The CTC-5 Contrast Control Modification

[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

[Penthode]

I see no reply from my posting on this yesterday: perhaps my message was too ponderous and I did not make myself clear? Let me try to launch this from another perspective.

Modern displays represent the dc or background level a lot better than most TV sets from the '50's and '60's. This need not have been since the sets from the late 40's were much superior in this regard. My 1949 RCA TV has a dc coupled video amplifier which represents the dc level amazingly well. And of course the TS630 design from 1946 included a dc clamping circuit which also yielded excellent results. But starting in about 1953, when set bandwidths became narrrower yielding less picture detail, the maintaining of proper dc in the picture was dispensed with.

Now on to color. The earliest RCA color sets (CT100 and 21CT55) had dc clamping after the matrix and just before the tube. I would assume the maintainance of dc in these sets to be pretty good. The CTC4 and CTC5 sets must have been originally fairly good. The CTC4 contrast control progressively shunts the signal with a capacitor to reduce contrast. This means that if the contrast control is kept fairly advanced, the dc should be coupled pretty well through.

The CTC5 however used a variable resistor in the cathode of the second video amplifier to reduce the gain. The problem with this arrangement is the interaction of the contrast control with the brightness control which led to blooming from overcurrent of the CRT. This must have been a problem because subsequent designs (CTC7 thru CTC10) used the variable coupled cathode capacitance which increases AC gain at the expense of losing the dc component. Alas, the CTC5 modification was done to copy the later series receivers hence reducing the dc component.

I have attached the circuit of the unmodified CTC5 video amplifier. I have marked the dc path between the first and second amplifiers in red. The AC path follows the red until where it branches off to the green path after which it recombines with the red. Note that the dc component is divided at the output of the delay line by R182 and R101A (the brightness control). Note also that as the brightness control is reduced, less of the video dc component reaches the next amplifier stage. This implies that the set up of the tube (background controls) should ensure that the brightness control is mostly advanced or as close to maximum as possible to ensure the dc component gets through. And without the electrolytic bypass capacitor used in the mod. and later series chassis's, the second video amplifier will have the same gain for the video ac and dc components. The division of the dc is not a bad thing because even at 50% reduction, only minimal dc shifts are noticable. Hence maintaining a percentage of dc is all that is really needed.


I hope you appreciate what I am trying to say. I am asking if others are aware of the dc level problem? (My test with the blanking bar will reveal this on any set, color and black and white. My 1949 RCA passes this test excellently in this respect).

[old_tv_nut]

These sets were not designed to have 100% DC coupling, as you stated (even before the contrast mod). Trying to set the bias to maximize it is mostly futile. If you want 100% coupling (or full restoration), feel free to redesign the circuit - but this of course is no longer "restoring" the set to original operation.

I agree 100% with the desire for full DC response for picture quality, but I don't plan to try redesigning the circuit to get it on my '5.

[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

[ChrisW6ATV]

I have two later RCA sets in working condition. The CTC-16 has pretty good DC restoration, but my CTC-16X has quite poor DC restoration, distracting to me after being used to color sets always looking good since 1979.

[Penthode]

Originally Posted by old_tv_nut

These sets were not designed to have 100% DC coupling, as you stated (even before the contrast mod). Trying to set the bias to maximize it is mostly futile. If you want 100% coupling (or full restoration), feel free to redesign the circuit - but this of course is no longer "restoring" the set to original operation.

I agree 100% with the desire for full DC response for picture quality, but I don't plan to try redesigning the circuit to get it on my '5.

I agree that the sets were not designed for 100% dc coupling. It was the inevitable cost trade off. And I am not suggesting that the set be redesigned.Although what I am trying to say is that those who make the mod are trying to do so.

I am now content undoing the mod and keeping the set as original. In the CTC5N Manual (T-6) instructions are given how to properly set up the background and screen controls as the set was designed. I find that this will help minimize but not eliminate the blooming problem when the contrast and brightness controls are not adjusted properly. Setting the background is reducing the 21AXP22 grid bias will compensate for the reduced cathode voltage when the brightness control is adjusted. I am more or less restating what is in the RCA instruction manual.

I perhaps did not make this clear, but setting up my CTC5 without the mod. as per the instructions, the dc level is not perfect but not bad. And I feel that it is better than before I undid the mod.

[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]

What's with Contrast pots in the cathode anyway? I don't like this as it would alter operating point of tube.
What's wrong with having a fixed operating point (in a nice linear region) and controlling the gain otherwise?
And does Brightness alter operating point also just to transfer change to kine cathodes setup?
Let's put the brightness control back up by the kine where it belongs.

[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 NewVista

And does Brightness alter operating point also just to transfer change to kine cathodes setup?

Or does it push operating range into non-linear, compressing or expanding blacks?

[Phil Nelson]

Originally Posted by Penthode

I'd be curious to hear back from those with CTC-7 and upwards chassis sets after performing my little blanking bar examination test

I have a working CTC-7 and CTC-11, but I'm not sure how to do what you're asking. With the vert hold control I can adjust to a slow roll, but it won't lock with that bar in the middle of the screen. It's pretty hard to judge average brightness, etc., with everything rolling.

Phil Nelson

[Penthode]

Originally Posted by Phil Nelson

I have a working CTC-7 and CTC-11, but I'm not sure how to do what you're asking. With the vert hold control I can adjust to a slow roll, but it won't lock with that bar in the middle of the screen. It's pretty hard to judge average brightness, etc., with everything rolling.

Phil Nelson

Yeah, I know it is tough. First adjust the brightness up slightly to make the sync bar grey. If you can hold the bar long enough in the middle of the screen, when the video goes from bright or a well lit scene to black, ideally the bar will remain the same shade of grey. The degree of the bar changing in brightness depend upon how good the set is maintining correct DC passage.

Expect a small change which is generally acceptable. If the diffence is very noticable, then the dc component has been lost. On most '50's Black and White TV's, the DC component is totally dispensed with and the difference will be quite noticable.

As I said earlier, your 1946 TS630 and my 1949 RCA will pass this test with flying colors....(err maybe "flying back and white" only).

[Penthode]

Originally Posted by NewVista

What's with Contrast pots in the cathode anyway? I don't like this as it would alter operating point of tube.
What's wrong with having a fixed operating point (in a nice linear region) and controlling the gain otherwise?
And does Brightness alter operating point also just to transfer change to kine cathodes setup?
Let's put the brightness control back up by the kine where it belongs.

I agree that putting it in the cathode path is a bad idea. But the operating point is actually governed by adjusting both the brightness control and the contrast control in unison. I however contend that removing the mod with the capacitive bypass contrast control and putting it back as originally designed helps better maintain the correct dc component getting to the CRT.

The earlier RCA black and whites had the contrast control in the plate circuit but this involved the pot having a number of taps for frequency compensation.

I think the best solution would have been to add a diode clamp just before the CRT but this would have required another tube.

[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.