1967 Zenith 25" (23XC38Z) working - almost - Videokarma.org TV - Video

Adam · #1 04-01-2007, 03:45 AM

With new electrolytics, this set has come back to life. In addition to the bad lytics it had a super weak 3rd IF tube (what was the source of my bad reception I had on this set back when it worked before, that I always thought had something do with the tuners). But the first time I plugged it in nothing came up and I heard this awful noise from the degaussing coil. It turned out the thermistor broke. I bypassed the thermistor and then it worked, but it was in bad need of degaussing. I did what I could with the back end of my soldering gun, but I couldn't completely degauss the corners. It also needs convergance, which I don't want to do until I get the set completely degaussed. The thermistor is 100 ohm/ 1W, where can I get one so I can get the degaussing coils working again? Here's some pics...

wa2ise · #2 04-01-2007, 04:53 PM

Quote:

Originally Posted by Adam

. The thermistor is 100 ohm/ 1W, where can I get one so I can get the degaussing coils working again? Here's some pics...

You might be able to find something usable from a VGA CRT monitor. If your set has the degauss coil in series with the thermistor, and connected to the powerline.

Adam · #3 04-01-2007, 05:08 PM

The degaussing coil is in parallel with the thermistor in between the output of the power transformer and the rectifiers.

orthophonic · #4 04-01-2007, 07:32 PM

Looks like the intro to the Time Tunnel, one of my favorites
as a kid.

Adam · #5 04-01-2007, 09:46 PM

I powered up the degaussing coils directly, and it didn't degauss the tube, it un-degaussed the tube worse, I wonder if the coils were damaged when that thermistor broke. I spent some time degaussing it again with the soldering gun, and got it all except for the bottom left corner. So I went ahead with the convergance. I remember a thread a while back on how to make your own degaussing coil I might try that next weekend. But anyway, here's some pics post convergance. And, yes that was the intro to the time tunnel, I can't get enough of those old shows.

jpdylon · #6 04-01-2007, 09:59 PM

I would give La Mesa TV supply in San Diego a call. I bought a service man's hand held style degauss for about 16 bucks. It has a built in switch on the line cord. Their number is 619-469-4220

If you want to make one, you'll need about 150-200 turns of 18 gage lamp cord, a 3 amp line cord style switch, and splice tape to wrap the wire up with. Make a 10 to 12 inch diameter coil.

Turn the coil on AWAY from the set, and slowly approach the face of the tube. Hold the coil about 6-10 inches from the CRT face and go in a slow circular motion from the inside to the outside. Slowly back up a few inches after each revolution. When your about 4 feet away, slowly move the coil up and away from the CRT when your at the end of a rotation. Only turn off the coil when you at least 4 feet away or more or you will leave purity stains.

I would cut the blue and green guns and cut the AGC so you have a red raster and check your purity. The rings on the CRT neck control the center purity. You will need to loosen the yoke and slide it forward or backward to adjust corner purity

Chad Hauris · #7 04-01-2007, 10:02 PM

For the degaussing effect to work right, the magnetic field needs to decay slowly. THe thermistor does this by decreasing resistance as it heats up, shunting current gradually away from the coil and directly into the rectifier diode. You may have cut off the power to the coil suddenly which can cause worse re-magnetization.

I have just bypassed the degaussing coil in these Zeniths and manually degaussed them with a bulk tape eraser and it worked fine.

If you can get a bulk tape eraser or real external degaussing coil and use it and withdraw it slowly, you should not need to degauss again unless the set is moved.

Checking purity is a very good idea after degaussing...sometimes what looks like a degaussing issue can be cured by adjusting purity. Make sure the purity is set up in the set's final location and that it is not rotated after setting it for best results.

Adam · #8 04-01-2007, 10:15 PM

""For the degaussing effect to work right, the magnetic field needs to decay slowly. THe thermistor does this by decreasing resistance as it heats up, shunting current gradually away from the coil and directly into the rectifier diode. You may have cut off the power to the coil suddenly which can cause worse re-magnetization.""

That explains exactly what happened. I'll call that TV shop in San Diego next week and see if they have any degaussing coils left in stock. Thanks.

Adam · #9 04-22-2007, 03:16 PM

Still haven't gotten around to looking for that degaussing coil, but did mess with the purity today and redid that center dot covergence. It looks noticeably better, but that bottom left corner still needs to be degaussed. Some pics...

wa2ise · #10 04-22-2007, 04:31 PM

You might, with a variac, be able to create the decaying AC magnetic field to deguass the tube. Turn on the TV, with the deguass coil in play, for a few tens of seconds, slowly drop the variac output voltage to zero. Then take the deguass coil out of play (short across it with a switch or such) and turn on the TV with normal line voltage.

Adam · #11 04-28-2007, 12:18 AM

Good idea, I would try it if I had a variac. It's on the long list of things I could use but don't have along with a good degaussing coil.

jpdylon · #12 04-28-2007, 12:28 AM

Variacs can be had at just about any industrial electrical supply, electronics salvage, and I've even seen a few at thrift stores.

They can be had anywhere for under 60 bucks new for a 5 amp --which is more than you'll need for a set.

The convergence on that zenith looks really good from here!

BTW, is that your CTC-11 (or is it 12?) in that avatar picture?

Adam · #13 04-28-2007, 12:40 AM

Philco that uses a ctc-15 chassis, I started a thread about it in early color.

I'm still upset about the time I passed on a cheap variac at a thrift, I was in a rush to get somewhere else and just left it, came back the next week and it was gone.

Adam · #14 09-06-2007, 05:33 PM

I tried that trick with the variac this morning and it worked. I turned on the tv with just the degaussing coil plugged into the variac, then turned on the variac then slowly cranked the voltage down.

vintagecollect · #15 09-06-2007, 06:57 PM

Nice picture shots Adam, looks like you put a lot of work into set.

Set has projection knob zenith used for a few years, nice design as number can be seen across room, try to get new knob light bulb. Cabinet looks of good wood too. I guess customers must of saved something if got cheaper laminate cabinet, I had one. Probaly should of reveneered over laminate would of been considable work. That sets a keeper.