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Author Topic: Rotator brake causes 1.0 Vp-p AC on coax braid  (Read 1470 times)
N4EDI
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Posts: 17




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« on: November 18, 2017, 02:51:39 PM »

I have a Ham III rotator on my tower along with a Moseley TA33 Beam and a Diamond 2 meter/440 vertical antenna. When I push the brake release button to turn the beam I get a 1.0 to 1.1 Vp-p 60 hz signal on the coax braids and the negative terminal of my power supply. It also gets on the usb connecting the radio to the computer which causes loss of connection. The coaxes are all grounded at the tower and go through lightning arresters which are grounded at the tower. All this and the tower legs are grounded to an 8ft. grounding rod. The radios and tuner are connected to a separate ground rod. I have tried connecting the 2 ground rods but that did not help.
I have unhooked the coaxes and checked and the unwanted signal is coming in through the ground or the coax's braids.
Any ideas? has anyone seen anything like this before?
Thanks
Ed Varner
N4EDI
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WB4SPT
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Posts: 502




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« Reply #1 on: November 18, 2017, 05:23:00 PM »

perhaps the ground return from the rotator is open.   The rotator uses 30V ac, and the return shows grounded to the case.  So, if the current is not going down the correct return wire, it may then return via the coax shields, etc. 
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AA4PB
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« Reply #2 on: November 18, 2017, 05:30:27 PM »

I was going to say the same about the return wire (pin 1). This wire likely ends up being in parallel with the coax cables as the wire is connected to the rotor case and to the control box safety ground on the power plug. If the wire is too small of a gauge or has a poor connection then you may have excessive AC current flowing in the coax shields. Does the position indicator needle jump or flutter when you press the brake release? The position pot center arm also returns via pin 1.
 
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Bob  AA4PB
Garrisonville, VA
N4EDI
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Posts: 17




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« Reply #3 on: November 19, 2017, 02:37:11 PM »

Thanks for the replies. The needle does not jump or flutter when the brake button is pushed. Would it help if I ran a wire from the tower ground rod to the AC house ground? They are about 25 feet apart. And also ground the radios at the house ground?

Thanks
Ed
N4EDI
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AA4PB
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« Reply #4 on: November 19, 2017, 03:51:27 PM »

Actually the NEC requires that the building AC power ground system and the radio's ground system be bonded (permanently connected) together with at least #6 wire. It would probably be a good idea to extend that #6 bonding wire out to the tower ground rod as well.
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Bob  AA4PB
Garrisonville, VA
K0UA
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Posts: 1466




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« Reply #5 on: November 20, 2017, 08:21:03 AM »

Don't you all reckon that the brake solenoid is defective in that its coil had shorted to the shell of the rotator? Either that or the rotor cable is touching the tower somewhere.   All of these grounding ideas are fine, but the problem is in the rotator or the rotor cable.
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AA4PB
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« Reply #6 on: November 20, 2017, 08:52:42 AM »

I believe his rotor is designed with a common ground for one end of the brake solenoid, the directional pot arm, the housing (and thus the tower), and the electrical safety ground in the control box power plug. My guess is that the house electrical safety ground is at a slightly different potential than the tower ground and that leakage current is flowing through the ground conductor in the rotor control cable. Getting the grounds all bonded together with a low resistance cable will divert most of the leakage current through that cable so that less flows in the control cable or the coax shields. Of the parallel paths, the one with the least resistance will conduct most of the current (Ohm's law).

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Bob  AA4PB
Garrisonville, VA
AE5GT
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Posts: 78




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« Reply #7 on: November 20, 2017, 03:20:47 PM »

Don't you all reckon that the brake solenoid is defective in that its coil had shorted to the shell of the rotator? Either that or the rotor cable is touching the tower somewhere.   All of these grounding ideas are fine, but the problem is in the rotator or the rotor cable.

Yes either the insulation breakdown on the solenoid or one side of the solenoid wiring is grounded ... and probably shouldn't be.  being low voltage wiring the latter is most likely and i would check with an ohmmeter first.
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WB4SPT
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« Reply #8 on: November 21, 2017, 05:29:36 PM »

hmmm.  Have seen the diagrams for Hamm, tailtwister, CDE and all use frame ground at the rotator.  Make the pin 1 wire either intact or bigger Grin

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AE5GT
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« Reply #9 on: November 22, 2017, 08:19:11 AM »

Alliance (hd73) and Yaesu , don,t appear to ground theirs . Yaesu shows just the case and one leg of the feedback pot. It looks to me that CDE grounded theirs at both ends , I really think that they should have grounded at just one end ,but there is nothing you can do about it except live with it.
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K1ZJH
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Posts: 3327




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« Reply #10 on: November 24, 2017, 01:09:24 PM »

Does the rotator work with the coax disconnected?

Since the coax braid and rotator common lead are in parallel, you will have some voltage developing across the braid over the span of the coax.

Refer to the manual, and do the resistance checks.  That will show whether the common lead has opened or developed a high resistance due to corrosion, etc.

But, with coax disconnected, I would expect that your meter would show the amount of AC voltage being dropped in the common rotator lead since the coax is providing a metering point for you at the rotator location and thus from what I reading you could be just reading the voltage drop across the common lead rotator cable between the shack and tower.  Those rotator brakes draw a lot of current. 

I've had problems where a bad rotator cable induced tuneable hum on received signals when the braid was forced to work as the common return for a Ham M.


Pete
« Last Edit: November 24, 2017, 01:12:56 PM by K1ZJH » Logged
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