Zenith roundie
 

some pics

CRT test good after brief wake up.

Is that a 24MC32? I'm working on one right now and my chassis looks just like yours. Mine's a 1966. I see that it's the "gel" PVA bonding that gets the green halo instead of the "mold" looking cataracts of RCA style PVA. My tube is a gel one too. Nice looking set, glad she's saved.

Edit: I just noticed the knobs broken off, I'll see how many spares I have left, I think I already gave them all away.

Great if you have any thanks :thmbsp:

yes thats the chassis number on the back.

Mark, I think the one you have is in slightly better chassis condition.

the whole CRT seems to have a slight greenish tint in addition to a slight green halo.

progress...

the color demodulators seem to be working, I have a loose focus lead (not sure if I knocked it out or if it was already loose, I had very poor focus until I pushed it back into the socket. The CRT socket is pretty loose on the tube so I have to be careful (it was loose when I got it). sound is good, sync is good. have to work the vert scan, prob a cap (there is a paper cap in there).

HV seemed a little low, about 21-22kv full on. Will have to look into that, maybe a low B+ maybe shunt tube (I checked the HOT current, about 220, but did not check th shunt 6BK4). the HV adj does work, just lacks head room. It looks completly orig, I see no evidence of any work being done.

I think I will try a new vert tube, and check that electro cap. then need to find the B+ and check it. This set uses a doubler so I guess if one or both of the of the doubler caps was a little weak the B+ could be down.

yep looks like the cap I was checking out is the one in the vert out cathode bypass. I will be replacing that automatically.

I will check the can caps with my cap tester, so far most of the zenith tvs I have worked on had good can caps.


there is also a "brightness range" adj pot called out on the schematic, I did not see one of those. on the chassis.

There are some of those white caps 1000v that I will prob get rid of, and one of the Black with red stripe, I will check it.

Not sure what to do about the focus pin lead. Any ideas there? just clean it up well and epoxy it in?

The fly looked good, no dripping wax or any of that nonsense.

I see the shunt tube has test points across a 1k resistor in the cathode circuit. I will assume a 1v drop is normal (assuming its still 1k). there is a 1.5meg resistor in the HV adj pot circuit, goes to ground. If that were to drift, I think it could cause the shunt to work hard, I will check that if the shunt voltage drop across the 1k it too high.

The finish on this thing is hard, like a photo finish, I don't think I am going to be able to make it look too pretty.

Oh I changed the video cathode follower which seemed to help the video some (the video amp did not). I am hoping if I can get the HV up the pic will be a bit brighter, its not very bright right now, of course the tube was a bit weak until it heated up, so its prob tired anyway.

found the test point for the shunt reg (right behind the damper tube). so I got the set on its side, service saver cover off, HV probe attached,HOT catode on the mA meter, and now a volt meter on the shunt test points. I will be firiing if of later after today to see where it is. I will prob just jump that vert out bypass cap for a quick test to see if that resolves the lack of vert sweep. I was thinking it could be a boost issue as well, since the vert uses boost voltage, so if the jumped cap does not work I will check the boost.

1.75v drop across that resisitor, equates to a 1.75ma, on other 6bk4 equiped set (the Packard Bell) they want that to be around 1ma, so I am going to assume this should be the same. So now I will poke around the shunt regulator and look for a problem. Not a lot there so I hope it will be easy. Maybe just try a new tube 1st? I guess it could be gassy.

what did the hv go up to with the shunt tube unplugged? Should be max 1 volt at minimum brightness, if its 1.75 at max brightness its way off. That like 30 watts dissipation through the shunt. That should turn that tube black fast.

did not pull the shunt tube yet. will do later.

I don't know about Zenith, but RCA lists shunt dissipation at 18 watts for most CTC-X(X) receivers. 6BK4 data sheet says 30 watts is the max.

disconnected the shunt tube, KV shot to 30kv so I have to assume there is a problem in the shunt. Tried a new tube, no change in Vdrop across the test points (1.7v) so I will focus on the few componets around the shunt, there is not much there so hopefully it will be an easy fix. 1st check will be that 1.5 meg resistor.

1.5 meg reads 1.7 so that is a bit out (5% resistor) but that does not seem far enough out.

I am going to see if I can get a direct read of the grid and cath voltage, I am thinking maybe the cathode is low, it goes to the 350v so I will start there.

