Tuby128, thanks a lot!
Didn't mean to bother you so much

I got a good picture using the TV tuner card. But it seems that my modern composite-HDMI converter didn't like the color EP signal (it converts BW composite quite well).

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I have problems with the color composite signal.
Now in winter time the 230V on the wall connector rapidly changes. Probably the electrical grid in our street is not that good.

 I measured with multimeter, every time when in the wall socket the 230V drops with 7-10V, I can see some brightness change on the TV when I use Composite.
This is not the old CRT, now I am using a 2000s SonyTrinitron.

 This kind of brightness change does not happen, when I use the R-G-B mode signal.

 I suspect, that the 12V circuit of the Enterprise is not stable. The Monochrome Composite signal is made by the PNP-transistor, which uses 5V. That signal seems to be stable.
The color composite I made, was using the 12V to amplify the signal to 75ohm load.

 Maybe it is time to change all the electrolite capacitors on the Enterprise board. On the other side the "Brick like" power supply has probably some electrolite capacitor, which also need some refresh.

I think the best solution modernize the machine with EPower/RePower, and L2 module. For a stable power voltages and stable Nick clock.