1976: 19” Zenith Chromacolor II (chassis 19HC48)
 

This is my first Zenith. I rescued it from an electronics disposal center. I am very impressed with the sharpness of the focus and dot pitch of this TV. I now understand the appreciation for the Chromacolor II delta gun picture tubes. I need to adjust the dynamic convergence, but when I do, the picture will look even more sharp.

One issue that I am having with it is the contrast seems to flicker between a very low contrast and the contrast that I have the TV’s “picture” dial set to. What is causing the contrast to cut out like this? If I adjust the picture dial, it typically causes the contrast to flicker back to normal.

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Look on the 9-88 video module. That long white pkg gets cold joints
& the pix will flicker. When ever you see them in a Zenith solder
them. Newer sets have more.
If not that clean & tighten module connections.

73 Zeno:smoke:
LFOD !

Thanks, I will give it a try. I spent some time touching up convergence and am very impressed with the vertical sharpness. I can’t believe Zenith’s CRTs had a focus this sharp. The horizontal sharpness needs work though as can be seen from the grid test pattern where the dots at the center of the grids have streaking to the right. Even if chroma gain is set to black and white, the streaking persists. I know Zenith used electrolytic capacitors to AC couple the video signal path. So I will try replacing those with polypropylene film capacitors. Any other idea what to check for this streaking?

I dunno about this circuit in particular but, in most cases, this is a video amp signal with insufficient bandwidth (this is common with newer TV's with outrageously high 15k video amp load resistors).
Sometimes makers opts for lower bandwidth for making noisy or grainy images less obtrusive to users.

I have an 16" Toshiba from 80's with those 15k video amp load; I put another 15k in // and readusted bias etc. Far better sharpness from the video input (I added a S-Video input). But if we want too high video response, things like Y delay coil ringing starts to appears, so a compromise is needed, since all of this is calculated for the reduced bandwidth from a composite signal.

Ah, I see some probably ringing also, besides the probably response limitation.

Also, if you are using antenna input (I suppose), the IF response can influence to a certain point.

That is often seen with cross hatch. Try backing off the brite, contrast
& sharpness. There are also 1 or 2 more video controls. One on
the 9-88 & another low on the chassis in SOME models.
On the 150-190 IF marked C-2 adust. Try bumping them but remember where they were.

Zeno:smoke:

I tried adjusting brightness, contrast, peak pix (sharpness), and G2’s. The fact that sharpness can be adjusted from smearing at full counterclockwise to ringing at full clockwise and the streaking doesn’t go away, combined with the fact that according to the circuit, video peaking is in the luma module’s IC, seems to imply that the streaking is occurring in the video signal path before the luma module’s IC. The C2 adjust is a pot that controls the detector level, right?

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I adjusted the C2 adjust using the Zenith service manual’s guidelines so that the composite video signal at test point C1 is 8.1V DC when the AGC test point E is grounded and the tuner is set to an unused channel. The streaking persists.

So I used my cheap $50 handheld oscilloscope from AliExpress to view the composite signal at test point C1 for one horizontal line of the grid test pattern. It seems to show ringing distortion, but it may just be an artifact of my crappy handheld scope. I will confirm tomorrow using a real oscilloscope.

If the ringing is real, then what does it mean? Does it mean the only way to fix this is with an IF alignment?

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This may be completely normal and you are just looking for a cleaner response than is possible when the signal goes through an IF filter with its necessary sound traps.

Check with ordinary program material and see if there is anything terribly objectionable.

People's memories of how analog TV looked have faded these days when they have gotten used to HDTV and computer monitors.

Edit: besides sound traps, the color subcarrier trap can cause some luminance ringing, depending on its bandwidth. Wider bandwidth = less rigning but more smear, and vice-versa - it's a design choice that was made for regular program material, not games or test patterns.

I don’t have this ringing distortion on my 1967 Magnavox T931, which is a fully tube VHF, IF, luma, and chroma demod chassis. So I am confident I can get less luma distortion on a solid state chassis. I know it isn’t going to look like an HDTV.

Aside from this ringing distortion, everything else about this Chromacolor II is stellar! The dot pitch and the focus are better than all of my other consumer SDTVs from the 1960s through 2000s. Why did TVs take a step backwards from the Chromacolor II’s dot pitch? Even Zenith seems to have taken a step backwards after the Chromacolor II.

Check fine tuning. Check for bad components around the delay line.
Then if nothing wrong there, consider full alignment.

Fine tuning and AFT are not the cause. I checked and re-checked. I do see an 0.47uF electrolytic capacitor used to AC couple the luma signal immediately after the delay line. My 1960s tube type TVs use a gumdrop polyester/polypropylene film capacitor in the equivalent place in the luma signal path. I will check when I get home.

I have a B&K415 for doing RF, IF, and chroma alignments. I have also studied B&K’s user manual, which is 100 pages long. It seems intimidating, but I will learn how to do a proper alignment eventually.