another zenith combo

[DaveWM]

there is a .1 1kv in the damper circuit to HV regulator bootstrap boost source, going to replace that (cant check it at the rated voltage).

on the side pincusion, I have been looking at the schematic, I think I see how it works, looks like a pulse from the vert out is fed into the screen of the horz out, I am assuming its a sine wave that would increase the output of the horz out (and therefore the drive to the yoke), with a time constant that would max the drive at the mid point of the vert sweep. this would result in a pin cushion adj of the horz sweep. Just a guess but I cant see any other reason for the way the screen of the horz is tied to the vert, and it makes sence as I cant see any direct pin cushion adj to the horz yoke drive after the HOT. the top an bottom pin cushion adj (increase the vert sweep top and bottom) looks to be a bit more complicated using a tube with what appears to be a phase reversed horz pulse for the bottom and a direct pulse for the top. I have seen other methods used that have a transformer and a passive approach (no tube involved) for the top and bottom. I assume they use a screen voltage pulse on the HOT for side pincushion as well, just have not had to study them as this is something new to me.


The HOT screen has a large value cap (40uf) on the screen, so I will start by checking that, its does not go to ground and has such a large value when you consider the freq of the horz, so I assume it not a bypass cap, but rather part of the RC network to create the sine wave from the vert pulse I mentioned above. oh well I will check it later in the week.

I prob should have started by just going thru all the lytics 1st, but lately I have had pretty good luck with them being ok, so I stopped checking.

I did flip the set on its end and looked, there was a cap that had been piggy backed to a filter can, it was done a while ago based on the vintage of the cap. I figure if on can section is bad there is a better chance that the other sections may be bad, unless its a low volt hi capacity seciton on a multisection that also has High voltage sections. When that is the case I have seen the HV be fine.

Anyway I dont think the screen cap is inside a can, should be easy to check.

[ctc17]

What Im thinking is that sense most of the convergence and pincushion comes off the flyback if the wave shape gets off even a little bit the whole thing goes nuts. This one something takes a dump when it warms up and the convergence and pincushion get all twacked out then it starts blooming. I can turn the brightness down to where the blooms stops but the convergence is still all off. Its like a big loop and if any one thing gets screwed up it effects everything.

[DaveWM]

well the large value cap checked out fine. Back to the basics, non linear horz sweep (wider at the top than the bottom). I will start with a new 6U10, then try scoping the wave forms in the horz osc section, voltage/resistor checks etc....

[DaveWM]

I still have a tendency to run off looking for the oddest possisble problem, need to remind myself to check the easy stuff 1st (deflection prob, check deflection tubes). I checked two caps (cardboard single sections 400v 10uf and 40uf) both tested very well. again a tribute to the caps used. I did see something odd. one of the doublers (the one with the grounded can) shows as a two section cap on the schematic. the one installed is a 3 section with one section cut off, the other section bridged with a separate cap, on one section active. Very odd, I will see if I can read the values off the cap, perhaps it was an early replacement.

On the HV pulse regulator, there is a 68pf cap that feeds the pulse from the horz osc to the grid of the regulator. I think I will scope both side of that cap and see what is there. I would be shocked to find a problem with that little cap, but the HV regulation is def not right.

[DaveWM]

this one is getting interesting. couple things, the HV reg is still not right as I have to have it set fully CCW to just get it to 25kv.

The horz scan is about 1in wider at the top than the bottom. I was reading my serv manual and found a comment about the pin cushion and the vert pulse off the vert out screen. This is what I was looking at above, and checking for a cap. They mention a cap in the screen circuit that should be checked but do not mention which cap. I figure it has to be one of the ones that is not found on a roundie which does not have pincushio correction. pretty sure I have check them all but will look again.

When I scoped the screen grid of the Hot I get a def vert freq pulse, not exactly sine wave but sorta, has a lot of hash on it which I assume is horz freq from the hot. I am going to compare this to a working zenith with pincushoion correction and see how it looks to be sure.

while checking around the HOT checked the drive voltage and pattern. I get about -55v with a vtvm, and the scope showed a correct pattern at 150vPP.

sams and zenith service manual have it at 250-270v pp. Not sure if that is related.

oh and I will prob check the pulse from the horz osc to the HV regulator tube while I am at it, to compare, just in case that pulse is tied into the lack of proper regulation range.

