another zenith combo - Page 3 - Videokarma.org TV - Video

DaveWM · #31 02-07-2011, 12:45 PM

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 · #32 02-07-2011, 12:56 PM

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 · #33 02-09-2011, 09:15 AM

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 · #34 02-09-2011, 03:10 PM

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 · #35 02-09-2011, 07:14 PM

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 · #36 02-09-2011, 09:21 PM

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 · #37 02-10-2011, 06:11 PM

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 · #38 02-10-2011, 06:56 PM

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 · #39 02-10-2011, 10:09 PM

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 · #40 02-10-2011, 10:23 PM

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 · #41 02-10-2011, 10:37 PM

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 · #42 02-10-2011, 10:44 PM

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

DaveWM · #43 02-10-2011, 10:47 PM

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

DaveWM · #44 02-11-2011, 07:32 PM

looking very good now. Just have to check the HOT cathode current. Screen current is right on the nose at 18ma, grid voltage is -62v. I found two bad disk caps on was the coupler cap to the grid of the HOT

and the other looked like a HOT screen decoupler, both brown disk. The grid cap would barely open the eye, the decoupler would open it about 1/2 way. Both were toasted somewhat by a large 5watt resistor (literally), they had brown burn spots on them. I replaced the grid cap with a 1kv dipped film cap and the bypass with a 630v .01 yellow tube cap. I am suprised the grid cap worked as well as it did. there is a .001 1kv disc cap off the plate of the horz discharge tube. Guess I should check it out as well. Its a 10% and I think it is related to the amplitude of the horz pulse from the discharge tube. I am still trying to get the PP up to 250v, its about 200v now.

DaveWM · #45 02-11-2011, 08:24 PM

had to clip out the .001 1kv disc to test, tested fine for cap, maybe a little low value, no leakage at 450v (as high as I can go), replace it with a .001 1kv just to be sure. Still can't get the PP any higher at the plate of the horz discharge. getting tired. I will check the HOT current tomorrow, will also try swapping out some other 6U10's to see if that makes any diff. Over all the pic is looking good, have not touched purity or convergence, but could watch it the way it is. Still need to check the remaing can caps.