Zenith roundie

[DaveWM]

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.

[DaveWM]

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).

[DaveWM]

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.

[ctc17]

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.

[DaveWM]

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.

[DaveWM]

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.

[miniman82]

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.

[rcaman]

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

[DaveWM]

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).

[DaveWM]

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.

[DaveWM]

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

[DaveWM]

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.

[DaveWM]

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.

[miniman82]

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...

[DaveWM]

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.