The "Dirty"
Chassis Condition
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Technical Support Information
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A “dirty chassis” condition occurs when a piece of equipment has voltage or ground noise present on its chassis and/or its signal ground reference even after balanced power has been applied. This technical bulletin outlines the various types of dirty chassis conditions and the techniques used to solve these problems.


Locating a dirty chassis

To locate a specific piece of equipment that has a dirty chassis leave all audio/video connections intact then plug in and turn on all of the equipment one piece at a time. When objectionable noise becomes audible, visible or measurable, the last unit turned on has a dirty chassis condition.


4 Types of “Dirty Chassis” Conditions:

1.  Noise is present with signal ground and chassis ground not connected

When the signal ground (pin 1 in a balanced audio system or the sleeve in an unbalanced audio or video connection) is not connected to the chassis ground, it is probably because the manufacturer has determined that the chassis power supply has made the chassis too dirty to reference to the signal ground. Often, manufacturers or technicians lift pin 1 from the chassis in this manner to avoid noise which is often the case when unbalanced power is used. It is easy to check for this condition with an ohm meter -- simply test for continuity from the signal ground to chassis. For balanced power applications there should be continuity. If there is no continuity, try jumping from the signal ground to chassis. The device should get quieter. If it doesn’t, another type of dirty chassis condition may be present that requires a different solution.

2.  Noise is present when chassis ground is not referenced to AC ground

When an equipment chassis is not referenced to the AC ground, it is often because the unit has an ungrounded 2-prong AC cord. In order to avoid grounding noise, some manufacturers avoid referencing the chassis to the AC ground altogether. The solution is to connect the equipment chassis to the AC ground with a #12 ga. or #14 ga. copper wire. Often the simplest way to ground a chassis is to strip the paint off of the inside of the rack ears and off one side of the rack rails with a grinding stone and a drill. Then, sandwich a ring terminal between the stripped rack ear and rack rail and run a copper wire from the ring terminal to the AC ground.

Occasionally, some equipment does not have continuity between the chassis and the AC ground even if it has a grounded 3-prong AC cord. It is easy to check this with an ohm meter. Simply test for continuity between the chassis and the ground prong on the AC plug. For balanced power applications there should be continuity. If there is no continuity, run a jumper wire from the chassis to the AC ground and the device should become quieter. If it does not become quieter, this is an indication that a different approach to the noise problem is needed.

3.  Noise is present with audio ground, chassis and AC ground connected

When a device has continuity from signal ground to chassis ground and AC ground but there is still voltage or noise present, there are simple procedures to follow that should correct the noise problem. If the unit has unbalanced audio signal connections the best approach is to balance and isolate the unbalanced audio with an audio isolation transformer or direct box and route balanced signal to a balanced audio input connection. An alternative approach when using unbalanced audio or video is to use an unbalanced to unbalanced isolation transformer. Both of these techniques serve to isolate the ground of a unit with a dirty chassis from the ground of the rest of the system thereby eliminating ground noise contamination.

Often, CATV lines exhibit a dirty chassis condition, the dirty chassis in this case being the CATV company's equipment. Similar to the above situation with noisy a/v equipment, this problem readily resolves with an RF isolation transformer. One can be built by soldering two balun transformers back to back and connecting the coax cables at the unbalanced ends.

4.  Noise is present with audio ground, chassis and AC ground connected and an audio isolation transformer is being used for interconnecting to the system.

When a piece of equipment has so much voltage or noise on the chassis and/or signal ground, it needs to be insulated from the rack rail with plastic insulators. In some rare cases, an AC ground lift adapter is also required to cure the condition.

There are also a few units that radiate so much EMI/RFI that other equipment will pickup noise from the stray field generated by the device with the dirty chassis, particularly if there is an unshielded patch bay or other unshielded gear nearby. These units need to removed from the rack completely or the sensitive gear that is picking up the noise needs to be moved away from the source of EMI/RFI. Fortunately, these seriously affected chassis are quite rare. Often replacement is the most logical course of action to take.
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