Agile Modulator, Always Overdriven?
 

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.

Your lytics are bad. I've had it before on a number of Blonder Tongues...The lytics get so bad that the power supply hum on the signal path is stronger than the video and the overmodulation lights are on no matter what the pots are set to or even if the signal is unplugged...
I don't know if it is cap plague era or not but most BTs with original lytics have bad lytics.

I had to replace all the lytics in the three that i have to get them working properly. Not too bad of a job or expensive.

Understood, thanks. I'll pull one open and order some replacements. Doesn't look like too much of a challenge to rebuild.

Sure enough, that was the issue. Looks like both of my dead modulators are now running fine!

Now if only I could figure out what was causing the scrolling dark bands across the screen. I thought that would be the filter caps but those have been replaced and I still see them. I may grab an AC line filter to see if that changes anything. I'm sure it's some sort of RF interference.

To minimize that I run my modulators off a 120V isolation transformer with the ground pin path of the plug open. My media PC also has its ground pin path broken. Cable TV coax ground and power outlet ground don't play nice together in that room...