Film-Tech Forum ARCHIVE: shocking platter

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Posted by Philip Wittlief (Member # 1724) on 09-28-2003, 09:21 PM:

I've got an ORC platter that is not grounded properly. There is a voltage on 'ground' i.e. the platters, the tree. From what I have measured its an AC signal about 80V RMS when the motors are running. When the platter hasn't been used the voltage goes away and it takes a minute or two for it to build up to the 80V once the motors starts running. Any ideas on what can be causing this?

phil

Posted by Manny Knowles (Member # 1171) on 09-28-2003, 09:31 PM:

Is there a ground pin on the power cord or has someone taken it out?

Posted by Philip Wittlief (Member # 1724) on 09-28-2003, 09:39 PM:

Yes, the plug is grounded.

Posted by Mark Gulbrandsen (Member # 72) on 09-28-2003, 10:32 PM:

Phil,
80 volts is alot to have floating in any ground and something is seriously wrong to the point that I would not operate the platter till it is repaired. IT IS IN A VERY DANGEROUS STATE RIGHT NOW! If the grounding system was working properly the fuse on the platter may well blow fomr that much leakage. Either the grounding scheme in the platter is seriously wrong, or there is a grounded outlet that is seriously wrong.
It may be something simple, but then it may be something more seriousand I'm not about to predict what the problem is....but it is just plain dangerous.
Mark @ CLACO

Posted by Paul Mayer (Member # 355) on 09-28-2003, 10:45 PM:

Sure sounds like there's a hot and a neutral crossed somewhere, perhaps a slow RC section has been created somehow and shorted to the frame since the voltage ramps up when the motors run and bleeds off when the motors stop. Like Mark said, very dangerous.

Is this a sudden occurrence? Was something recently replaced on this platter?

Posted by Paul G. Thompson (Member # 655) on 09-29-2003, 12:53 AM:

Take that thing to cold water ground. That thing could kill you.

I would question the effectiveness of the ground in the wall plug. If the ground was intact, there is no way you could get shocked since the frame or tree would be at ground potential.

You have a busted wire somewhere....

Posted by Darryl Spicer (Member # 711) on 09-29-2003, 01:00 AM:

Have an electrician check the plug that the platter is plugged into. The ground lead on the plug may have come loose. ANother possability is issues with the buildings ground conections. I wouldn't screw around with it Get an electrician in before there is a fire or personal injury.

Posted by Brad Miller (Member # 2) on 09-29-2003, 04:01 AM:

It's one of the brown tree ORC platters, isn't it? (Not the orange ones). I had that once before and I'll be damned if I can't remember what the solution was. Maybe it will come to me.

Posted by Jason Burroughs (Member # 68) on 09-29-2003, 09:30 AM:

Brad, was your electrifying experience with the ORC platters by chance the ones that came out of the AMC Prestonwood? Their ORC platters did the exact same thing, actually injuring someone. They were tossed out in the dumpster once some new SPECO platters were installed and someone came by (not you) and picked them up to install them in their theater (First class operation I'm sure [Eek!]

).

Anywho, there's a possibility you service those exact same platters. It seems like that there was an issue with one of the motors that cause the problem, that was shorting to ground.

Posted by Gordon McLeod (Member # 33) on 09-29-2003, 09:33 AM:

I suspect that the ground of the outlet isn't connected
Also I suspect that if it is the older style with the variac on the takeup there is a connection on its frame to the brush assembly or it is induceing it into the frame that isn't grounded

Posted by Philip Wittlief (Member # 1724) on 09-29-2003, 01:27 PM:

Its not the outlet, its something that is in the platter itself. All the other equipment had on the same outlet is properly gounded. And it had been on a different outlet before causing the same problem. At that time I thought that the outlet was out of phase, but now its obvious that something is wrong inside the platter.
Yes it is one of the brown ORC's.
I'm aware of the dangers of the platter. I'm an electrical engineer and had many classes learning about the dangers of elecricity, so I'm careful not touch anything else at the same time. Although I have of course, which how I figured out the problem. And the shock is not nearly close to a 120V line.

Brad, hopefully you can remember what the problem was with your platter. Does anyone have the manual or schematics for the brown ORC's so that I know what I'm looking at if I open it up.

Phil

Posted by Jack Ondracek (Member # 1466) on 09-29-2003, 02:10 PM:

Phillip, there's something inherently wrong with the situation as you describe it. If the grounding from point A to point B is connected properly, there's no way you can be measuring that kind of voltage between the same points. Something is not hooked up right. As an electrical engineer, you know you should be able to touch pretty much any two metallic items within your reach without risk... assuming proper grounding has been accomplished.

Further, the voltages in a platter are relatively stable within 10 seconds or so after the projector starts up & the platters reach their operating speeds & stabilize, so the idea that it would take two minutes... or even one to build up the kind of voltage you describe sounds bizarre.