It would seem that if the over all B+ is low the boost would be low also, but maybe the relationship between the two could be upset? The boost is the source (thru a resistance series and the HV pot) for the shunt grid bias.

hmmm, after checking the cathode (right at 350v) I decided to check the Vdrop on the shunt resistor again, it was 1v the .8v then .3v just kept dropping. the HV did not go up. the HV control pot is still working. Its like it was losing HV headroom. The fly current did not move off 250ma, dont know if that is right, but seems like its in the ball park.

going to let it cool down and try again. It still has the orig damper and HV rectifier tube in.

jumped the cap, no change, lack of vert, new tube next. Vdrop right around
.6v not sure why the inconsistant readings (trying to compare on the same brightness settings and scenes).

new vert out tube, no help, next will be a new vert osc. Darn it as a termistor in the vert height circuit, I hate those. From there it goes to the boost voltage, AND there is a .1 cap to ground. That would be a prime leaker.

May also explain the odd stuff going on at the shunt (a leaky one that gets worse with heat would kill the boost). that one will get subbed (I do that routinely on those white ceramic tube 1kv caps) they are about 50/50 on being good or bad with leakage. I am pretty sure they are just a paper inside.

Welcome to another time eating vampire. This one has all the best of the ones we have been fighting going on. I have a working set with this chassis and it is very stable. Be patient. Ill send you the sams when I get home.

yea, not typical zenith this time. I got a sneaking feeling I am going to find some leaky caps. there is surface rust on the chassis, makes me think its seem some dampness (in florida that is hard to avoid if not kept in an air condition enviroment 24/7/365.

lots of weak or leaky caps in this one, brown drops, a
.1 .22 and .47 all in the vert circuit, much better height now.

the contrast control seems to have no effect, I the video amp tube has a large value cathode bypass on the contrast pot, I think its in a can but hard to get at and hard to trace as I still have the chassis in the set.

still getting odd results on the fly, its back to 1.7v drop on the shunt, the cathode voltage is ok (shunt tube), but hard to get a reading on the grid, even my vtvm is too much of a load on the grid, when I hook up the voltage must drop cause all of the sudden the HV shoots up as the tube gets cut off. so I am back to trying to figure out why the grid voltage is too high, looks like I will have to pull the chassis out and see if I can figure out whats going on. sure seems like it should be that HV pot and 1.5 meg meg to ground. the HV pot works so I figure it cant be open, it just does not lowering the grid voltage enough to reduce the current thru the shunt.

I dont know why last night the shunt current started dropping, but I did not leave it on very long this time, so maybe its a heat related thing.

Huh, maybe I should replace the brownies in the vert circuit of my CTC-9. It's always had a slight lack of height, and the vert linearity could be better too.

i replaced the brownies on my ctc10 vertical circuit and the height and linerity came back just like new. steve

Thats why I like my cap tester, great for checking those caps, shows up weak ones vert quickly.

Interesing note on cap testers, this got me once so now I always remember to disconnect the video feed from the dvd player to the TV. I have no idea why but if you leave it connected, sometimes a cap will read weak (fuzzy eye opening) if one leg was lifted for the test (so one leg is still in the circuit).

There is a .1 1kv white tube cap that test ok but I can't test over 450v so I may just replace it anyway.

Not looking forward to pulling the chassis, it looks like its going to be a pita, lots of tied off wire cables. This seems to be a low end zenith based on all the missing (unused) adj pots on the back of the chassis, and the cheapie IF tubes (6BZ6 on the 1st 2).

Thinks its time to check the cathode voltages, CRT seems dark, even with the lower kv (22kv tops HV adj as high as it will go, still 1.6v drop (1.6ma) on the shunt tube) it seems like it should be brighter. The fact that I am not getting any contrast control makes me think the video amp is not right, perhaps cutting off the CRT. looks like I will have to pull the chassis soon to get much further.

a few more checks, the CRT cathodes 260v and the screen grid around 650v so all good there.

progress, at least on the picture, had a open cap in the contrast circuit, much better pic now. Purity and convergence all jacked up, prob from my flipping it around so many times, I may have bumped the static convergence, but the pic is much better. Purity is a mess, but I can work on that later.

the cathode cap was in a can so good chance the others in that can are bad (screen bypass on the video amp was one, not sure about the others).

replaced the cap on the shunt (.022 grid to cathode) thinking it may be leaky, it was not, no change in shunt current still high and hv low (21-22kv).

I am going to take another look at the 1.5 meg to ground in the grid circuit of the shunt. It now seems like that is the only thing that could account for to high a grid voltage, unless for some reason the boost voltage is way, but not sure how that could be.

