Duplicate, see below post.

Looking at the signals on C1 and C2 to me they look the same, with just the signal at the sound detector attenuated some. How important is the DC offset for each of these? Would it work to splice in the same amplified video signal from the op-amp to C1 but then include a dropping resistor to connect to C2? Is there a need for coupling capacitors between these?

https://i.imgur.com/EIJa6aV.png

Here is what I measured:

• C1: 0.23 VAC
• C1: -2.98 VDC
• C2: 0.10 VAC
• C2: -1.14 VDC

Also the scope traces look like the waveforms are the same:

https://i.imgur.com/iwqKl2t.jpeg

I think the DC coupling from C2 to the AGC/sync circuits is crucial.
I think normal operation is that AGC sets the (negative ) sync peaks to the desired level, thus setting the overall amplitude and the DC level of the half point of the sync pulses for sync slicing. It looks like the two sync/AGC tube sections are DC coupled, meaning that they depend on the signal going negative from ground to put the sync slicing point at the correct level.

I think what might work is to build a DC restorer/amplifier circuit for the input that puts the bottom of sync tips at the correct negative voltage at C2 (about minus 4.5 volts?) while amplifying the signal to 4 V p-p. You will need a negative supply for this.

Edit