As far as I know, there are no caps in these platters... certainly not in the orange tree versions I have, which makes the idea of any kind of "build up" seem impossible.

So.... more info?? What are the points you're measuring this voltage between?... the platter & a power raceway?... the motor and the tree??... what? Are you actually watching this voltage build up on a meter of some kind?

When you get the chance, unplug the platter & measure the resistance between the ground plug and the platter (at various points). Assuming the ground connection is reaching the platter at all, maybe you can find some part that might be floating. At least you'd have a point to start from.

There is one spot that I could conceive a shock risk might occur. If you have those vacuum cleaner motors on your platter, make sure the front plate of the motor housing (the part with the switch) hasn't worked loose & is floating. There's a lot of stuff crammed in behind that plate... and the switch contacts & ceramic resistor pins are very close. The diode is mounted to the plate itself, so there is a direct connection that might be a potential problem if the screws holding that plate work themselves out & the plate loses direct contact with the rest of the motor housing.

Posted by Philip Wittlief (Member # 1724) on 09-29-2003, 03:07 PM:

Jack said "If the grounding from point A to point B is connected properly, there's no way you can be measuring that kind of voltage between the same points. Something is not hooked up right."

I know, that's my point. And now I'm trying to figure out what is not hooked up right.

My estimate of 1 or 2 minutes may be off a little bit, but its not a consistant thing. Today, when I turned everything on, there was already 80V on the platter. Before I threaded up about 30 minutes later, the voltage was only a couple volts (was looking on a scope and didn't take an exact measurement)

I'm measuring with a DMM and a scope the voltage between the casing of the platter and the metal box of the outlet, which is properly grounded. So I know that where the chasis is supposed to be connected to ground, it is not. But I don't know where that connection is on these platters or if there's multiple connections. Or what is putting the voltage on there since it is loose. It would seem to me that its more than just a ground wire that's loose, because something would have to take all that energy to bring it back to ground. Unless its a voltage that's being induced onto the chassis which then I suppose the ground would keep that from happening.

The platter won't be in use for 2 hours later today, I'll take resistance measurements and look to see if I see anything else suspicious.

Posted by Brad Miller (Member # 2) on 09-29-2003, 03:45 PM:

quote:
Does anyone have the manual or schematics for the brown ORC's so that I know what I'm looking at if I open it up.

Yes I just got ahold of a manual for that unit. However it has lots of fold out schematic pages and the manual itself is quite huge. I am going to have to photocopy some of the stuff before being able to scan it, so it may be later in the week when I can post it.

Posted by Edward Jurich (Member # 1787) on 09-29-2003, 05:49 PM:

Unplug the platter. Using an ohm meter on the 20,000 ohm (or whatever scale you have that's close), measure from each AC plug prong to the platter. Neither of the flat prongs should read. The ground prong should be close to 0 ohms. Raise the takeup speed control arm all the way up and measure again. If either of the flat prongs read a resistance to the platter you have a wire touching somewhere.

Posted by Darryl Spicer (Member # 711) on 09-29-2003, 06:21 PM:

Could it possably be a bad ground in the plug its self. Are there capacitors in this unit that could be causing problems. When you turn the platter off I am assuming you are using a breaker. I don't claim to be an expert in electricity but I would think that if it is holding 80v with the power off there is an issue with either a capacitor or ground for draining off the voltage. These are just some thoughts.

Posted by Philip Wittlief (Member # 1724) on 09-29-2003, 06:32 PM:

Darryl is right, its a bad ground in the plug or cord itself. I just opened it up and the first thing I checked was if the ground connection was loose. When I got an open between the ground terminal in the platter and on the plug, I disconnected the ground wire from the terminal and it was still open. I also at that time measured from the terminal to the platters and it was near 0 ohms. So it turned out to be nothing but a shorted ground lead in the cord/plug. Although I don't see any physical wear in the cord, so I assume its actually on the plug end. I already replaced the plug and everything seems to be fine. I'll double check while the next show is running, but I'm sure that the problem's been fixed.

Thanks for everyone's help and input.

phil

Posted by Jack Ondracek (Member # 1466) on 09-29-2003, 09:17 PM:

quote:
Thanks for everyone's help and input.

Everybody send Philip a bill for a hundred bucks! [Big Grin]

Posted by Gordon McLeod (Member # 33) on 09-29-2003, 10:32 PM:

Some dim wit may have reversed the neutral and ground wires as well
The green bonding ground must go from the chassis of the platter to the ground pin
One other possibility is the ground pin of the plug may not be bonded to the ground of the box
In some locations electrician rely on the screw connection mounting the receptical to the box
In Ontario a wire must run from the ground terminal of the receptical to the bonding ground of the box
If there is any painted devices in the circuit then a bonding wire is required right through