Patch cable problem?

[Andrex88]

exactly I use PVC screws, between the rails and the modules I inserted plastic strips in order to clearly isolate each strip with two holes for the screws to pass through, theoretically the aluminum rails where the modules are fixed are mounted on the frame using strips of wood so I guess they are isolated maybe I'll try the link where I bought it where you can see the photos https://www.bax-shop.it/flight-case-...oppia-porta-19

[Tom Phillips]

exactly I use PVC screws... so I guess they are isolated...

If you want to be sure you can Ohm out each chassis to the others. However, you will need to do that with all the patch and AC plugs disconnected.

[Andrex88]

Could you please explain how should I run the test? do you disconnect the patch cables and the power supplies cables and how should I position the tester leads between one module and another to verify that there is no resistance? the problem is that the module frames are all painted and it is difficult to find a bare iron point where to place the tips.

[Tom Phillips]

Could you please explain how should I run the test? do you disconnect the patch cables and the power supplies cables...

Yes. All the patch cables and power supply cables should be disconnected before making the measurements. This is because the purpose of the test is to determine if the individual equipment chassis are electrically floating with respect to each other. If they are not floating then there will be ground loops formed via the mounting rails and the chassis.

...and how should I position the tester leads between one module and another to verify that there is no resistance? the problem is that the module frames are all painted and it is difficult to find a bare iron point where to place the tips.

I suggest the following ideas.
1) For the units that have a removable line power cord. Unplug the power cord and use the center ground pin of the chassis mounted connector as your test point. If the equipment has been built per code and has not been altered then that pin is hard wired to the chassis metal.
2) For the two units that appear to run on wall wart power, I would use a sharp tipped probes to poke thru the paint in an inconspicuous location to get make a chassis connection. I have also made connections by connecting a test lead to an X-ACTO knife blade and then probing with the blade tip.

Once you have identified usable ground points then measure the resistance between each chassis to each of the other chassis in your system. If they all measure infinity / "OL" then you will know that your isolation screws are doing their job properly.

[Andrex88]

Thanks a lot, I will try your method

[Andrex88]

i did the test you told me, all the modules are isolated from each other, they all mark OL between the ground pin from the socket to the frame of the module under test and to the other module frames, tried all. The thing that I noticed and intrigued and this I turned on all the modules of the rack then I tried to turn off the G FORCE but nothing happened but by disconnecting the 230v power cable of the G FORCE the hum decreases drastically, the GK is also off nothing changes but if I disconnect the rear 230v power cable the hum is lowered even more to normal and acceptable levels. there must be some problem with the grounding, but at this point I'm wondering shouldn't the samson be filtering the network? tomorrow I want to try powering the transformers power strip with an extension lead before the samson to see what happens.

[Helmholtz]

The thing that I noticed and intrigued and this I turned on all the modules of the rack then I tried to turn off the G FORCE but nothing happened but by disconnecting the 230v power cable of the G FORCE the hum decreases drastically, the GK is also off nothing changes but if I disconnect the rear 230v power cable the hum is lowered even more to normal and acceptable levels. there must be some problem with the grounding, but at this point I'm wondering shouldn't the samson be filtering the network? tomorrow I want to try powering the transformers power strip with an extension lead before the samson to see what happens.

What is the G FORCE? Manual, specification?

[Andrex88]

sorry the G Force T.C electronics and a multi-effect that you can also see in the photo of the rack

[Tom Phillips]

i... they all mark OL between the ground pin from the socket to the frame of the module under test and to the other module frames, tried all...

That is not as expected. Each individual unit should measure zero Ohms resistance (Or close to it) from the line socket ground pin to the frame of the module under test. It's a primary purpose of that earth ground pin to be connected directly to the frame of the unit. Therefor, something is amiss with the equipment or your description or your meter.

[Andrex88]

That is not as expected. Each individual unit should measure zero Ohms resistance (Or close to it) from the line socket ground pin to the frame of the module under test. It's a primary purpose of that earth ground pin to be connected directly to the frame of the unit. Therefor, something is amiss with the equipment or your description or your meter.

sorry, I made a description error in the previous post, from the ground pin of the power socket to the 0 ohm chassis, from the ground pin to the other chassis it marks OL

[Tom Phillips]

sorry, I made a description error in the previous post, from the ground pin of the power socket to the 0 ohm chassis, from the ground pin to the other chassis it marks OL

Understood. The chassis grounding and lack of extra ground loops through the mounting frame tests out OK. Even though the original problem has not been solved, this exercise has ruled out one potential issue. At least that is some progress.

[Andrex88]

exactly, the chassis are all isolated from each other, the thing I don't understand is that if I remove the power cables from the Gallien Krueger and TC electronics the hum is almost greatly reduced and yet the power cables all come from the samson, regardless of whether these two modules are on or off when the power cord is pulled the noise is removed, if I can explain in simple words if the noise we hear in the video was a graduated scale of 100% if we remove the cord first the GK will be removed approximately 40 % of the noise, if we remove the TC G force it will remove another 40% of the noise produced, I would say that removing 80% of the noise is not bad. Something to do with the ground of these two modules.

[Tom Phillips]

...the thing I don't understand is that if I remove the power cables from the Gallien Krueger and TC electronics the hum is almost greatly reduced and yet the power cables all come from the samson...

With high gain systems even the small resistance of the power cord can create a ground loop that causes hum. the fix can be difficult sand sometimes expensive to implement. The fix can also be unsafe for the case where people install ground lift plugs. If I come across a good article about fixing the problem I'll post a link.

[Helmholtz]

With high gain systems even the small resistance of the power cord can create a ground loop that causes hum...

Not quite true.
Whenever two class 1 (safety grounded) units are interconnected via an unbalanced signal cable a ground loop is produced.
The loop is along the two mains ground conductors and closed by the shield of the interconnect cable.
This loop acts as an antenna for magnetic fields from e.g. power transformers.
The induced noise voltage depends on the loop area and its orientation.as well as the proximity of such noise sources.
The induced voltage causes a loop current which in turn produces a voltage drop along the shield of the interconnect cable
This voltage adds to the signal and causes hum/buzz.

Ground loop noise can be reduced by separating chassis ground and signal ground (by a resistor) within class 1 equipment.
The resistor lowers the loop current and thus noise voltage.

A short ground wire directly connecting the chassis' of class 1 equipment involved can lower the loop area and thus noise.
Similarly a good contact between each chassis and the rack frame might lower the noise.

[Tom Phillips]

...A short ground wire directly connecting the chassis' involved would significantly lower loop area and thus noise.
Similarly a good contact between each chassis and the rack frame should lower the noise.

Good points you made and this last point you made in your post would be relatively easy to try.