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.

Hey guys. I'm pretty confident that my vertical output transformer is not operating. How can I test and verify this with a volt meter? Thanks

I got that Sams in the mail, a day late. I jotted down the yoke part numbers if you need them (not in front of me though.)

There is a trick for testing the vertical circuit on older bw sets, not sure if it will work on this though. You connect a cap (like a .047) from the filament to the vert osc. tube grid. This is where I have to check my notes-if you get deflection at one grid it points to the osc xsfmr, the other grid it is the output xsfmr.

You can disconnect the leads from that transformer and see if it is open with an ohmeter. That photo sure looks like a yoke. I would think many Zenith yokes would be close enough. When I was around 18 I had a couple Zeniths, one very close to yours, another that was probably among their first rectangular models. I had the newer one working fine but couldn't coax any life out of the oldie. One day I decided to swap the chassis. Biggest issue was that the yoke connectors were different. I didn't have a schematic to go by (and probably couldn't have read it, anyway) so I did the best I could splicing them. I crossed my fingers and actually got a raster, but never did anything else with it.

---

Think it would be best for me to wait for the sams that Bryan is sending me (thank you veery much Bryan) and lets see what I find. Also Charles is looking to see if he can find a yoke. If I could scope the waveform off the vertical transformer, I could start there to see whats going on hopefully.
Thanks guys for all your help.

Good luck Shawn, this one sounds like a real dog. I hope she runs for you.

Yes sir this ones a pickle! But thats the challenge. It's all good fun. Especially since it looks like my daily driver looks to be my Ctc17 RCA since the KV-36XBR800 pooped out on me again. So I'll be fixin me a Sony after this one.

What is it with those damned Sonys? On the Zenith, you're right about it being fun, it makes it much more enjoyable when it's working right after all the stuff that's wrong.

Is the sony flashing a code on the standby light?I fixed one that i dragged home by typing sony and the number of flashes into a search on the internet.Someone pointed me right to a defective ic.

Hey guys,
I've been poking around in thais chassis tonight with a meter and a sams and a cold one (pepsi).

Well im finding a load down of the 390v B+ supplying pin 10 of the 6JC5 vert output. There should be 322v there and at startup the voltage drop goes from 390 to 223v. Im finding this all over the 390v B+ for the sections requiring it. Actually at the source PS cap I get 400v and after a 22k 1/2w resistor (that is dark and overworking) it drops to 223v. So something is really loading down the 390v. I cannot seem to pinpoint but ill keep checking.

Oh yeah, about that 22k resistor. It measures 10k and I soldered in a new 22k in place. It's getting really hot.

Oh and I disconnected the yoke and the voltage drop is the same.. Also I cannot hear the cycle of the vert out transdformer like I can with other sets.

Hey guys. I spent mote time last night tracking down the gremlin in this set.

Off the filter can 1 square 40mfd 475v is the circuit in question. This can goes immediatly to a 22k resistor that is badly scorched beyond reading the color codes. Sams manual stateds this resistor is a 22k R136. This reststor out of circuit tests 10k. This supplies the 322v to pin 11 of the 6JC5. With this resistor in circuit at inital startup 400v creeps to 223v at raster. If I replace the resistor with what the SAMS states aS A 22 k then voltage at raster of pin 11 is 177v. So in my experience when a resistor heats up is loses resistance and climbs in value.

So theriotically (ya i messed that word up) if I put a 5k resistor in circuit of R136 then my voltage of pin 11 would increase more to my liking? Im stumped... And at this point I am ready to call it quits on this chassis...

Any info or input would greatly be appreciated.. This circuit is where I installed the 100mfd 50v cap backwards. And I have tested everything in this circuit I can )and with my equipment shy of an oscilloscope due to the fact that I dont know what to probe) I have tested and seems in spec.
Thanks!

Can the SAMS manual be wrong?

What happens if you pull the vertical output tube out? What else uses that supply. I wouldnt worry to much about the resistor for now, something is loading that supply down.

Dont trash that set! I would be willing to drive up there and save it.

Sorry to hear that it's being difficult... I've been preoccupied with this big fire here in Ventura County, so I didn't make it down to LA this week to check into a yoke. That same shop owner probably has the original Zenith manual, and I can probably take it if he does.

I've seen mistakes in Sams on many occasions, and usually it's an engineering update that took place after the Sams was published. Sams usually put out SSD pages in a later folder (supplemental service data), and that would show up in a newer Sams index. But I have the feeling that it's just a weird, hard-to-find problem. If I can get the manual, I'll take a look at it (no promises), and see if I can offer any pointers. Since the fire looks to be less of a problem today, evacuation is lifted as of last night, so I'm probably going to get back to normal activity by tomorrow, and should be able to go down to LA early next week.

Charles

Oh no I'd never trash a set. These puppies are gems. I'd just start another r project. If I pull the veritcal out there is still a loaddown and the flat vertical line does not meet the end ofthe CRT horizantally. I might still consider the yoke if the vert pin 11 didn't load down with the yoke unplugged but it still does.

Charles I prayin these fires get put out.. You live in a beautiful area and it's a damn shame for those who have lost it all. I feel for you guys down there. No rush on the set of course, it keeps me busy and of course always brain food.
Thanks for checkin it out though!

How about the horizontal output tube? Or remove the damper tube. If I had the schematic I would be a lot more help.

Hmmm I thought about it last night but never pulled them yet. I'll give it a shot.

Ok just noticed something tonight.. I pulled the 6JC5 vert out and got my vertical flatline like I figured would happen... But.. Pin 11 now has 390vdc with a new 22k resistor in r136 with the tube pulled where I was getting 233v w/ tube pulled and old resistor in place. So I have sufficcent DC on Pin 11 (too much but b/c tube is pulled so this is normal w/ no load on circuit)..

Here's the kicker. My vert flatline falls horizontally short on the right side about 2". So something is also loading down the horiz circuit too. Already changed 80mfd 475v caps and swapped wires on 30mfd 475v where the prob is so I know caps are not the issue even though they register good w. esr readings. So.. Even w/ yoke pulled off circuit and 6JC5 in place I still have a loaddown in the circuit. If yoke is shot would this cause a load as if it were disconnected from the circuit too?

What if the vertical output transformer had some shorted turns?

Thats a really good question! How do you suppose one finds out if it is shorted? Is there a way to ohm out the windings or such?

I've seen shorted vertical output transformers in Packard-Bell B&W sets, but I don't remember seeing one in a Zenith. But anything's possible.

Charles

Could a shorted vertical transformer affect horiz sweep circuits? I suppose the plate voltage comes from the same circuit.

I suppose it could if it was loading a supply voltage down. Let me think about testing for a few minutes. The sams should show an ohm reading but that may not be good enough to tell you if only a few turns are shorted.

Hmmmm, I just thought of something.. Im going to disconnect the supply voltage to the transformer and see if I het a horiz raster to fill the screen.

I just realized something else. If I flip the service switch to setup, this disables the vertical transformer's power source right? Well the horiz raster is still shrunk on the right..
The yoke still passes through the vert transformer even when the transformer's disabled in service mode. If I disconnect R136 off the 30mfd 475v filter that is the trouble circuit, horix raster is restored. This is the resistor that is overheating too...