Zenith 14DC16 Colour question/problem - Videokarma.org TV - Video

maxhifi · #1 12-20-2010, 01:42 AM

Hi All, new forum member here.

I have a Zenith 14DC16 portable hybrid TV set with single button space commander remote control. Have had it since 1994, when I picked it up for free. I have used it fairly sparingly over that time, and the only repair it has required, has been a replacement of both tubes in the VHF tuner.

I have the sams package for the set, and basic knowledge about how TVs work. Audio/radio have been hobbies of mine for years, but I have only done minor repair work on TVs.

In general the set is in excellent working order - the picture is sharp and free of noise, and the picture geometry is more than decent for such an old TV. The picture is bright, and when the image is a solid colour, it is consistent over the entire screen area.

Question is as follows... when watching certain shows, especially The Simpsons, it's extremely hard to get the tint to stay the same over the course of a show. In one scene, they look yellow, then the next a bit green, and in another a bit orange. In all cases I can adjust it to the right colour using the tint control, but then later on it will be different. Not all programs seem to show this effect, but when it is happening it is really annoying, and detracts from the otherwise perfect operation of the set.

I am targeting the 3EJ7 burst amplifier, and the 9MN8 G-Y, R-Y, B-Y amplifier as possible culprits. If either of these tubes are weak, or operating in some non-linear region, I can see how this would upset the colour. I have no tubes to use as substitutes, so thought I'd see if I could get a second opinion before I start ordring parts. There appears to be no electrolytic capacitors in the colour circuitry, and I am really at a loss as to what else it could be.

Any input from someone experienced with TVs of this era would be greatly appreciated.

ctc17 · #2 12-20-2010, 01:58 AM

Does that set have the quad detector diode tube in the color circuit? Also GH8s and KT8s cause issues like this. The bright solid colors in the simpsons really test the old color sets. Their yellowish color seems to be one of the hardest colors and results in red bleed on most of the old sets I see.

maxhifi · #3 12-20-2010, 02:06 AM

Thank you for the quick reply!

So it might be something I just need to live with.

Colour section is hybrid, as follows

3EJ7 pentode Burst amp --> transistor amplifier ---> IC Demodulator ---> 9MN8 tripple triode voltage amp --> RGB grids of picture tube.

And add a number of probably germanium diodes to above

old_tv_nut · #4 12-20-2010, 09:07 AM

Just guessing wildly, but:

Could it be the burst gating pulse is too wide and the color sync is picking up the left edge of the video? If you record and play back some Simpsons, you could try seeing if the variation occurs according to the color at the left edge of the picture.
If this set has a horizontal hold control, try turning it slightly to see if there is a hue variation.

ctc17 · #5 12-20-2010, 12:03 PM

If you want to try a 3EJ7 I can send you one.

Chroma alignment is something I want to work on. I have a great magazine article going through the alignment of a brand new Zenith and how much better it can be made.

I have 2 identical rca ctc35 chassis and one displays the simpsons perfect and the other has the red bleed. Thats using the same crt, yoke, convergence etc...so I know its the chassis. I tried switching the tubes between the two chassis and that didnt fix it. Its an alignment or cap issue somewhere.

maxhifi · #6 12-20-2010, 11:07 PM

old_tv_nut:

Thank you for the suggestion!
I am using the TV with a digital cable box - it's strange, the horizontal hold control has almost no effect on anything. When I first got the TV, and was receiving over the air signals, the horizontal hold control was fairly touchy. So far The Simpsons hasn't been on, so I haven't had an opportunity to see what it does with colour. Next time it comes on, I am going to give that idea a try.

ctc17:

Your website is very interesting! I clicked on the link hoping to see what a ctc35 looks like and ended up spending 45 minutes looking at photos of tvs and other stuff. Drove through Death Valley for the first time myself this summer. Anyhow, would appreciate the 3EJ7. Last night I dug through hundreds of tubes, without any success finding one! I think it's at least worth a shot. Another thing about this TV, is my TV-7 tube tester is incapable of testing *any* of the tubes in the set! That's usually my first step in repairing anything with tubes in it.

ctc17 · #7 12-20-2010, 11:18 PM

Does your tube tester show a 6ej7? Use the settings for that and reduce the filament voltage to 3.

radiotvnut · #8 12-20-2010, 11:21 PM

If you want to test later TV tubes, get either a Sencore "mighty mite" or one of the many B&K tube testers in the 600 or 700 series. These will test the later TV tubes, as well as older octal, loktal, and miniature radio and amplifier tubes. Also, these testers have a "grid emission" test; which, is very important for testing TV tubes. These testers were made right up to the end of tube TV production and between one of these and your TV-7, you should be able to test most any tube used in consumer electronics.

maxhifi · #9 12-20-2010, 11:41 PM

ctc17:

Why didn't I think of that! Yes, the tester does list 6EJ7. Minimum good reading is 39 -- my tube reads 10!

radiotvnut:

Passed up on one of those B&K testers about 15 years ago and regretted it... this Zenith is my only tube TV, so haven't really had a reason to seek one out, but will keep my eyes open.

ctc17 · #10 12-21-2010, 12:16 AM

Ill try and dig one out in the morning. Dont get to excited, good chance it wont make any difference.

maxhifi · #11 12-21-2010, 11:56 AM

ctc17:
It's worth a shot... Not expecting miracles. I thought the TV was worn out before I replaced the 2HA5 in the tuner. Now am adressing the less significant issues.

ctc17 · #12 12-21-2010, 02:10 PM

well bad news...I just pulled out the 4 I have and all of them test bad. 20-40%, I know its not the tester because I tried a new 6EJ and it checks 75%.

Grab one off ebay, they are only a few dollars, you can get 5 new ones for $10 so its not a rare tube at all.

maxhifi · #13 12-21-2010, 02:53 PM

hey thanks for trying!
I think i will pick up a full set of tubes for the set on ebay, minus the horizontal output tube. It looks like most of them are pretty cheap and easy to find.

bgadow · #14 12-22-2010, 10:12 PM

For what it's worth, I'm practically up to my ears in NOS TV tubes, if you (or anyone) is ever looking for something. Just send a PM with what you need and I'll see what I can put together.

maxhifi · #15 12-23-2010, 02:57 PM

Thank you for the offer! I wish I had read that yesterday, I just placed an order for a full set of tubes for my TV, minus the horizontal output ($$$)

I have another question about this TV. What is the "peak pix" control supposed to be adjusting? On my TV, when you turn it down, the TV gets blurry, and the picture is best at the opposite extreme.