Diagnosing ground issues...

alex1fly

alex1fly

Well-known member
How do y'all diagnose ground hum? I've been doing this for nearly 20 years and still occasionally I run into a ground hum that I can't defeat - the classic "touch the bridge and the hum goes away" hum. Just seems variable, whether or not I end up with ground hum after doing some wiring. Latest issue is a P bass, pickup and jack and bridge and volume pot and little tab in the shielded control cavity are all grounded to the back of the tone pot. I'm clearly missing something in the wiring and/or troubleshooting process. Any tips/advice?
 
GuitarDoc said:
Are you sure the bridge is grounded? Did you check it with a meter?
Click to expand...

Well the wire is under the bridge plate. I'm pretty inadequate with a meter though... probably a good skill to try and improve.

Does this check out as a good procedure to follow in this case?

HUNTING DOWN GROUND PROBLEMS:


If you’re experiencing Ground problems on your guitar, there’s an effortless way to hunt them down. If you don’t own one already, invest in a Multimeter – you can purchase a decent one for $25. Follow the following steps here:
  1. With your guitar’s electronic cavities open, turn your multimeter to the D.C. Resistance setting, about 20K.
  2. Hold one terminal on the back of the volume pot, (B on the above strat pickguard image)
  3. Use the free terminal to touch every metal piece, and pay attention to the reading of the Multimeter.
If your Multimeter reads “0.0”, you have a solid connection – there is zero resistance between the two parts. If your Multimeter reads “0.L”, you have a severed connection, and this is at least one of your problems. You’ll need to run a ground jumper to make sure the part gets appropriately grounded.

Note: Make sure you perform this on every part of the guitar, including the Bridge, Switch, and Output Jack Sleeve Tab.
https://www.fralinpickups.com/2018/11/12/understanding-guitar-grounding/
 
Also say you do find that there's a bad connection between the bridge and the back of the pot, even though there's a wire in place that should be grounding properly - what do you do? Run a new wire? Remove the old wire? Just resolder the connections?
 
You aren't necessarily doing anything wrong. Our bodies are about 80% water. We conduct electricity, and RF, quite well. You hold a guitar up to your body. We generally wear non-conductive shoes, and/or stand on a carpeted or wood, or worse yet, synthetic floor. When you touch the bridge, you bring your body to the same ground potential as the guitar. In other words, you "ground" your body.

Sometimes, it's simply unavoidable.
 
ArtieToo said:
Sometimes, it's simply unavoidable.
Click to expand...

What? Heresy :)

How do you get around it though when you have a customer bring you a noisy guitar and everything seems to be wired properly? Do you ever scrap the electronics and start fresh?
 
alex1fly said:
How do y'all diagnose ground hum? I've been doing this for nearly 20 years and still occasionally I run into a ground hum that I can't defeat - the classic "touch the bridge and the hum goes away" hum. Just seems variable, whether or not I end up with ground hum after doing some wiring. Latest issue is a P bass, pickup and jack and bridge and volume pot and little tab in the shielded control cavity are all grounded to the back of the tone pot. I'm clearly missing something in the wiring and/or troubleshooting process. Any tips/advice?
Click to expand...

Is the shielding in the electronics cavity complete (all around, not just the pickguard)? If not you need to abuse your body as a shield via grounded strings.

For that matter is that shielding grounded?

If the pickup has a base plate, ground that.

Finally, the wire from pickup to cavity needs to have a shield or be shielded, too, if you really want noise free without touching the strings.

If the noise gets better when you touch the strings your strings are grounded. Life-threatening nonsense if you ask me.
 
Last edited:
alex1fly said:
What? Heresy :)

How do you get around it though when you have a customer bring you a noisy guitar and everything seems to be wired properly? Do you ever scrap the electronics and start fresh?
Click to expand...

Sometimes, I just have to explain it to them, while showing them, with a meter, that everything is grounded properly. But, even more often than that, it does it at wherever they're playing, and not in my shop. That's even more frustrating.

Sometimes, they call me when they get home, and say they fixed it by changing to a different cable.
 
uOpt said:
Is the shielding in the electronics cavity complete (all around, not just the pickguard)? If not you need to abuse your body as a shield via grounded strings.

For that matter is that shielding grounded?

If the pickup has a base plate, ground that.

Finally, the wire from pickup to cavity needs to have a shield or be shielded, too, if you really want noise free without touching the strings.

