Shielding For A Test Point?

[GByxbee]

There is no general guitar forum for this question that I could find, so I'm placing it here. I have a Guitar/Organ that I'm wanting to install a test point in the rear cavity so I don't have to pull the pickguard off to measure the output, a very tedious job! This will allow me a test point for the organ output and a ground for all measurements to reference to. Since this test point (signal) is open-ended, should the shield be connected at both ends, or just one end? If just one end, which end? The signal out might be 300mV tops.

Thanks!

 

[Jon Snell]

Connect a wire to the ground point you wish to use, the other end to your test point.
That is it. No current is flowing so no hum is induced.

 

[GByxbee]

Connect a wire to the ground point you wish to use, the other end to your test point.
That is it. No current is flowing so no hum is induced.

Jon;

I made a mistake in my question. There will be two test points. One will have signal to it and the other will use the shield of that wire for a ground reference test point. I'm just wondering if the signal test point might act as some sort of antenna. It would act like a dangling disconnected wire, but shielded. The ground (shield) will be connected to signal ground and to the foil shielding stuck to the back side of the pick guard. Right now there is no connection between the foil shield and signal ground and never was from the factory.

 

[Jon Snell]

It shouldn't act as an aerial because if it is less than 10" long, (Quarter Wave Dipole would be good for 160MHZ)and your test gear won't even see that. Any shorter and the frequency will go even higher.
There should be no need to shield the ground wire.
The grounds on a guitar should all connect together at a single point ideally. But, as I said, there is no current flowing so no hum will be generated.

 

[GByxbee]

It shouldn't act as an aerial because if it is less than 10" long, (Quarter Wave Dipole would be good for 160MHZ)and your test gear won't even see that. Any shorter and the frequency will go even higher.
There should be no need to shield the ground wire.
The grounds on a guitar should all connect together at a single point ideally. But, as I said, there is no current flowing so no hum will be generated.

It shouldn't act as an aerial because if it is less than 10" long, (Quarter Wave Dipole would be good for 160MHZ)and your test gear won't even see that. Any shorter and the frequency will go even higher.
There should be no need to shield the ground wire.
The grounds on a guitar should all connect together at a single point ideally. But, as I said, there is no current flowing so no hum will be generated.

Jon;

I'm not shielding the ground wire. The shield will be acting as a ground wire. I'm just trying to make it neater by saving the running of a wire dedicated to the ground test point. Instead of running two separate wires for signal and ground, I can run one. So the shield of the wire will be acting as a shield, and a ground reference test point. Thanks!

 

[elpico]

Jon;

I'm not shielding the ground wire. The shield will be acting as a ground wire. I'm just trying to make it neater by saving the running of a wire dedicated to the ground test point. Instead of running two separate wires for signal and ground, I can run one. So the shield of the wire will be acting as a shield, and a ground reference test point. Thanks!

Yeah running another wire next to the shield, that's connected to the shield, won't help anything.

 

[2L man]

You must connect both ends of shield and it operate as a "reference" for signal.

 

[elpico]

You must connect both ends of shield and it operate as a "reference" for signal.

To what?

 

[2L man]

To what?

Between Signal Input and Signal Output whatever they are! Sometimes when "reference" come thru Chassis or Shielding the Cable Shield is connect only other end to avoid "ground loop".

You know all this!

 

[Peegoo]

@GByxbee

Sometimes this stuff is not so obvious, and talking about it does not get the circuit built.

Sounds like you know how to operate a soldering stick, so:

1. Assemble your board with unshielded test points and see wht the noise floor is like.

2. Add shielding to your test points and see what the noise floor is like.

Use a scope for testing, and try the amp in various locations because EMI is everywhere and levels vary widely.

And please report back with your findings because this is how we make progress!

 

[elpico]

@2L man

It's a guitar

If you connect the inner conductor of a shielded wire to the pickup's signal lead and the outer shield to ground, then run this shielded wire to some random place on the guitar, what are you going to connect the other end of the shield to, wood?

 

[2L man]

@2L man

It's a guitar

If you connect the inner conductor of a shielded wire to the pickup's signal lead and the outer shield to ground, then run this shielded wire to some random place on the guitar, what are you going to connect the other end of the shield to, wood?

There must be two measuring points for one signal! Obvious measure place would be guitar jack! Strato and Tele jack plates are screwed to wood. I have installed shielded cable to jack and soldered other shield end to potentiometer case.

 

[elpico]

We know you need to touch the meter probes to both the inner and outer conductor to get a measurement. Touching probe tips to the wire isn't "grounding" it. What he asked was "should the shield be grounded at both ends" and you seemed to be telling him yes. There's nothing to ground the other end to.

Strato and Tele jack plates are screwed to wood

So they don't fall off! Lol

Anyways, it sounds like he quickly realized the question didn't make sense and is asking a different one now, would there be a noise difference between running separate ground and signal wires out to where he can measure them vs doing the same thing with coax. (it shouldn't, but a lot easier to just try than guess)

I don't know why he doesn't want to measure it at the cable, maybe it's an active guitar, or he's concerned having the controls between the pickup and measurement point will change the measurement too much. Who knows, but this is what he's doing and he's got a question about it.

 

[dogmeat]

to be clear, whatever you do, the shield only grounds at one end. and the shield(s) all connect to a common ground plane. you can get away with using that as a test point for the ground side of the signal as all the grounds are common