Injecting video in DuMont RA-102?

[old_tv_nut]

The 4.7 is 4.7 microfarads, right?
The + side should be connected to the transistor base, but the way you have it drawn, the negative side is connected to the base. Which way do you really have it?

That transistor should be OK for video frequencies (Ft is 250 MHz, which means the internal capacitances reduce the gain to unity at 250 MHz).

Are you using a 10x probe on the scope? A 1x probe may add too much stray capacitance and reduce the high frequency response. You can detect this by watching the picture while you apply and remove the probe.