HV progress. I think I had incorrectly check the 1.5 meg to ground, I jumped in a 2.2 meg and everything works as it should. Can't say for sure that the 1.5 is bad and my 2.2 is just compensating for some other problem until I get a sams resistance reading as I think there are other part making a in circuit reading inconclusive.

If I had pulled the chassis to check it I prob would have found this a while ago, its just a major pita.

Maybe I'm wrong here, but 22kv sounds fine for HV. All my RCA sets run at 22.5, so you can't be too far off base...

well if were not for the setting being at MAX hv yes I would prob be ok, I like to have some adj that lets me set it a little above that with room to spare.

The other issue was the excessive current thru the shunt tube, that is normal now.

I still need to check the HO current with the new setup. If the resistor does indeed check to have drifted I will have my answer. After that I will address the HOT, and any excess HV being made. One solution would be to increase the screen resistor a bit to lower the overall HV to reduce the shunt load to around .8 to .9 mA and have HV up to 24kv.

The screen resistor seems like the way to go to deal with increase line voltages. I am somewhat hesitant to just go re-engineering these old sets, but if the line is consistantly 125v it may be worth putting in some easy to reverse change to make it more friendly to modern Higher voltages.

Of couse the easist approach would be to just use the variac, which I do on some sets that get a lot of time.

These round tubes were meant to be operated at 22.5kv, so there's no good reason to put 24 on them.

yep sams says 22.5-23.5kv. I have to scan the sams, do it tomorrow. Got stuck exploring the abandon prison and got home late.

Seems perfectly happy at 110v still holding at 23-24kv with reg current down to about .9ma, HOT current dropped to just under 200ma at the reduce voltage.

That was with the shunt grid resistor jumped, without it, the max HV was 19k and the shunt tube was 1.3ma at the reduced line voltage. I hope to pull the chassis sometime in the next few days to do a proper check/fix of that and also check/replace any other faulty caps (like that video amp cathode bypass cap.

Just trying to decide best way to pull the chassis, will have to make notes of lead dress etc... as there are wires all over the place, tied back to various points, a lot using the CRT mounting for tie points.

almost got it out, not as bad as is it seems, the UHF has the power switch and most of the hard wiring back to the chassis, I can leave the VHF in place and just disconnect the IF, AGC, B+ and filament leads. Have one less piece of stuff dangling. I am sure there is a way to mount the UHF and all the pots to the main chassis, maybe that will become obvious once I pull it back.

whew that was fun, its out now, that pesky 1.5 was reading 2.2 and its a 5% so I guess that was the problem after all. going to be testing cans, but my guess is they will all need to be replaced. On stuff I dont care about how the cans look, I generally just bone saw off the can and stick radial caps down from the top, that is the fastest way and keeps all the lead dress untouched.

Since no one will be seeing it anyway I generally do not bother to put the can back on top. effectively the can bottom acts like a term strip with the caps mounted on the top.

Most of the chassis I have done work to have the cardboard covers, maybe you could have someone send you a few to glue over the top of the stuft cans?

generally yes the doublers have the cardboard, and I do exactly that when I have extra covers.

So far the 1st can 4/4/100/200uf 475/475/475/50v only the 100uf 50v was open, the rest test like new. For now I am just going to bridge the 100uf and finish checking the rest. I am bit short on my 450v caps, so the plan will be to check the remaing cans, and see if I can get it going, then I can make up an order and get the proper caps.

well every other can cap tested fine, oh well nver hurts to check. I have checked all the brightness stuff since the CRT was a bit dim, all looks good. So to recap no pun intended,

200mfd 50v cap on video amp tube.

1.5 meg resistor on shunt tube.

I cleaned up the pot controls, including the on off volume that was inside the uhf knob it was sticky.

I sure hope there is no diddliing needed, the coil forms have what looks like dried cheese in them, my guess is the forms have started to break down.

I will put it all back together tomorrow, set it on the variac (to keep the HOT current low, use 110v ac line) and do a complete setup.

hmm the only thing I did not check was some of the caps on the convergence board, there was a bumble bee in there as well as one of white tube caps. I will check that out as well, my guess is the bumble bee will be leaky.

Anyone have exp with the yokes with the fishing line coming out of them? from what I read there are for centering the pic, but I have never had a set with them.

Old wax, my CTC-7 had that going on too. Hit them with a hairdrier before you attempt to adjust them, or the form will get damaged.

Sense the crt is weak this would be a good candidate to increase the HOT screen resistor. Right now you just got a lot of HO wattage being wasted in the shunt tube because the crt cant use it.

It would be a good set to experiment on. You should be able to get the cathode down around 200ma or lower at line voltage and not see a performance change.