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I cant see it being agc, most of the AGC rca stuff I have run into is either it works or its set incorrectly. incorrectly would be a under contrast pic that I would not describe as out of focus but more like lack of contrast. Over load it and you get a dark pic or loss of sync, but again more like too much contrast.
The only defect I recall in AGC was on a zenith with an open cap in the circuit, it would manifest as overly critical range (complete wash out of pic to complete loss of sync in about 1/8 of a turn of the pot.
Maybe the OP can duplicate the prob with the old modulator and this time get some video of it. Its nice to see different problems and solutions, may come in handy in the future is someone runs into a like problem.
I am still wondering exactly what it looks like, focus mean something very specific to me and I just cant get there with signal source. I wonder if the OP ever tried adjusting the focus control to see if there was an optimum setting somewhere if going either way it would get worse. That would tend to indicate the focus voltage control is working and getting the correct voltage. Then if it went out of focus would touching up the control bring it back. Again to establish focus voltage as the issue.
If it was smear from IF response then the focus adjustment should still work just getting the most in focus smear, with out of focus smear on either side of the adjustment optimum setting.
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