1970 Zenith 20BC50
 

Hey guys,

Do you know the value of the resistor (cooked in half) that comes off pin 11 of the 6LB6 Horiz output tube? starts with orange it looks like.

This set had a vertical linearity issue after warmup, think it's a power resistor. Ceramic green I believe, it's wasted as well, but I installed a cap backwards and destroyed a small 1/2 watt resister off the Horiz out tube..
Thanks
SHawn

I don't have that exact schematic at my fingertips, but looking through a Tekfax I have here at home (my Sams are at my work location) yields differences - On one chassis, (12BC50) there is a 470 ohm 1/2W resistor tied to pin 3 and 11 on the 6LB6 on one end and to a yoke plug interlock on the other. Through the interlock, it ultimately goes to a 390V source.

On yet another set, the same two pins of the 6LB6 are tied to a 3.3K 7W resistor, and then to a 270V source.

If it was a 470 ohm 1/2W (yellow violet brown color code), it would darken the yellow band to orange....


Hope this helps,

Hey Brian, you were spot on with that 470 ohm resistor. That brought my picture bac. Im back to the orig issue. There is a 17k green ceramic heat resistor coming off the other side of the yoke interlock (i believe). The odd thing is if I put my VDM on the rersistor when the set is on, i get full raster! But then it smokes the 470 ohm resistor coming off pin 11 of the HOT..

BTW heres what im getting. If anyone has any ideas please let me know. Im stumped here..
Thanks!

http://i739.photobucket.com/albums/x...9/IMG_0111.jpg

Wow, thats a unique symptom! I'll check out the schematics. So do you have a full screen on initial warm up? How long before this happens? Do you hear a distintive rather loud pulsing sound coming from the vertical output transformer?
Just out of curiosity, do you have pics of this set posted somewhere?

Hey Doug,
Never have a full screen, actually it seems as if it got worse. As it comes up this is how it is. Now if you turn up the brightness, then it blomms to the point to where there in no raster at all. No pulsing sounds either out of the vertical output trans. I do have vertical hold in this condition too. I have pics on photobucket..
Here's another of the set.

http://i739.photobucket.com/albums/x...9/IMG_0083.jpg

000

Zenith was famous for shorted yokes in their hybrid sets, but I remember usually it was only the vertical windings. That looks like something is WAY loading the whole power supply down. There are sometimes some ceramic disk caps inside the yoke cover, which rarely but sometimes do short. If the yoke is shorted, it can wreck the flyback. Did that problem start right away when you got the set home? I last powered it up about a year ago, and it had a full picture. Just focus problems, and this, that & the other... I wonder if something got jarred, such as a screw falling onto the chassis and lodging between a couple of those strip terminals? That's happened to me many times on those Zeniths.

Charles

Actually it started with a vertical linearity issue. As the set warned up the lineraty went nuts. I found a leaky 50v 100mfd cap. I replaced it with a new one backwards. I since straightned it out and this is the result. The cap did not blow up or anything but there is a what seems to be a 17 k high wattage green resistor about 2" long gets mighty hot right off the bat. I probed it with my VOM and when I touched one leg the raster came back. But that in turn took out a 470ohm resistor that goes off pin 11 of the HOT and the other side to an orange wire going up the the yoke. The other orange wire from the yoke goes to the 17k resistor. But I think I originally screwed something up by replacing that cap backwards. I'm thinking either I destroyed the yoke or the vert out transformer or maybe its simple.

Perhaps you cooked a transistor with the reversed polarity of the cap. The 20BC50 is mostly all tubes but it does have a vertical blanking transistor and a transistor for the pincushion correction which can be affected by improper cap placement. At this stage, I'd take a guess thats what may have happened if this was not a symptom beforehand.

If you want to rule out power supply issues for reassurance, check for these two B+ voltages:

390v at the half circle (80@475) on the filter can.
250v at the square (30@475) on the same filter can.

Ditto for sure. Any 'trapezoidal' pattern is almost 100% indicative of the yoke being shorted.

Bill(oc)

Ok cool thanks for the input guys.I'm going to check the voltages on the caps tonight. I hope the yoke is not shorted. That would be a pita to find. I've got bigger fish to fry. Some dude smacked my wifes door on her car and bent the hinges so it won't close.

I hope you don't have a shorted yoke, and that you find the problem soon... But if it stays unconfirmed, and you come down this way again soon, you're welcome to bring the chassis and we'll hook it up on my test jig. That's a quick & dirty way to confirm the yoke or not. OR, if another AKer up in your area has one......

Charles

Well, it's not looking good.
I have 400v at the half circle and 250v or so at the square. I havent done the research but is it possible to try the yoke from another Zenith set to confirm? I have a 1967 25XC38 I believe it is.

Well I pulled the vert blanker and pincushion correction transistors. No change. Actually what's more strange is I can't even hear the vert output xformer at all. Im wondering if I am getting proper voltages to the yoke and vert transformer.. I almost thinking that half the chassis is dead.