How to get tube type RCA to look as good as Zenith Chromacolor 2?

[Chad Hauris]

Recently I have been working on a 1967 RCA CTC-28. It has a new replacement Hi-Lite picture tube put in in 1976 which tests very good on a tester. I have put in all new electrolytic capacitors and rectifier diodes and checked power supply voltages, measured HV and regulator current and horizontal output current and it all looks good.

I have a pretty decent color picture on it but it's not nearly as good in my opinion as the picture on the 1979 Zenith Chromacolor 2 we have in the living room using a very similar 23" delta gun picture tube.

On the RCA the contrast doesn't seem as good (picture can seem washed out) and it seems almost impossible to get perfect convergence. On the Zenith the convergence and linearity are perfect all around the screen...I never thought a delta gun tube could have such good convergence.

I have worked with RCA tube type color sets for around 20 years now and have almost always had problems getting a really vivid picture with good contrast and getting really good convergence. I'm wondering if it may be still be due to weak tubes somewhere or bad maroon capacitors?

I only have one RCA tube type set which has produced a picture that measures up to the Zenith and that is a portable CTC-22 which spent its life in an air conditioned office and probably wasn't used much. I know the console RCA's can produce a better picture than I've seen on most of them but don't think I've found the secret yet to doing it!

[jpdylon]

If I were to make a rough guess, The main reasons why the chromas make better pictures are as follows.

1. Lower operating, fully regulated B+ voltage. Between heaters and high voltage, tubes generate heat - alot of it. This heat wears out components. When your IF strip is closed up - the RCAs are generating a hotbox for the IF strip - whereas the chromacolor remains cool - making components last longer. I'm sure if you replaced everything in the IF strip and realigned - your video response would sharpen up quite a bit.

2. Full DC restoration. Most of the RCAs of the mid to late 60s had only partial DC restoration. It was less expensive to design and was not as demanding on the high voltage supply. However partial DC restoration does not yield as vivid contrast.

3. The chromacolors have a superior high voltage supply that is fully regulated. The tripler unit also takes a load off of the horizontal output stage by requiring less current of the output stage. The solid state rectifier an regulator are not subject to "sag" like their older tube counterparts. When you crank the brightness or contrast and the full demand of the 25k is needed at high beam current - the solid state HV supply does it! Try full DC restoration and high contrast on a mid to late 60s RCA and it will bloom to black. Even the 4 tube chromacolor hybrids had the solid state power supply - just no DC restoration. Older sets with DC restoration like the CTC-2b and CTC 100 had double shunt regulators in the high voltage supply for much better stability and a brighter picture - but again it was more expensive to make.

4. The color circuitry of the chromacolors uses an IC to process the color. It operates at a low voltage and is less likely to fail. You could make the tube color set work nearly as well with component replacement and alignment - but I don't think it will ever be quite as good. I've tweaked out both my 11 and a CCII and they come close - but the chromacolor II beats it out with slightly better demodulation on top of the video amp DC restoration.

5. The convergence assemblies for the chromacolor II were made at the pinnacle of Delta Gun technology. By the mid 70s manufacturers had pretty much perfected manufacture of the gun assembly and shadow mask, as well as understood what magnetic influences were necessary to achieve that perfect convergence picture. There are some RCA sets of the mid 70s with delta guns that I've seen with near perfect convergence as well. 1966-67 around the time the 22 and 28 were made, was in the early days of mass produced delta gun rectangular CRTs. At that point I think they were more concerned with banging out the high demand rectangular CRTs than perfection. I'm sure more people were still riding on the high of color television considering it was still somewhat pricey.

6. Safety glass! Pull off the safety glass or bonded face of any color set of the 60s and you'll see the glass is heavily smoked. This was to reduce glare on the face of the tube - but severely blocks light! The CCII and other later sets with black matrix tubes eliminated the need for smoked glass as there was less of the possibility of light reflecting off the shadow mask causing glare. Want a brighter picture with better contrast? Just remove the safety glass and you'll get about 10% brighter, but if light hits the tubeface it becomes really difficult to view blacks.

Just my thoughts. Interesting post Chad! Got my brain juices flowing.

[Username1]

Funny thing your thinking about something me and a friend at college did years ago. The town I went to school in back in '89 had one Zenith and one RCA dealer on opposite ends of main street. I was taking some circuit design classes with the Zenith guy, and both of us were interested in the picture detail and clarity difference on the Zenith and RCA sets. At the time back when I knew all the design analysis math, the both of us set out to figure it out. What we found was in the circuits we put through the math, was that in the Zenith circuits they matched stage impedance better. Also what is commonly called "bounce" which is a minor second order response was higher in the Zenith sets. On a Square wave, bounce looks like overshoot on the transition edge of the square. On a lot of sets the Sharpness knob produces the same effect on screen when you watch the edge of white lettering that seems to have a dark trailing edge on crt sets. Now since we only did the math, and did not correct or rebuild, modify circuits all I can say is that the math seemed to indicate impedance matching, and since there was less picture detail and sharpness in the RCA I would say the mismatch was probably inductive. But this is all over 20 years ago from what I remember. Both sets were tube, and models we both had access to at the time.
The math involved reflected impedance through stages, and bode plots something I don't even remember how to start now a days..... Thinking about it now, I bet the RCA sets had more negative feedback to prevent the second order response. I bet now that there is a lot of computer based design analysis software, that this would be a good thing to explore. Old RCA tube manuals had a lot of the information in there to add the tube characteristics to the design software. You kinda have to think of the tube as a MOSFET because tubes are not current amps, a lot like the mosfet.

