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Hmmm...
It will be hard to see a signal on the LEDs because the LEDs are very non-linear. So I would expect the blacks (with the FET cut off) give a voltage equal to the power supply, and then if you draw even a little current, you see the LED voltage drop, and this doesn't change linearly with current, so it appears to be clipped. The resistor you have will add a linear voltage drop at the drain. This in itself should not be a big problem for getting a good picture if the resistor is not too big, as whites should not be clipping in the FET, that is, the drain voltage should not drop so low as to equal the source voltage. You should verify that this is true. What are the voltage ranges of the signals at gate, source, and drain? (Be sure to measure with the scope DC-coupled.)
What else can be wrong? Maybe you have a problem with current sharing between the LEDs - you could try putting some resistance in series with each one (maybe 100 ohms?) instead of one common resistor and see if that helps.
By the way, when you get a visible but bad image, does it look like the circuit could be oscillating? It is sensitive to putting your finger or a scope probe on different parts of the circuit?
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