1970 Zenith 20BC50

[freakaftr8]

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

[bgadow]

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.

[andy]

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[freakaftr8]

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.

[zenithfan1]

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

[freakaftr8]

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.

[zenithfan1]

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.

[joemama99]

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.

[freakaftr8]

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.

[freakaftr8]

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.

[freakaftr8]

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!

[freakaftr8]

Can the SAMS manual be wrong?

[ctc17]

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.

[kx250rider]

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

[freakaftr8]

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.