward to now and the situation has totally reversed! OK, overall electronics is quite cheap these days, but mass produced units are far cheaper and, sad to say, usually better.

Sometimes, I think the only reason I still build stuff is out of pig-headedness.

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It wasn't me!
(Well, actually, it probably was)

Then lets raise our glasses to pigheadedness

Andy

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When the going gets weird, the Weird turn Pro.

Quite Will,Zen.

But there is a serious side here IMHO. We cannot go on consuming 3.4 planets or whatever the figure is. We need to return to a world in which things are maintained and fixed. I have had two instances in the last year or so where in one case a £106 lump of traff steel and copper had to be binned for the want of a £1.00 top whack PCB. The second was a Dell PSU for want of a plug!

We must make things so that they can be repaired and manufacturers forced to supply spares to do so.

Is there someone I can report these companies to? Name and shame as it were?

Very true Dave.

So much goes to landfill for the sake of a board/component. And it's all part of a world where things are hopelessly entwined. We wouldn't have the affordable computers and networks we are using for this discussion if it wasn't for surface mount tech and robot part pickers. But when a part fails, in most cases there's no alternative but to replace the board.

OK, I can see how we got here, but I don't like it either. I have a handful of old watches, 3 70's Tissot's and a 30's cocktail watch, all mechanical and spares are still available to keep them running accurately. And every 10-20 years all they need is a service from a watchmaker.

I hate this culture that encourages people to spend £100's on a new guitar when all they need to do is spend £10's on servicing and maintaining their perfectly good old guitar. And that's why I enjoyed helping a friend make a replacement sash window on saturday, the rotted frame we replaced was over 60 years old and the one we made was constructed in exactly the same way and will last another 60 years. That was SO satisfying.

When I was growing up every working man had a toolbox. He could hang a door, service the car, build a brick wall, lay a path, fix a leaking tap... the list is endless. I'm just glad I learned to do the same.

Andy

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When the going gets weird, the Weird turn Pro.