There is another way to inject a signal.
The german "Funkschau" offered in the 50s a nice DIY-device.
This little assy covers a tube "EC 92", father & mother socket for the CRT,
and a few resistors, caps, 2 switches, cables and a piece of alloy.
It is a little generator which produces horizontal and vertical bars.
The device gets its voltages from the CRT plug.
To operate it, it has to be pushed on the CRT contacts and the plug coming from the TV chassis is conected with the backside of the assy.
Now you can switch beween vertical and horizontal bars on the screen,
as far the voltages to power a CRT are all there.
Regards,
TV-collector