If the noise gets better when you touch the strings your strings are grounded. Life-threatening nonsense if you ask me.
Click to expand...

The shield seems complete, it's all black in there. Standard Fender factory job. I swapped out the pickup and bridge, now there's a hum unless I touch the bridge/strings. Guess I could try a shielded wire to the cavity.
 
alex1fly said:
The shield seems complete, it's all black in there. Standard Fender factory job. I swapped out the pickup and bridge, now there's a hum unless I touch the bridge/strings. Guess I could try a shielded wire to the cavity.
Click to expand...

Well, if you swapped the bridge, it's possible that you didn't get a good ground connection to it. You've "been doing this for 20 years" and don't have or don't know how to use a multimeter?! Get one. Like was said, put it on the resistance setting (ohms, 20k), put one lead on the back of a pot, put the other lead on the new bridge or the strings. If it doesn't read continuity (0 ohms), then it is not grounded. If it already has a ground wire, make sure the solder connections are good at both ends of the wire (the pot and the claw).
 
Solder the wire to the bridge. Get a fat soldering iron if you don't have one which can provide lots of heat, scuff up the bridge, and use flux.
 
GuitarDoc said:
Well, if you swapped the bridge, it's possible that you didn't get a good ground connection to it. You've "been doing this for 20 years" and don't have or don't know how to use a multimeter?! Get one. Like was said, put it on the resistance setting (ohms, 20k), put one lead on the back of a pot, put the other lead on the new bridge or the strings. If it doesn't read continuity (0 ohms), then it is not grounded. If it already has a ground wire, make sure the solder connections are good at both ends of the wire (the pot and the claw).
Click to expand...

OK thanks - this is what I needed. I've had trouble reading help articles and applying them to my specific situation.

Reading between the bridge and the back of the pot where the bridge ground wire is soldered , jumped right to 70. This means it is not grounded, right? I then took the ground wire off the pot and took a reading between the bridge and the wire end. The reading slowly climbed up to 16. Does this mean the wire under the bridge is not making a good connection to the bridge?
 
alex1fly said:
Reading between the bridge and the back of the pot where the bridge ground wire is soldered , jumped right to 70. This means it is not grounded, right? I then took the ground wire off the pot and took a reading between the bridge and the wire end. The reading slowly climbed up to 16. Does this mean the wire under the bridge is not making a good connection to the bridge?
Click to expand...

Yeah, should be a solid zero.

I wonder why you read 70 Ohm and 16 Ohm, though. Maybe something else is screwed up.
 
uOpt said:
Yeah, should be a solid zero.

I wonder why you read 70 Ohm and 16 Ohm, though. Maybe something else is screwed up.
Click to expand...

Yeah. This is usually the point where I throw my hands up and take the instrument to somebody, lol. All I know is that I got this instrument out of storage and the buzz is crazy when I'm not touching the strings - but also has a present buzz in a recording.

What about putting both multimeter leads on the bridge? Should that give a continuity reading of zero? Because I did that (also did bridge to strings) and got a reading of 1.7.
 
alex1fly said:
...

What about putting both multimeter leads on the bridge? Should that give a continuity reading of zero? Because I did that (also did bridge to strings) and got a reading of 1.7.
Click to expand...

What does it read if you just touch both multimeter leads together? If that is not 0, check the multimeter manual for a procedure to zero the meter.
 
Bad or weak battery & the leads not making absolute contact to create continuity can be a reason you are getting the random 1.7, 70 or 16ohms reading, experienced that before myself. There is the zero error to account for always & keep a note of the value of it.

Instead of measuring the ohms if you used the buzzer/continuity/diode (?) setting on the DMM then that is easier to check for ground faults.

The one way to get rid of the hum for good would be to take a wire and connect one end to the metal outer sleeve of the instrument cable and other end to a metal plate and you stand or sit on this plate to ground your body. I suggest not doing it though, instead keep your bare foot on top of a pedal made of metal body and connect that between the amp and guitar ;) Ofcourse that wont work at a gig but should help at home.
​
 
Thanks y'all for the input. Unfortunately I can't get rid of the hum. I even moved the ground wire to the back of the volume pot and am getting a reading of zero between the bridge and the volume pot now. Bad for me, but good for the tech that'll get to make a few bucks off of my inadequacy :)
 
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