Agile Modulator, Always Overdriven?

[Dubis7]

I've got four Blonder Tongue agile modulators, all of the AM60 variety. Two work fine, but two seem to have the exact same problem. They broadcast severe interference and distortion, enough that a signal isn't visible, and the "overdrive" lights are constantly lit. I'm using the same input (the exact same model DTV converter box) as I am on the two working units, and have verified that the problem persists when I swap in a known working converter box. Adjusting the controls on the front has no impact.

Does anyone have experience servicing these, or an idea of what I should be checking? I've heard of the filter caps going bad in these, but this seems extreme for that.

Perhaps a service manual would be the right place to start, if I knew where to get one.