Zenith 4-tube hybrid Chromacolor questions

[RetroHacker]

Well, I've been out of the "game" for a while, haven't been seriously collecting TV's, although I still have my collection... just haven't been actively hunting them in a while. But, last week, I found this Zenith Chromacolor out at the curb, and it followed me home.

The set is fairly dusty, but not bad overall. No major burn marks or signs of horrible failures, so I powered it up. Sound works, no picture. I can hear the horizontal section running, but it sounds "off", probably not the right frequency. I'm getting about 700v on the output of the flyback once the tubes warm up.

I know these sets are famous for burning up triplers, and it looks like the focus divider has been replaced before. Just for grins, I tried swapping out the 6U10 since I had spares, and no change.

Now, I'm not sure which model this actually is. The back cover, on the inside, lists 19DC10,11, 12, 18, 21, 22, but nothing on the chassis itself clues me in as to which of those it actually is. How do I tell? Also, I don't have the schematic for it yet, but I'm probably going to go dig that up soon.

Any advice? I haven't really worked on one of these hybrid sets in a long, long time.

-Ian

[DaveWM]

hmm, I would start with the basics, see if you have the correct drive to the HO tube. I you have a TV analyst that is what I would do first (plate drive).

[Jon A.]

Helluva nice curb find. Space Command set? I believe I see a tuner tab wheel for stopping on or skipping channels in there. In regards to the chassis number, did you look on the right side, adjacent to that big probe? Saw it there on a hybrid once before.

[old_coot88]

Originally Posted by RetroHacker

...I'm getting about 700v on the output of the flyback once the tubes warm up.

What point are you measuring that at? Where the flyback feeds the tripler? If so, how did you avoid frying the meter?
The fly should be outputting a peak pulse of 8 - 9 KV there (unless the tripler is shorted and lugging down the fly).

To tell if the tripler is bad, disconnect the fly from the tripler, and see if you can draw a healthy 'air arc' off the disconnected lead, using an insulated screwdriver. The arc should be about a third the length of the arc you'd get off a non-triplered
flyback.. or about 3/8 inch or so.

I'm betting on a bad tripler.

[tvcollector]

I wonder what the front of the set looks like..

[RetroHacker]

Originally Posted by old_coot88

What point are you measuring that at? Where the flyback feeds the tripler? If so, how did you avoid frying the meter?

Correct - I haven't disconnected the tripler, I was just measuring there. I was using a high voltage probe, it divides the voltage by 1000 so I can get a reading on my meter without blowing it sky high. So, my reading of .717 volts would indicate 717 volts at that point.

I'll disconnect the tripler and see if I can get healthy voltage there.

Another thing I noticed is that the damper socket is pretty cruddy looking, it'll probably have to be replaced, but I don't have any 12 pin sockets on hand.

I'll try to get a picture of the front of the set soon. It's not a Space Command, sadly. I would love to find one of those!

-Ian

[Jon A.]

Oops, bad advice, never mind.

[jr_tech]

Originally Posted by Jon A.

Not trying to insult anyone's intelligence here as errors of omission happen, but make sure that's a *grounded* insulated screwdriver.

He said "air arc", which to me implies that the screwdriver is not grounded. A grounded screwdriver could short out the B+.

jr

[old_coot88]

Originally Posted by Jon A.

Not trying to insult anyone's intelligence here as errors of omission happen, but make sure that's a *grounded* insulated screwdriver.

Nein, Nyet! No need to ground it. Simply hold the screwdriver by the insulated handle. The arc is drawn by the innate capacitance of the screwdriver 'free floating' in the air. The screwdriver is in effect 'grounded' capacitively to RF.
Though this reactive capacitance is very small, the arc occurs because it's driven by high level RF spikes (the 'fly-back' spikes).

I b'lieve maybe you're thinking of grounding a CRT to discharge it.

Originally Posted by RetroHacker

...I was using a high voltage probe, it divides the voltage by 1000 so I can get a reading on my meter without blowing it sky high. So, my reading of .717 volts would indicate 717 volts at that point.

Actually that's just reading the averaged-out DC level at the output terminal of the fly. It won't show the RF spikes riding on top the DC. Those are what the tripler uses to generate the HV for the CRT (stacked on top of the 717 volts).

[rca2000]

Originally Posted by jr_tech

He said "air arc", which to me implies that the screwdriver is not grounded. A grounded screwdriver could short out the B+.

jr

And KILL the fly too..NOT easily replaceable now--either...

Those flys seem to hold up--at least in my case--so not sure too many replacements are floating around...

[Jon A.]

Originally Posted by old_coot88

Nein, Nyet! No need to ground it. Simply hold the screwdriver by the insulated handle. The arc is drawn by the innate capacitance of the screwdriver 'free floating' in the air. The screwdriver is in effect 'grounded' capacitively to RF.
Though this reactive capacitance is very small, the arc occurs because it's driven by high level RF spikes (the 'fly-back' spikes).

I b'lieve maybe you're thinking of grounding a CRT to discharge it.

Hm, I didn't know that a screwdriver that isn't wired to ground could cause an arc at all. I was thinking of the screwdriver being held close to but not actually touching the HV lead.

[old_coot88]

Originally Posted by Jon A.

...I was thinking of the screwdriver being held close to but not actually touching the HV lead.

That's exactly what you do with the ungrounded screwdriver. Once the arc is struck, draw the screwdriver slowly away and see how long of an air arc it'll pull.
Remember this is RF. No air arc is possible with rectified (DC) high voltage.

[Jon A.]

Originally Posted by old_coot88

That's exactly what you do with the ungrounded screwdriver. Once the arc is struck, draw the screwdriver slowly away and see how long of an air arc it'll pull.
Remember this is RF. No air arc is possible with rectified (DC) high voltage.

Ah, gotcha.

[zeno]

If the damper socket looks fried they went left & right on these.
Part # is 78-1888-01 . Also the big disc cap under it often burned.
To replace with any 12 pin Zenith socket drill out the center rivet,
remove unused pins & the 2 mounting pins. Remount with old
grommets & rivets since you will never find them.
If you have 700 V plate odds are tripler or damper. FBT's never
went on theses. Only changed one out of what must have been
over 1,000 I worked on. It was a BIG kit.
Jon is correct, the chassis # often stamped by HV adjust control.
When done clean the mung from the anode, it will arc on the first
humid day.
Always nice to see my old "friends" being worked on.

73 Zeno

[zeno]

Originally Posted by Jon A.

Helluva nice curb find. Space Command set? I believe I see a tuner tab wheel for stopping on or skipping channels in there. In regards to the chassis number, did you look on the right side, adjacent to that big probe? Saw it there on a hybrid once before.

In this case the switch is either / or / and the UHF B+ switch,
AFT defeat switch, dial light switch. Or it may be blank !!

73 Zeno