Zenith 23XC36 Blue G2 pot wiring

[Carmine]

All,

Recently took possession of the aforementioned set. It works, but I am unable to do a proper gray-scale set up because the blue G2 pot has a short.

I removed a pot from a junk chassis. Problem is, I can't be sure of the notes I took to wire the new pot in. The only thing I can be sure of is the center terminal is white w/blue tracer. Might anyone have/post that section of a schematic that would show how the G2 blue pot is wired?

Knowing the values would also be nice just so I could be sure the new-old pot is good.

[ctc17]

You had another thread on this set I scanned it and posted it in. It should still be there

[Carmine]

Originally Posted by ctc17

You had another thread on this set I scanned it and posted it in. It should still be there

You are correct, I have posted about this chassis before, however this is a different set. The schematic section you posted unfortunately doesn't show the drive controls.

The set I posted about earlier this year was a ratty-looking non-remote set. I couldn't get too deep into troubleshooting because I needed to get it put away at the bottom of a stack in my basement. If it wasn't just a tube change, I didn't have time for it.

However, I just purchased this set in semi-working condition. I'm planning a chassis transplant on the Zenith Emperor which can be seen just behind it. (The Emperor chassis has innumerable hacks in a number of sections.I'm amazed it never caught fire.) The crazy thing is, a few years back I bought an empty cabinet at an estate sale. It has been serving as a DVD-and-junk storage cabinet in my living room. Now I actually have the correct set to fill the cabinet, but it already has a very good cabinet, plus I need the guts for my Emperor. That will leave me with two identical empty cabinets.

Scene in my living room this morning:

[ctc17]

Pm me you email and ill do it in a bit

[Carmine]

Thanks... I found the old threads about the hacked Emperor and the empty cabinet

My how time flies.

[ctc17]

Did you get it? Sometimes i wonder about email

[Carmine]

Originally Posted by ctc17

Did you get it? Sometimes i wonder about email

Yes I did. Thank you very much.

[Carmine]

Ok, I was able to wire in the new/old screen control. I set all three controls about mid-way and turned it on. It came up with colored snow, about what I expected. It ran for about three minutes while I connected a DVD input. Suddenly I heard a loud "snap", and the screen got slighly darker, but otherwise everything continued as normal.

I went around the back to begin adjusting screen controls and found that none had any effect on the picture. Tipped it back on its side, expecting to see something charred where I had just been working, but nothing stood out in the area. I moved wires around, making sure nothing was crossed.

Set it back up, popped in a DVD and it has a better picture than it did before I changed the blue pot (screen was either overdriven with blue, or lacked blue). However, the screen controls still have no effect.

Attached is a photo. It shows green, but that is basically what it should show. (As you can see from the DVD player) DVD didn't want to respond to the remote, and I was too tired to wait for a fleshtone scene.

Any idea where I should start next to get the controls working again?

[ctc17]

its a real simple circuit. All 3 pots are tied together. 300 volts on one side and 1200 on the other. Verify those are there then go from there.
Maybe the 1200v boost is gone, maybe r198 went open. Easy to check. You really need a meter that goes up to 5kv to be safe.

[Carmine]

It may seem a simple circuit to you, but this kind of troubleshooting is really stretching my electronic muscles. Most of my electronic ability comes from a H.S. class, and automotive electrical systems (which is much easier; wires are color coded and usually can't kill you). However, I get a lot of satisfaction from this, so I imagine it's fun in a perverse way... lol.

Anyway, thank you for suggesting the idea of R198 being open (although on the scan it almost looks like it could be R398. Must be an artifact). I followed an orange wire that "input" at one of the blue pot's outside terminals (How are these numbered?). This wire led me to a large resistor. My H.S. electronics knowledge told me a 1 watt resistor would be on the fat side, and that the color code works out brown-red-yellow-silver, or 1200 ohms-10% tolerance.



On my 2K scale, it shows open. It doesn't appear damaged on the outside. However the photofact shows "120k 1W". Am I being too literal to read this as "120 thousand"? In my book, this should be 1.2K ohms. I shall replace this resistor, and start the TV on its side, observing the area surrounding my new work just in case a wire is pinched... Although I know this TV chassis is relatively HUGE and easy to work on, it's still much smaller than the work I am accustomed to doing repairing auto wiring.

[Norbert]

That resistor is indeed 120K ohm and is correct per the color code. Brown=1, Red=2, Yellow=(4) trailing zeros thus 120,000 ohms and the size is indeed a 1watt unit, so the schematic and the chassis are coinciding with each other.
Try increasing the multiplier on the meter to get a reading. It would be highly unusual to find an open carbon resistor.

Just to confirm, do you have the .001uF disc cap installed between the center lug and the outside leg of the pot? (this cap would be attached to the same outside leg which has the wire that goes over to the 120k resistor)

Considering the controls had effect prior to the repair, is there a possibility of a solder blob that fell shorting something out?

Also, look at those spark gaps and did you modify or bend any of them out of the way? Lots of times on the Zenith's, the spark gap is nothing more than a piece of light gauge 2 conductor "zip wire" (like that of a lamp cord) with the one end clipped off, you'll see the wire just sticking up out in the chassis making you wonder what it does. Just so you know, with the end of the wire clipped off (like it should be), the two wires remain close together thus creating a spark gap. It's easy for them to get overlooked. A shorted spark gap can kill your G2 voltage rendering the control non functional.

Do you have B boost of 1200V on the other leg of the pot?

[Carmine]

Sorry been busy, and could only wish for more time to work in it. Sigh.

Anyway, I don't know why I made that error on the color code. Maybe it was because I was reading it @ 6 AM. It measured at 134.4k on my 200k scale.

Norbert, I did verify the tiny little disc cap was not only connected to the center connection of the blue pot, but also eventually ended up at the 120K resistor.

As for a falling solder blob, I doubt it would effect the G2 controls. I did the work with the set on its side, so any blobs would have fallen into other areas of the set. If a new problem occured, that would make sense as a cause.

I cannot find the spark gaps... Are they visible from the top of the chassis, or underneath? What purpose do they serve?

I'll have to get another meter to read those voltages. Mine only goes to 1000v.

[wa2ise]

Originally Posted by Carmine

.

I cannot find the spark gaps... Are they visible from the top of the chassis, or underneath? What purpose do they serve?

Some spark gaps look like small ceramic caps. Some of those have slots cut into them. Some ceramic caps have an extension up top (the edge of the disc away from the leads) with a slot cut in it. Other spark gaps are formed inside the CRT tube socket.

Spark gaps keep some of the CRT electrodes from getting overvoltaged, or to keep stray high voltage from the CRT from overvolting the CRT drive circuits. .

[Carmine]

Since my meter only goes to 1000v, I used a HV probe just to see if it would read "anything" on either side of the pots. I do, although since both sides appear to move the needle roughly the same amount, I would guess the 1200v side is low?

Still haven't found the spark gaps.

I also noticed something about the schematic voltages from the pots to the CRT. The red shows 870v, blue 940v and green 840v. Red being the least efficient phosper, shouldn't it have the highest voltage?

[ctc17]

Sounds like the boost voltage is missing. Check the parts on the flyback board. Maybe the boost diode or resistor popped.