Zenith 14DC16 Colour question/problem

[ctc17]

Its just that video peaking. Its like sharping pictures on the computer. I always leave them all the way up.

Does it use a xLF6 for the output tube?

[maxhifi]

it has the option of 20LF6 OR 26LX6

Currently the set has a 26LX6, with red printing on it, marked Zenith Service, with a 1978 date code. It appears as though the horizontal output, damper, and vertical output were changed when the set was around 5 years old. The set has no symptoms of a bad horizontal output, so I will hold off until either it needs it or I hav e reason to believe it will improve performance

[maxhifi]

Update on this set:

Have been brushing myself up on TV theory, and discovered some peculiarities about this TV. For one thing, it has RGB screen controls, but not drive controls. It seems to me that this is a design compromise, which makes it difficult to compensate for uneven aging of the CRT cathodes. Specifically, when the screen controls are set for proper white balance, the tint control has to be adjusted fairly far off centre towards red, in order to get accurate skin tones. Is this really a compromised design, or is there another way to go about setup?

Remembered I had another tube tester packed away. It is an EICO model 667, and once I found a comprehensive setting chart online, I was able to test all of the tubes in the TV.

Weak tubes include the final video amplifier, and also the 10Z10, as well as the colour burst oscillator. The only tube which tests in the "red" zone of the tube tester is the aforementioned colour burst oscillator. Luckily the horizontal output tests well into the green zone, at 120%

Out of curiosity I tested the old tuner tubes, 2HA5 and 4GJ7, and both tested in the "yellow" zone of the tube tester. In the set however, they completely ruin performance, the picture becomes overwhelmed with noise and snow. It makes me wonder what effect new tubes will have in other positions which are currently reading weak good, or marginal.

The set is displaying a few symptoms which seem to indicate a weak CRT, according to what I have read. Will reserve judgment until all small tubes are good, but symptoms are as follows:

1. Green appears a bit weaker than other colours, grass is always a bit browner than on other TVs. Nothing awful, but all the same apparent

2. When contrast control is anything above mid level, text on the screen has a bit of a shadow, “blooming” type appearance. It is still legible, but there is a definite loss of sharpness. This effect is worst with text on a dark background, such as movie credits or TV guide, and does not seem to affect normal video.

[ctc17]

I havent figured that brown grass thing out yet. It seems to be one of the more common chroma issues. When you use a true ntsc color generator the yellow and green are all screwed up. I have 3 identical zenith consoles and one of them does it along with several others so I know its not the chassis design.
Its on the list of issues to figure out but it could be several months. My first step was going to be to sweep the bandpass and see what it looks like. The tint being off center could be a weak osc tube, cap or alignment issue. Most of the sets we deal with seem to have the tint all the way to one end. If your having the same green issue I am you will never get it right.

The Z10s always seem weak. The only difference you will notice is louder sound and maybe less buzz if you replace that one.

[maxhifi]

Of the three chromacolor consoles, have you ruled out that it could be a problem with the CRT?

I am inclined to think in my case it is not, in my case, because the RGB screen controls are in the middle of their rotation in order to get good white balance. Nothing is pegged. I do not have a CRT tester though, so can't be sure.

Once my set of new tubes arrive, I am going to put some more effort into this. I think in the mean time I need to do more reading about how NTSC colour works.

[ctc17]

Yes, its defiantly the chassis. I exchanged the chassis of two of them and the brown grass issue followed the chassis.

[maxhifi]

Tubes for the set showed up last week. New colour burst oscillator seems to have cured problem. Simpsons look great now, tint control is a bit more into the red than I would like to see, but colour is stable and minimal red bleed. CTC17 has a great video about how to dial this out, and at this time I don't have the equipment or experience to do a colour bandpass alignment, but I feel as though I understand the issue a lot better than when I started into this!

Right now am in the process of acquiring an RCA 8PT, and will cut my teeth aligning that before I try it with a colour set. Luckily the Zenith has solid state IF amplifiers, so I didn't throw those out of whack by changing the tubes!

Set has a new problem. Everything is fine with brightness at normal, but when you crank it all the way up, the picture gets larger, and less bright. Why would brightness affect vertical size? Don't remember it doing this before I changed the tubes.

In general though it's back in top shape, have been watching it every day again.

[compucat]

Originally Posted by maxhifi

Set has a new problem. Everything is fine with brightness at normal, but when you crank it all the way up, the picture gets larger, and less bright. Why would brightness affect vertical size? Don't remember it doing this before I changed the tubes.

In general though it's back in top shape, have been watching it every day again.

The problem you are describing is called blooming and is usually caused by a weak high voltage rectifier tube. That tube is often located inside the metal high voltage cage with the flyback transformer. I have seen these tubes get so weak that when the brightness is turned up the picture disappears completely and the screen goes dark. Zeniths are great sets built for long reliable service. Keep at it and when you get the bugs worked out it will be a superb performing set.

[maxhifi]

Thanks for the explanation! I actually just changed the zenith HV rectifier to a NOS GE, which may be what brought on the problem. I bought a set of tubes for the set, and changed all of them... thought it couldn't hurt after nearly 40 years, even though about half of them tested well into the green on my EICO 667. I'll put the old one back in tomorrow. I hope to keep the TV going until the picture tube becomes too weak to
be useful... at that point I'll have to search for a replacement tube, or a lower hour TV to get one from, which looks like quite a challenge in this day and age. I always assumed I could just have it rebuilt, when needed, so never paid much attention to securing a replacement.

[old_coot88]

To expand a bit on the blooming problem (no pun intended), as the HV drops, it is "attracting" and accelerating the electron beam less and less, allowing the deflection angle to increase to the yoke's full deflection ability. Raster size is actually a balance between the HV and the yoke's deflection power. Since color CRTs draw much higher beam current than B&Ws, they require a much "stiffer", regulated HV supply. Bill(oc)

[maxhifi]

I put the Zenith tube back... and problem solved. I wonder why the new tube was weak? This Zenith does not have a regulated HV supply, maybe it can get away with it on account of its small size?

Simpsons are on now... I can't believe how good the colour is with new tubes... set has a new lease on life.

[maxhifi]

This TV is going to drive me crazy, but not before I get it sorted out. I've noticed a new problem with it. On some stations, when there's a lot of white in the picture, it tends to flicker... it's especially noticible with white text, and at the top and bottom edges of the picture when there's a 16x9 program playing. I don't remember it always doing this.

[maxhifi]

Put the original 6MV8 Zenith sync separator back, and worked the AGC control back and forth a bunch of times to clean the tracks a bit. TV seems back to normal, or at least a lot better. Have learned a lesson here about not changing tubes all at once!

[mwplefty]

Originally Posted by maxhifi

Put the original 6MV8 Zenith sync separator back, and worked the AGC control back and forth a bunch of times to clean the tracks a bit. TV seems back to normal, or at least a lot better. Have learned a lesson here about not changing tubes all at once!

I have a 1972 Zenith 19CC19 (there's a picture in my VK photo album). I'm assuming that your set is a 1973 model judging by the chassis number.

[maxhifi]

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