The CTC-5 Contrast Control Modification

[NewVista]

If you want to see elegant, look at Admiral (simplicity that works great)
If you want to see berserk, look at Motorola ( their crazy early 50s B&W)

What is C402 doing coupling signal back to the screen divider?
Other end of brightness goes all the way to H-out grid just to find -35v?
Both Brightness & Contrast controls are weird, the Contrast Mod is
a good start but there's scope for more improvement as DC performance
is poor

[Penthode]

Originally Posted by NewVista

What is C402 doing coupling signal back to the screen divider?

Curious circuit. The signal on the unbypassed screen grid will be anti-phase to the cathode coupled signal. The RC network off the screen grid is also interesting: perhaps the circuit was to improve the frequency/phase response of the amplifier? I shall give this more thought

Originally Posted by NewVista

Other end of brightness goes all the way to H-out grid just to find -35v?
Both Brightness & Contrast controls are weird, the Contrast Mod is
a good start but there's scope for more improvement as DC performance
is poor

The other end of the brightness control is through a resistor to a negative voltage source. The size of the resistance defines how much of the dc component is fed to the grid of the second video amplifier. i see on later chassis's (CTC10 for example) the resitance is larger and the end is tied to a more negative potential. This facilitates mor e dc component getting back.

[NewVista]

Originally Posted by Penthode

Curious circuit. The signal on the unbypassed screen grid will be anti-phase to the cathode coupled signal. The RC network off the screen grid is also interesting: perhaps the circuit was to improve the frequency/phase response of the amplifier? I shall give this more thought


.

If feeding forward, C402 would be adding in-phase What are they up to?
Through both stages, a lot of response tweaking C401, C407, L401, L402, L403, C110, C404, L404, L405

I have to question how well ~1meg DC path can maintain clamped video

[DaveWM]

as long as we are talking about the mod, what the 22pf removed from the plate to screen and the change in the 330 to 390pf all about? c404 and c410

[NewVista]

Originally Posted by DaveWM

as long as we are talking about the mod, what the 22pf removed from the plate to screen and the change in the 330 to 390pf all about? c404 and c410

The 22pf would help avoid RF oscillation & 390 would provide same level of video peaking as before Mod?

[frenchy]

My CTC-5 is unmodified and I agree it's harder to avoid blooming, but then it is a very early color set where they hadn't worried about keeping that in check as much as later on. Plus at least I've never had retrace lines showing up at really high levels like my later sets can tend to do. Hah maybe that's because the blooming will kick in before that point anyway. : ]

I usually seem to end up with the brightness cranked pretty high, then keep tweaking the contrast as high as I can get it without getting bad blooming on a white screen. That will usually nix any isolated blooming in a band like if there is a lot of pure red on the screen. I'm smart enough to watch this set either in the dark or very low light level so not really a problem for me.

I don't think I've yet seen the complete schematics for the mod but perhaps I did and lost them, can't remember! Does anybody have a copy of them? Was this a mod specified by RCA themselves or just some outside sources?

[Penthode]

Originally Posted by NewVista

If feeding forward, C402 would be adding in-phase What are they up to?
Through both stages, a lot of response tweaking C401, C407, L401, L402, L403, C110, C404, L404, L405

I have to question how well ~1meg DC path can maintain clamped video

I suspect there was a lot of fiddling to get a reasonable frequency responce and perhaps compensate for the behaviour of the delay line.

Besides adjusting the brightness by changing the second video amplifier bias, the brightness control also varies the amount of dc component fed to the control grid of the second video amplifier. R407, both halfs of R101A plus the resistor not depicted attached to the horizontal output tube negative bias form the divider chain. I think the 1 megohm dc path is not an issue because the input of the tube is high impedance at DC anyway.