[dobulee]

Well, a little bit of heat, and I was able to move the slug slightly. That combined with the horizontal drive trimmer gives me decent linearity.

I got out my DIY pattern Generator and threw the grid pattern up -- plus that gave me a good chance to put the scope on a few interesting points in the set.

The grid pattern looks pretty decent. The vertical linearity is now worse than the horizontal - stretched slightly at the top. That's a problem for another time.

MadMan's discussions on 'ghost / smearing' on his Admiral, as well as the discussion on DC restoration prompted me to look at a few things.

This RCA chassis provides a convient synch negative point to inject composite video right at the grid of the 1st video amp, the captures below are annotated to say if the capture was made with RF injection, or composite video injection.

1st up: Lookint at the Grid pattern with direct composite video injection into the video amp. The linearity is passable. The slight 'bowtie' shape of the horizontal lines didn't used to be there before I had to install an aftermarket deflection yoke :/ Anyhow. Look at the scope, the bottom trace is composite video input, and the top trace is the crt grid. 2 things to note: (1) the video amp obviously has a fairly sloppy underdamped response. The ghosting seen on the screen is clear as day in the CRT grid waveform. Each 'bar' on the composite video is followed by several cycles of underdamped oscillation on the grid. Maybe RCA made this as a compromise to try to squeeze some extra bandwidth out of this 12au7 video amp (I don't have enough TVs to know if low mu triodes were often used as video amps or not...I'd guess not). (2) The bandwidth of the video amp itself is pretty poor (remember the IF stage is out of the mix). Note that the colorburst on the composite video is completely stripped off of the grid waveform. At least this lets me know that I shouldn't expect miracles by tuning up the IF section. The video bandwidth in this chassis is at best a paultry 3MHz.

2nd up. DC Restoration. This is a grid driven set, with DC restoration happening as part of the sync clipper. In fact, a positive going triode grid functions as the DC restoration diode in this configuration. So how good is the DC restoration? Not very. I have taken several captures, an all black screen, an all white screen, and then a center white bar. In all captures the scope settings are the same... the reference and gain are identical.

All Black Composite input to vid amp: The synch tip is at the center scope division. The black level is about one division up (22V). The screen appears gray at best, with retrace lines visible.

All White Composite input to vid amp: The synch tip is now maybe 10V below the previous level, but due fairly high impedance in the diode/cap restoring path, the black level is now only a few volts above the synch tip. The black level has drifted down to ~2V. So, even with DC restoration by the 'black' level at the CRT grid drifts almost 20V under the most extreme cases.

White Bar Composite input. to see DC restoration in action (imperfect though it may be) look at the center white bar. The scope settings are the same, except now were triggered on the vertical, not the horizontal. Note how uniform and flat the 'black' and 'white' sections of the composite input are, while the same portions of the grid drive 'angle' in as the circuit drifts toward the new equilibrium. So, even with DC restoration shown in the schematic, the action leaves a little to be desired.

Finally, I"ll leave you with the response to an NTSC color bar pattern, first going through the RF/IF stage, and then with direct composite injection. With RF injection the color carrier is entirely stripped off the signal when it hits the CRT.

With direct composite injection, the response of the grid is sharper, but the video amp still fails to pass the majority of the color carrier. However, enough 3.57MHz color info remains to give a faint appearance of marching ants on the CRT.

Anyhow. Just a few fun waveforms. I'm not sure how much better this KCS47 chassis can look. Might be polishing a turd, as the expression goes!

Were triode (vs pentode) video amps common? When did they get phased out?