Zenith Roundie Still Not Running
 

I installed the vertical output transformer that I got from Zenithfan1 and the transformer works fine. When I turned the set on after installing the transformer I have restored vertical function. I had the set laying on its side for servicing. Since it was working, I turned it off and was going to stand it upright so I could make the height and linearity adjustments. When I urned it on, it worked for about two minutes and then I heard arcing and the raster started to disappear. It turned out that the damper tube started arcing. Now I have no HV and the horizontal output tube is toast. I was able to get it going again for a short time by turning the set on its side, and tapping the damper tube so I know the vertical is alright and I suspect that the damper shorted out when I turned the set over. Nothing appears to be out of order with the high voltage section or the work I did. A year ago I replaced all the power tubes when I did the restoration. I would have expected to get at least a few years out of these tubes.

Can damper tubes fail like this? I'm beginning to wonder if these early color sets are really all that reliable. The picture quality is unbelievable but am I going to have problems with this set every year? How do those of you with a fleet of early color sets keep them all running or do you just pick out a favorite and give it all the frequent service it requires? I'm going to order the tubes and get it going again. I hope it stays fixed for more than a year this time or am I being unrealistic?

I've never had it happen to me. There are sets that operate the damper sideways, but maybe that particular tube had a defect that caused a short to develop while on its side? Or just a plain bad tube? That should be among the more reliable tube chassis color sets, but then, even the best of them couldn't get near the track record of a solid state set.

On my Zenith roundie, a 24NC31 model I had a B+ power resistor that supplied B+(270V?) to the oscillator section and hortizontal output tube fail. It opened, killing the bias. Horz output tube died with the plate glowing red. My damper tube survived the ordeal. I know my issue was different from what you are experiencing, but I'd thought I'd throw this out there.

Ah, a key to me finding that resistor was that it also killed the B+ to the audio output section as well. Kind of odd, loosing HV and audio.

Replaced resistor and horz output tube got me working again.

Hope you get your running. Mine has been a daily watcher now for 4 years, with a few minor hicups one of which is told above. I did loose a damper tube in the 2nd year of opererating it. It failed peacefully, and replacing it brought my HV back. It has slight vertical jitteryness, and I know some caps in the vertical section will need replacing.

Lets hope when you get this sorted out that you'll have a worry-free Zenith roundie.
Good luck,
Matt Davala

I love roundies, but as an everyday watcher,they are not reliable. I remember going on service calls when the roundies were about 15 years old, and it was not uncommon to have to make a house call 3 times a year. I remember those pieces of crap CTC-38's which would have been around 9 years old at the time and there were some we got stuck going out every few months.
Now granted, the roundies were the best the manufactures could put out at the time, and I would say a Zenith would be way more trouble free than a CTC-16, but they are what they are. Manufactures didn't want a bad rep, and which is why they strived for improvements until reaching solid state. To give these sets credit, they were often the ONLY set in a household, run 10+ hours a day, often in an un-airconditioned environment in the summer (which took a big toll).

I'm not out to "slam" tube sets, as I collect and love them as well, but one can not expect trouble-ree service, which for some, is part of the fun.

Good luck on yours, and hope it will be trouble-free for awhile.....

As a recommendation when repairing them used 105 degree rated capacitors and install a small, quiet, low voltage 4-5" ventilator fan to keep cool air circulating.

Heat kills.

Man, sorry to hear the set is still not working. I'm glad the transformer worked well though. Hope it's just that tube shorting out. I wonder if there's a wire or part shorting out against the chassis, or does so when moved a certain way. I've had that happen with point to point chassis. Sometimes it is hard to see a problem like that. I hope you get it fixed.

Possibly check to make sure ground straps from the CRT bezel to the chassis are tight.. My Magnavox did something similar until I repaired it. It took out my damper!

Damper tubes were a common failure in Zenith sets. A quick check for a damper tube is to slightly remove it from its' socket while the set is on and if the pins arc to the socket, it is conducting and should work in the circuit. They can also arc internally when bumped and should be replaced if they show these signs.

The damper tube was arcing internally. There were a couple of occasions where after tapping the tube I could get the set to work but not for long. I suspect the tube shorted somehow. The elements appear to be close together in these tubes anyway. I'll order replacements for the damper and horizontal output and see where I go from there. I do think that will fix it though.

for soom reason i seem to remember Zenith had a few color chassis that had a tag next to the damper tube which specified a certain tube and no subs could be used . But thats from memory on a past set .

I know this is kinda off-topic but Mark, your picture page is awesome.

Got it fixed
 

:banana:I replaced the damper and horizontal output tubes and now the Zenith is working again. I also replaced the vertical output and oscillator tubes since I could not get the height and linearity adjusted right. I suspect the output tube might have been damaged by the failed transformer. I did replace some caps in the vertical circuit including a black beauty and one of those white ceramic caps which I believe is just a paper cap inside. The set would often for a split second pull in from the top and go back to normal as though the vertical were about to colapse. Maybe those caps in the circuit killed the vertical output transformer. I ran the set for an hour last night after touching up a few adjustments and there were no signs of vertical problems. Everything is working just as it should. I will post screen shots later. I am still amazed at the color quality of this set. For me at least, the novelty of a roundie just never wears off.

YAY! I'm glad you got it running! Can't wait to see pics. Also glad the transformer worked out well for you. I am currently restoring a set like yours but in a different cabinet, I think. I can't see the bottom of yours, is it the one with the screw on legs and the arched piece of wood between the front legs? If so, we have the same set. The 25MC33 was one of Zenith's best roundie chassis in my opinion. Good luck and I hope she runs for many more years for you! I also agree with you on the fact that the novelty of a roundie never wears off.

Compucat's TV has the arched wood support and screw-in legs:
http://www.audiokarma.org/forums/sho...t=harry+poster

I was jealous. Likewise Zenithfan. If you're interested in trading yours for something, or want to sell it, let me know...

My set does have the screw on legs and arch in the front. The arch, like the legs is easily removeable. The cabinet on mine is rather plain and about as compact as it can be. I'm sure it can be considered a small console. Keep us posted on the restoration with lots of chassis pics.

Sure will, we have the same set! I really like that cabinet style. I have to replicate the arch piece on mine, it is missing, otherwise the cabinet is nice. Doug just sent me a PDF of the blueprint he made on autocad so there will be no problems making the piece. I will start a thread about the set when its done, should be in a week or so.

That arch is not complicated. It is strictly a cosmetic piece and not required for structural support so don't worry if you can't get it exactly right. I went to your site and I just love the pics. My favorites are the Zenith roundie combo and the Zenith metal cabinet tabletop set. Someday I would like to get a Zenith metal cabinet set.

Here are the latest screen shot since repairing the Zenith. I have only put two hours on the set since repairing it. I'm a little hesitant to use it much in case something else breaks.

Looks great. That must be one of the popular models. so far, three of us have this one. Anyone else have one? I too like the 25MC33 chassis. Certinaly the most common zenith roundie chassis one will find these days. Good luck with it. IT should last another 40 years now.

Very nice! It has a great picture! Good job fixing it up. Very glad to have helped you. With that print of the arch, I will be able to accurately make a new one, I know it's not all that important but I want it exactly as it should be, I'm kinda anal about these sets sometimes. When I restore one it bothers me when something is not right or "original".

i like that zenith , wish i could find one like that

Yea i have one also that I havent worked on in a few years. I broke a couple slugs in the convergence board trying to adjust them with an allen key. (convergence is WAY off and awful). So im seeking a new convergence board. otherwise the set is junk.