[Penthode]

Despite my CTC-5 having had the contrast control modification done many years ago (after 1959 from the date code on the capacitor), I was unhappy with the DC level variation in the video. Despite being warned that I would not like the result, I thought I should try putting it back as original to see how it would perform.

Before the modification and with the contrast control advanced, the picture DC variations were quite wide. You can see this for yourself by adusting the vertical hold control so that you can see the vertical sync/blanking bar on the screen. As the program video changes from high to low average brightness scenes, the blanking bar would vary considerably in brightness. Fades to black were instead fades to grey.

When the contrast control is adjusted for reduced contrast, I felt the DC level variations reduced but were still there. No longer is the capacitor cathode bypass in place which eliminates some AC coupling. However, there is still a large degree of AC coupling to the second video amplifier grid, despite the DC path through the brightness control.

Changing the circuit back to the original has its pros and cons. DC variations are reduced but still noticable. (This is the PRO now for the CON).

The downside is the interaction between brightness and contrast controls. You have to be pretty adept in your adjustment not to cause blooming. It is a pain but satisfactory adjustment can be done.

The best solution would have been to put diode clamping at the output of the second video amplifier. It is strange that the later '50's mentality was the maintaining proper DC level wasn't important.

I'd be curious to hear back from those with CTC-7 and upwards chassis sets after performing my little blanking bar examination test to see if the DC level maintenance is as bad on later sets. I recall my Zenith 24MC32Z had a DC level improvement modification over the 24MC32. So it would be interesting to hear back from owners of these sets as well.

And of course the CTC2 chassis sets I believe had the diode clamping so these sets should be the reference, if I am not mistaken.

Cheers,

Terry