Film-Tech Forum ARCHIVE: Strong AP platter runs faster when nonused platter take up motor is activated.

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Posted by Eric Robinson (Member # 2966) on 04-17-2006, 03:48 PM:

I service a theatre which has made me aware of a strange behavior exhibited by Strong AP3 platter systems. (not DMC, not SCDC, just the regular old variac with the phase control brain plates)

The projectionists would use a rubber band to hold the take up roller on the platter , not in use, so that the motor would just free wheel while the other two platter fed out and took up the film. Doing this seemed to cause the feed out platter to be more responsive, yeilding a feed out which didn't lag or speed up and slow down.

I spoke with Design and Manufacturing (Potts) who commented that they have heard of this behavior also but didn't know why this would happen after inspecting the electrical schematic.

Anybody have any insight on this?

Posted by Monte L Fullmer (Member # 2797) on 04-17-2006, 09:58 PM:

He holds up that roller with a rubber band?

What about just using one brain..the other one phase control in the brain could be bad. (replace those phase controls with microswitches..)

Why not just simply shut that deck off with the makeup switch in the up position, thus shutting that motor off.

Or, unplug that motor and see what happens. .

Motor could have a shorted diode in the frame.

For, the variac is controlling ALL of the electricity going into that tower, and if one motor is weak, it depends on the other to aid it along.

Disconnect each motor to see if the speed picks up..in as well as using one brain.

Some ideas for you. - Monte

Posted by Eric Robinson (Member # 2966) on 04-17-2006, 11:37 PM:

The projectionist uses the platter system in a normal way regarding the proper use of the feed out and take up platters. Additionally, the projectionist lowers the motor on the third unused platter so that it does not contact the platter. The projectionist then uses a rubber band on the unused platter to activate the unused motor by pulling the switching take up roller in. This is the last roller on the tree that the film comes into contact with before it winds up on the platter ring.

The projectionist has learned to do this trick from someone and I have witnessed that it does seem to make the brain plate more responsive. The downside; however, is that this action is masking a problem with the platter system, most likely a brain plate which needs a new phase control circuit. Additionally the free wheeling motor is spinning for no reason which will wear down the motor brushes and shorten its life.

I was just curious why this technique seems to speed up the response of the feed out platter??

Hopefully I have not made this more confusing.

Posted by Monte L Fullmer (Member # 2797) on 04-18-2006, 02:52 AM:

quote: Eric Robinson
most likely a brain plate which needs a new phase control circuit.

..the LED in the unit is getting tired..I bet if you took off that cover, you'd see the year of manufacture of that phase control.. - definitely time for a new phase control unit.

What you need to do then is to 'condemn' each part of that platter with these steps then;

1- use only one brain..the good one.
..if this doesn't return the platter to normal..then -
2-disconnect the motor from the tower that isn't being used.

What happens when you hook up the MUT to the payout deck and control the motor speed with the MUT's speed control .. and NOT have that unused motor activated?

I'm pointing to a weak phase control unit that is causing this..

Try the one brain trick first..

-Monte

Posted by Eric Robinson (Member # 2966) on 04-18-2006, 07:40 PM:

Thanks for all of your helpful information Monte.

I suspect that the phase control LED has faded or perhaps the solar cell detector has lost its sensitivity.

I had suspected this as a problem from the beginning.

I guess my objective for this post was to see if anyone knew why the platter system acted differently when an unused motor was activated.

Posted by Monte L Fullmer (Member # 2797) on 04-19-2006, 12:05 AM:

sorry...it's like when a headlight burns out in a car..and you then replace that headlight with a new one. That new one is going to send the bulk of the current to the old one since the resistance in the new one is higher due to the newer filaments (and a stronger phase control..) and current will always like to find the least resistive route.

Why, when a headlight goes out, you replace them in pairs.

Doing the motor trick balanced that current flow... - Monte

Posted by Eric Robinson (Member # 2966) on 04-19-2006, 12:10 AM:

Yes. Its like a parallel circuit with two loads where one load has less resistance. The load with less resistance will consume more current. I'll try to match that with my circumstances and see how it pans out.

Posted by Jack Ondracek (Member # 1466) on 04-19-2006, 01:36 AM:

quote: Eric Robinson
The projectionists would use a rubber band to hold the take up roller on the platter , not in use, so that the motor would just free wheel

Something like using the lever to disengage it from the platter?

Putting a second takeup motor across the variac would add to the load, causing the rollers to drop & the variac to increase voltage to get the takeup speed back up. That would also raise power to the feed platter. So... maybe it could be an LED... but couldn't it also be just a simple phase control alignment or platter timing issue?

Posted by Eric Robinson (Member # 2966) on 04-19-2006, 10:48 AM:

I don't think this scenerio is possible:

quote: Jack Ondracek
causing the rollers to drop & the variac to increase voltage to get the takeup speed back up

because the only way the variac can increase voltage is to cause its own armature to move. Even if the variac was able to move its own armature, it would then be in a position which is out of sync with the projector. That is it would be taking up too quickly or too slowly leading to an eventual problem.