BTR-800 Ground Loop

[Repeater]

Hi all, we have a Telex BTR-800 connected with a ClearCom wired system. The integrated systems work well together with no noise or hum until I ground the Telex box, with the green wire. Lifting the green ground eliminates the hum, but this is not the way to continue. Any ideas on what to look for?

Thanks, Laryn

 

[FMEng]

Check to see that the I/C switch on the Telex back panel is set to "Clearcom." If that doesn't work, let us know.

 

[Repeater]

Yes it is set to Clearcom.

 

[Stookeybrd]

Lifting the green ground eliminates the hum, but this is not the way to continue. Any ideas on what to look for?

Why not? The BTR-800 base station has a grounded IEC cable for its power connection. If the receptacle that you are plugging into is properly grounded then the BTR is properly grounded. Are you attaching the green wire to your rack rail or entire rack? Are you sure whatever you're attaching the green wire to is properly grounded?

 

[Repeater]

Why not? The BTR-800 base station has a grounded IEC cable for its power connection. If the receptacle that you are plugging into is properly grounded then the BTR is properly grounded. Are you attaching the green wire to your rack rail or entire rack? Are you sure whatever you're attaching the green wire to is properly grounded?

Because the green wire I'm lifting is the equipment grounding conductor for the base station in the AC line cord. Lifting that green wire and un-racking the base station and letting it float electrically cures the hum. Connecting the green ground <or> racking it brings the hum back.

 

[museav]

Hi all, we have a Telex BTR-800 connected with a ClearCom wired system. The integrated systems work well together with no noise or hum until I ground the Telex box, with the green wire. Lifting the green ground eliminates the hum, but this is not the way to continue. Any ideas on what to look for?

Because the green wire I'm lifting is the equipment grounding conductor for the base station in the AC line cord. Lifting that green wire and un-racking the base station and letting it float electrically cures the hum. Connecting the green ground <or> racking it brings the hum back.

There is no user accessible green wire on the BTR-800 or the associated IEC power cord so are you saying you modified the unit or power cord to defeat the safety ground?

 

[Repeater]

There is no user accessible green wire on the BTR-800 or the associated IEC power cord so are you saying you modified the unit or power cord to defeat the safety ground?

Yes, I modified the power cord, for diagnosing the problem.

 

[museav]

Yes, I modified the power cord, for diagnosing the problem.

You could have used a 'cheater plug' as a quick check without physically modifying anything, now you really need to not use the BTR-800 until you can replace the power cord.

It sounds like a ground loop issue. Can you use an extension cord to try powering both the BTR-800 and the Clear-Comm master station or power supply off the same circuit if not the same receptacle?

You might also want to check to see if Pin 1 of the wired Clear-Com lines is tied to the shell of any of the connectors on the cables or any related wall plates, if it is then break that connection.

If that does not resolve the problem you might want to consider Clear-Com: Partyline, Digital Matrix, IP and Wireless Intercoms, it is intended specifically to isolate two connected systems but apparently costs around $189.

 

[Repeater]

You could have used a 'cheater plug' as a quick check without physically modifying anything, now you really need to not use the BTR-800 until you can replace the power cord.

It sounds like a ground loop issue. Can you use an extension cord to try powering both the BTR-800 and the Clear-Comm master station or power supply off the same circuit if not the same receptacle?

You might also want to check to see if Pin 1 of the wired Clear-Com lines is tied to the shell of any of the connectors on the cables or any related wall plates, if it is then break that connection.

If that does not resolve the problem you might want to consider Clear-Com: Partyline, Digital Matrix, IP and Wireless Intercoms, it is intended specifically to isolate two connected systems but apparently costs around $189.

Brad:
Yes a cheater plug. I didn't have one handy so I yanked the third pin out of the Edison male. We have IEC cords by the dozens around here.

I tried powering both systems from the same rack-mounted strip and no change. Still hum.

As for pin 1, that may be the cause. All four wired comm channels that we have installed in-house have that pin grounded somewhere. It's not grounded by the ClearCom power supply because I checked that. I have not pursued it beyond that. We have a number of rack-mounted comm stations, and several dozen wall and floor pockets that give access for connecting comm packs. It may take several hours to check those, although pin 1 may be grounded at/near one of the rack-mounted units. I don't know why the installers would have grounded it anywhere else, or at all for that matter.

The MT-1 for Partyline appears to be only for ClearCom systems, although I have not called them to ask for sure.

 

[FMEng]

I think you found the cause. The com wiring should be floating. It could be as simple as someone stripped too much of the cable jacket and left a long shield or drain wire contacting a metal box or cover. Checking will go a bit faster with a power screwdriver.

 

[Repeater]

Alright, thanks guys! Looks like I'll get the screwdriver and ohmmeter out.

 

[museav]

The MT-1 for Partyline appears to be only for ClearCom systems, although I have not called them to ask for sure.

You are running the BTR-800 in 'Clear-Com mode' so where that connects to your wired Clear-Com system is basically the same as connecting two Clear-Com devices.

 

[Repeater]

<brain working now> But, of course! Thanks Brad.