[Username1]

jpdylon makes a few good points about how IC's made for much better circuits. We picked similar sets with tube circuits, and obviously did not do the entire set. Many of the better designed Zenith circuits also helped make a better picture overall. Now that I have completed those courses there are lots of things we did not know yet, like noise, distortion, PS regulation, and the DC Restoration, and coupling of stages, especially in the convergence circuits. Also you have to consider one thing not discussed; What was acceptable and its true that people were no the "high" of having color, and if the color level was too high or mis-adjusted, then were people really going to notice if the red or blue was hanging out around the edges?

[Username1]

In that college town we also had a very high end stereo store, and I remember in one of the magazines they sold in there, I read a story about some guy that had a problem with the design of some amplifier. It was a review magazine, and they were very thorough. They sent square wave through the amp, and these guys redesigned the amp and changed a lot of the design parameters and produced a better amp and then the company began selling that amp with a letter designation after the model. It was really high end like $1000. single channel, you had to buy 2 for stereo. But that story was one of the things that got us on the tv circuit thing. and although to the naked ear you could not tell the difference between the original and the redesigned amp, the difference was all math, and what the O'scope showed. Funny thing was a lot of low end amps sounded really good on higher end speakers. Same is true for the tv's the display tube did make all the difference, one thing we did not get into, so maybe Zenith had a better output device, and designed accordingly.

[andy]

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[miniman82]

Originally Posted by jpdylon

Older sets with DC restoration like the CTC-2b and CTC 100 had double shunt regulators in the high voltage supply for much better stability and a brighter picture - but again it was more expensive to make.

The CT-100 schematic begs to differ...

http://www.antiqueradio.org/RCACT-10...Service_Manual

[jpdylon]

Originally Posted by miniman82

The CT-100 schematic begs to differ...

http://www.antiqueradio.org/RCACT-10...Service_Manual

You're right. I was under the impression form memory there were two. Guess not!

I type corrected.

[ctc17]

My ctc16, 17 and 35 all have excellent pictures Bright color, contrast and brightness.

Setup is very critical. AGC can make it washed out.

That Zenith should be a black matrix tube set and that can make a big difference as well as all the above mentioned reasons.

[kx250rider]

I've put a Zenith 23V Chromacolor tube into CTC-38s, and it's a world of difference! Part of the problem is the RCA non-glare glass, which diffuses the CRT's light output and blurs the picture slightly, along with diffusing the room lighting.

Charles

[ctc17]

Give this a try Chad, feed you composite video signal from your source directly into Pin 7 of the 6LF8 1st video amp. Its the tube closest to the middle of the chassis in the IF strip. (im assuming its basically the same as the 17 and 35)
You can pull the tube and stick a small piece of telephone wire in next to the pin and replace the tube without removing the IF shield.
You should get a nice colorful good contrast picture with no noise.
If you see a drastic improvement something in the front end is off.

I just spent 2 weeks performing an IF alignment on a 35 and am very familiar with the front end.

On the convergence...test every part on the convergence board. If you want it to be good and not drift the selenium diode pack must go. Standard 1 amp rectifier diodes work find. I have been able to get mind good except the last inch all the way around. It all depends on how much time you are willing to spend.

Can you post some pictures?

[Chad Hauris]

Thanks a lot for that advice! I will definitely try it.

When I first tested the set I had just a blank raster and found that one of the IF tubes was totally bad...I replaced it and then was able to get a picture.

I was thinking about just injecting video directly and completely bypassing the tuner/IF stages as I just have used a DTV converter or VCR with the set. I would just need to build a driver amplifier for the audio like using a 6AV6 or 12AX7 tube. I wasn't sure though where the best place to inject the video was but I will try putting it in to the place you said.

I'll also try replacing those diodes on the convergence board too.

http://www.youtube.com/watch?v=z6ll5ByKAiU

Here is a video I made of the set. At about 5:41 you can see the picture quality. It just sort of has a hazy quality to it.

[DaveWM]

if if has a cathode bypass cap on the video out 12BY7, check that. looks like a 50uf for better contrast. there is also a sharpness pot, did you try that?

[andy]

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[jpdylon]

definitely purity troubles at the bottom right corner.

The picture looks washed out. Lack of video amplification maybe, or poor frequency response? Looks as though the chroma is way too strong too. How does the contrast compare on the B&W picture?

A weak crt would give that chalky appearance, might check that first.