[kf4rca]

Very likely that first video stage is getting some bias from the IF. Not unusual.
Attached is the schematic for the video input circuit I used for an RCA CTC92. I found this circuit on the web and modified it some.
The pot labeled DC sets DC level which equates to black level.
I would inject the video signal after the sound trap as the sound trap rolls off everything over 4.5 MHz.
Maxim used to give out free samples. But I think that's now only available to Asian EE students.
Another source of video chips is Linear Technologies (now a part of Analog Devices). Not sure what their sample policy is.
Something to remember, if you go the op amp route, you will most likely need a bipolar power supply. Whereas the transistor circuit requires only one.