I have NOT checked the yoke, did not really want to pull that off but prob will soon. Was also wondering if a convergence assy problem could be at work, dont know if that could effect the overall scan and increase the width a the top, seems like a long shot.

My plan now is to check the drive and that screen modulation of the HOT of a working set to confirm those PP numbers and the look of the HOT screen.

[DaveWM]

fortunitly the color demodulation is working fine, and over all the pic is pretty good. It does not like fine white lines (if I turn down the color its fine), and it seems pretty picky about fine tuning, just a bit more than some of my other zeniths of that era. Vintage programing looks best (seems the pic just looks better). I think back in the day perhaps the programming was purposefully done to make it easy for a set to produce a good pic.

[Bill R]

You might want to check that yoke before you go any farther. You said the raster is an inch wider at the top than the bottom. This usually indicates a yoke problem. It could be loading the HV section down. You may be able to ring the coils without removing the yoke from the tube. Pincussion problems cause the raster to bow in or out, but the top and bottom are the same width.

[DaveWM]

I hear ya Bill guess I was just hoping against it. I will go ahead and give that a try, I think I have a spare one laying around, if not I can borrow one from another set. It would be nice if that accounted for some of the HV problems as well.

[DaveWM]

brand new yoke tried, no joy.

Oh and putting a convergence coil assy back on the CRT backwards will really foul up the convergence. yikes....

anyway orig yoke back on. My next try will be with the scope on the HOT screen to compare the pin cushion adj wave form to another set.

I guess I could try swapping the convergence board as well, but only if some one here tells me that a convergnce board can cause the screen to be wider a the top by about 1 inch. I will hook up the cross hatch gen and get a pic later to share.

I was really really hoping the yoke would do it.

[DaveWM]

a few more PP checks, 200v on the horz discharge tube plate, per sams should be 250 per zenith serv man 270v so I am back to thinking I am just not getting enough drive signal to the HOT. Guess I need to check some more caps/resistors around that circuit, not sure if I am on a wild goose chase but I bet this is why the horz width is down some. I check the B+ 350 was 325 I am going to check my line voltage in, I did not think that was a big enough amount of a diff. Prob should check the B+ current while I am at it.

Geez I hope its not something that simple (low b+).

[DaveWM]

GOT IT!!!

the little bastard, it was an open lytic on the red lead of the vert out. That is where that pesky pincushion pulse is taken from. It was part of one of the can caps, so I guess its time to just go ahead and restuff them all. I know this would not have been an issue if I had just shot gunned the cans but then I would not have learned nearly as much about how things work. I checked the waveform of the screen grid of the HOT, before there was a vert freq wave riding on the DC, but it had a lot of hash in it. I jumped that cap and sure enough the top pulled in and the vert wave on the screen of the HOT cleaned up (no hash at all). The amplitude of the wave also decreased. If the darn sams had a scope patter on it for that HOT screen I would have been able to narrow it down a LOT faster. Anyway one more thing to know to look for when trouble shooting.

[ctc17]

restuff? I thought this was going to be a parts set? Just solder one across the bottom.

The first thing I do its check all the caps using a speaker and audio source. I series each cap with the speaker and listen to the music. Lower value caps filter the bass off, weak or open caps pass little audio. Its a cheap esr meter and it gets around resistance in the circuit.

Did this fix the horizontal performance issue?

NM, yea I see that cap, its a partial bypass off the b+ side of the vert out transformer. 18uf, what kinda special custom value is that. If you out something to big in there it would effect it the other way.

[DaveWM]

its so late I just bagged it after seeing it shape up. will look into the other stuff later.

That guy finally got back to me with that zenith, (the tube set nearby) so I will see if I can pick it up tomorrow, think its a 20y1C38

[ctc17]

Well David you are officially the Zenith kombo king. I have 2 and thats enough, I would really like another RCA combo.

[DaveWM]

yea the house and garage are starting to look like a dealership...