Finally I have one Zenith!

[DavGoodlin]

Same 12A10C50 chassis...wow. Then its a heavy beast too. I had a Zenith 1968 15" once, a true luggable with a metal cabinet. My Aunt had a 1970 15" Sylvania color as a bedroom set, of course not as heavy. I don't think I ever had to work on it... I saw a few of those RCA CTC22 15" sets and a Mag or two in the same category. Yours beats them all for true quality.

Thought I saw a transformer in your picture, Zenith did make a few "portable" tube sets without transformers 14CC15 IIRC. A tube Zenith that size is just too cool! For the jitters, the Hor osc tube 6U10?, or I have also seen HV arcing cause this.
My 16" set had an arcing puncture through the HV rectifier tube cup, so I placed it in the dishwasher to clean up, RTV silicone the hole and then put a 1/4" rubber disc under it.

The problem also seems like a bad electrolytic. Sometimes that drops the voltage. The B+ voltage drops as the HV comes up only because that stage draws significant current, making the lack of filtering even worse.

The issue could also be a bad ground on one of the terminal strips. I use a chopstick and poke at the end terminals, ones grounded to the chassis. I have seen this but not often compared to the number of Zeniths I worked on.

[Komet]

Thank you for the advices

You have right, I found a loss of capacity in the first electrolytic capacitor on the power supply...

Section 1: nominal 270, measured 95
Section 2: nominal 150, measured 70

Probably the jitter was caused by ripple from bad filter capacitor

[zenithfan1]

Yep, that'll do it every time. I was thinking a bad cap too when you described what it was doing. I look forward to seeing that little cutie all cleaned up with a good picture.

[dieseljeep]

I think the fact still remains, that you are running the set @ 50HZ instead of 60HZ, even though you are using a transformer to reduce 230 volts to 120 volts. In past practice, the electrolytics, had to be increased in value. I guess, that's the reason the US adopted 60 HZ, as our design standard. You probably have to increase the values of the originals, by at least 20%. That includes the voltage doubling cap, that is before the power supply diodes.

[DavGoodlin]

MMM very good point about the cap values, I also wonder if running 50HZ through a transformer actually increases the line current on the primary to the point where 100 volts would be better than 120.

Of course the turns ratio determines secondary voltage and that would be lower. Im sure this has been discuss here somewhere

[snelson903]

man that is a cool little tube zenith , i forget about the early portable color sets.

[Komet]

I have changed the main capacitor; and as dieseljeep suggested, I increased the value of capacity. I found some NOS capacitors at my lab, made in Italy by CREAS under Sprague license... One of these is a four sections, 200-200-75-50uF... I have put in parallel the 200 and 75 sections for use like the original 270uF capacitor, and the 200+50uF instead of the previous 150uF.

Now the set seems work good, in these days I have make a good setup

[Komet]

Ok, good news, CRT is ok

I have some problem with color decoder. Yesterday when I adjusted the color saturation I see the test bars changing color, like if I adjusted the hue control... After a suggestion by the old owner of this Zenith, I inverted the position of the two 6KT8 (1st color amp/cathode follower and sound amp/sound limiter) and now I have the correct chroma when I adjusted the saturation knob.

But seems I have an AGC problem, the saturation changing during watching, probably some tube has gone (like 6HS8 AGC/sync sep.)

[zenithfan1]

Kawaii desu yo! (^_^) Looks good so far, how much does the color saturation change during viewing?

[Komet]

It like if I turned the color knob from half position to full

[Komet]

Mh, I have wrong with the two 6KT8... I inverted again their position and no changes... Checked electrolitic cap on color/AGC but it are still good. When the saturation goes up I see several horizontal colored stripes on the video... Tomorrow I check with scope for ripple on the low voltage power supply. If I don't find anything I buying another 6HS8 or EF184 (burst amp)

[dieseljeep]

Originally Posted by Komet

Mh, I have wrong with the two 6KT8... I inverted again their position and no changes... Checked electrolitic cap on color/AGC but it are still good. When the saturation goes up I see several horizontal colored stripes on the video... Tomorrow I check with scope for ripple on the low voltage power supply. If I don't find anything I buying another 6HS8 or EF184 (burst amp)

I see by you entry's, that all the tubes in your set are six volt heaters. I have a newer model that has series counterparts of your tubes. IE, 4EJ7 and 4HS8 etc. I was having trouble with the tint range. I wasn't sure what coil needed to be adjusted, not having the proper schematic. I ended using a brass screw as a tuning wand. Putting it near the coil, showed me what coil needed touchup.

[Electronic M]

If you feed it a test pattern consisting of vertical color bars that run from the top to the bottom of the screen and then turn the saturation up does the color part of the image look like it has lost horizontal synch while the monochrome image looks fine?
If so then your chroma reference oscillator needs to be adjusted(it should be an adjustable slug tuned coil adjustment).

I'm familiar with chroma ref. osc. issues as I've just finished fighting them on a Silvertone roundy that for the longest time needed 1/2 hour for the color osc. to stabilize....I readjusted that thing half a dozen times, lived with the issue, then resoldered the the chroma board while in for another bug and that finally fixed it(I was actually happy about the PS bug as it led me to fix the annoying color issue I had).

[Komet]

Originally Posted by Electronic M

If you feed it a test pattern consisting of vertical color bars that run from the top to the bottom of the screen and then turn the saturation up does the color part of the image look like it has lost horizontal synch while the monochrome image looks fine?
If so then your chroma reference oscillator needs to be adjusted(it should be an adjustable slug tuned coil adjustment).

Thanks for suggestion - No, the color bars stay perfectly vertically when I turn the saturation up.
This evening I have checked a non polarized electrolytic capacitor in the circuit of the tube 6KT8 "1st chroma bandpass amp" - BINGO! On the capacimeter I read 1,962 uF instead of 1... and the value slowly goes down... replaced with a 1 uF poly, seems work good. Tomorrow I doing some final controls (I hope )

[Komet]

Ok, finally my Zenith works good! I used an economic camera and the screen seems like out of focus... but is all ok!