Jet drum sander

I've got a Jet 10-20 drum sander. I've had it and used it for several years. Today I used it for the first time since we moved and discovered a strange problem.
I was thicknessing some maple boards by taking them down to 1/16" oversize on my planer and then taking off the rest with a few passes through the drum sander. Something I've done many times with no problems.
The boards are 4/4, about 6' long and 4"-8" wide. I set up infeed and outfeed stands, each about 3' from the drum. The sander was set to take off about 1/64".
What I get is a board that is, for example, 49/64" about one foot in, 50/64" two feet in, and 51/64" three feet in. Then it starts getting thinner again until the other end. It's like the drum is gradually going up and then back down.
I've tried running either face up and either end in first. Seems to make no difference. I'm stumped. Help!
I'll call Jet tomorrow, but I thought I'd see if anyone here had a solution. Probably more expertise here than at Jet :-).
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Where have all the flowers gone? Pete Seeger 1919-2014

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On Mon, 24 Feb 2014 00:12:30 +0000 (UTC), Larry Blanchard

Are you properly supporting the board both going into the sander and out? If the board is bowing, I can easily imagine this sort of problem.
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"Larry Blanchard" wrote in message

This strikes me as an in-feed/out-feed support height issue... which could be due to an uneven floor. That is, though the stands may measure the correct height from floor to their top, the floor itself may not be level and in the same plane from stand to sander to stand. Using a 4' level between the sander table and the top of the in-feed/out- feed supports may provide a good clue as to whether this is the problem.
John
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The drum isn't going up and down, your boards are because your infeed/outfeed stands are off by about 1/32. Too low.
When the board is going in the leading edge is slightly up and the hold down is not strong enough to overcome the weight of the board to push it down level, opposite at the outfeed side. After the board is partially through the hold down can get it level until considerable is hanging out.
To fix, get the supports correct and/or adjust the pressure of the hold down rollers.
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dadiOH
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On Mon, 24 Feb 2014 08:48:38 -0500, dadiOH wrote:

I thought I'd been pretty careful setting up the supports, but I'll check again. Maybe try setting them a hair high. I was a little suspicious of the hold down pressure, but it'd always been sufficient in the past. Plus it's hard to get to them to adjust them, but I may have to do that.
Thanks to you and the others who replied.
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Where have all the flowers gone? Pete Seeger 1919-2014

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BTW, the same thing will happen if the board is a bit bowed.
The in/out feed I use on my Performax 16 is made with 2.5" PVC pipe as rollers. They are fastened in a frame which is hinged at the sides of the sander cabinet so that they drop down when I don't need them. Both sides are made in two sections enabling them to be extended to about five feet. The outboard ends are supported by a single leg which is adjustable in height via a sliding dovetail.
I mention this because I often sand long boards that have a bit of bow in them. Since the leg is adjustable I can easily adjust the height to accomodate the bow.
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dadiOH
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