Joe Zeglinski

    Wise words indeed, “Biker”. Thank you for clarifying this so eloquently,  for many of us.
    Since we are both retired electrical engineers, and based on your obviously far greater professional experience on “grounding issues” ... I wonder what you think about,  how many Astro Accessories designers,  tie their electronics devices DC Logic Ground Return DIRECTLY  to Chassis Ground, which in turn connects to Earth ground via AC/DC power supplies?
    For example:
   When  I purposely isolated my CPx’s “negative” terminal – (by wrapping the power cable cigarette lighter plug’s  side terminals with a slip of paper or tape) -  I have seen sparks fly from the CP3 (and CP4) serial port DB-9 panel connector bodies, or its panel screws, when my serial cable connector even comes close to mating the DB-9 on the panel. As proof,  I have even rung-out the resistance between the DB9’s logic ground pin and that same  connector chassis screws, and they are indeed directly connected – by design !
    Obviously, such a test is not a common thing to do, but I first discovered such sparking,  quite by accident,  when the ground wire inside my AP-900 (or AP-1200) DC power cable’s  cigarette lighter plug had a broken ground return wire inside, and I needed to re-solder it. I wondered why the CP3 still ran normally with that missing and broken connection.
    In fact, even with a purposely “isolated” DC ground  return connection, the CPx’s (and many similar controllers),  run perfectly well, displaying coordinates, data, etc. because the controller’s  DC power returns to its source supply via the CPx case/chassis, the mount, our body standing on dew damp grass,  as well as any other accessory’s improperly wired  DC ground/chassis returns. I have seen the same results – controllers running perfectly well with a disconnected DC Return,  when isolating DC Return terminals on AP’s CP3 & CP4, as well as on my RCOS RC-14.5 telescope TCC controller, and Kendrick’s Premier Dew Heater controller. The only device I have not seen such an improper DC Return tied to Chassis ground, design fault,  was when I ran the same test on my SBIG STL-11000 camera. They did it right, that camera’s case (chassis) is NOT tied to their board’s Logic Ground.
    I’ve warned everyone about such improper grounding danger to telescope equipment, but I guess this isn’t an issue until someone’s very expensive, possibly now irreplaceable,  controller(s) gets fried, and then they wonder WHY. This is how an approaching storm can quite easily blow out any such “astro electronics”,   caused by the  “regional elevated earth ground potentials” entering the devices via a controller’s improperly connected DC Logic Return terminals.
    Of course, the commonly quoted and sound advice is to “completely unplug all telescope electronics”, unless the telescope is being used at the time, and even then, any distant lightening storms are still many miles away. Not sure how such advice works with Remotely operated  observatories.

From: biker123@... [ap-gto]
Sent: Thursday, October 10, 2019 12:10 AM
To: ap-gto@...
Subject: Re: [ap-gto] MACH1GTO VIBRATING

The OP said he is getting tingling when touching anything on the mount, pier, CP 4 box, etc. that is NOT normal and could be an issue. I am a retired electrical engineer that worked many years for an electric utility in California. 
I assume you are running AC to a regulated  DC power supply for everything.
First thing I would do....get a phase check device from your hardware store to confirm if your house is wired correctly. This is an inexpensive little block that has the two flat prongs and the round prong like a standard electric plug.  The device will usually have three LEDs on it that will help determine if you have the house circuit wired correctly. You would be amazed at how many houses aren't wired correctly.
A little electrical class first. A standard  plug will have two flat blades one of which is wider than the other. The smaller blade is the hot leg, the larger flat blade is the ground. The round lug  is the case ground and should not be grounded and it definitely is NOT a ground as many think. Its purpose is to connect the case of your device to other device cases so they all have the same "electrical potential". This means that if you touch two different devices you won't get zapped. Often people will find the two flat blades are reversed because the contractor wired the outlet wrong.....or someone will use an old style two blade plug that has same size blades on each side.and plug in upside down. Sometimes people have very old house wiring that only has the two same size flat blades and no round lug.....and they want to plug a modern two blade/round lug device in. They get an adapter from the hardware store that is designed for that purpose. There is a pig tail or sheet metal piece that acts as the round lug when it is screwed into the center retaining screw on a wall plate. If the adapter is plugged in backwards or the case ground is not connected you have a problem.
If you touch the metal of a a device and get a tingle that is an indicator of this issue. You are standing on the ground and the path of least resistance is through your body. When I was young my families refrigerator outlet was wired backward which meant if one of us leaned against the metal door and someone else leaned against the metal edge around our kitchen sink and you touched each other you got a mild shock. Of course being kids we delighted in zapping our family members and friends ;-) That came to an end when I became an engineer and figured it all out. Kind of ruined my brother and sisters fun though ;-)
This may not be the final cause of your mystery bug....but something to check.  I would also take Roland's recommendation to heart. Run everything from a 12 volt battery and see if the problem goes away. You won't get a tingle from 12 volt equipment. But you could get zapped if you are using a 12volt DC to 120 volt AC inverter in the field for some reason. Those are notorious for bad grounds since they don't have one normally and can cause all kinds of gremlins.

---In ap-gto@..., wrote :

    Would it help isolate the problem source, if Martin would unplug each of the servo motor cables, one at a time?
     A vibration might potentially be caused by a stuck servo motor, a fallen screw/part inside the motor box, or its rubbing internally against its case/mount. Perhaps a wonky servo shaft. Might also explain the electrical tingle he feels from touching the CP4, if the short was caused by a motor power attachment,  mechanical failure, leading to a DC short to the case and mount. Otherwise, if there is still mount vibration when either motor’s power is isolated, then as you suggested, it may likely be a power source leak going to both motors via the CP4, causing the mount to conduct the vibration.
    Pure conjecture on my part, but I would eliminate each motor’s power feed, in turn, since a “mechanical vibration” can only emanate from a mechanical source and there are just these two,  on the mount. Besides changing the DC power supply, I suggest completely disconnecting all accessories on the telescope – just power the mount directly, in case a current leak is coming to the CP4 via some other device/accessory.

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