First Post-Restoration Glitch

[compucat]

My recently restored Zenith roundie has developed its first problem. The vertical is acting up. The vertical is non-linear, won't stop rolling and briefly collapses to a line for a fraction of a second. Everything else seems to be alright with the set. I replaced the vertical output tube during the restoration and the set has been working perfectly up until now.

Having restored many antique radios, I was anticipating some kind of near term failure as is common when a forty plus year old piece of electronics is back in regular service. The vertical circuit does not look too complicated. One thing comes to mind, however, there is a black beauty cap rated at 0.015 uf, 1000V that I did not change. I had no replacement for it and since the set worked fine, I left it in. It is located in the feedback loop from the vertical output tube to the oscillator. The way the set is acting makes me very suspicious of this cap. This set does not seem to use hardly any of these caps, I'm surprised there are any at all. Is this a likely culprit? Any advice from experienced roundie restorers would be appreciated.

Up until now, this set has been absolutely wonderful.

[compucat]

Well, whatever was failing has gone completely. I have no vertical sweep at all now, just a misconverged horizontal line. I guess it is time to measure some voltages in the vertical circuits. I sure wish this had not happened. I was looking forward to enjoying this set for a while.

[reeferman]

(For informational and educational purposes) Zenith roundies used a dual disc (3 lead) capacitor in their vertical circuits that would play hell with linearity, but I've never seen one cause loss of vertical.

[Ampico-kid]

Hi;

I recall back in late 1964 our family bought our first color television. It was a Zenith roundie and I believe the chassis was a 25MC33. It was an exceptionally reliable set for the most part, but I do recall sometime during the first few months we had the TV the vertical did exactly as you described. It would roll vertically and then collapse to a horizontal line and then snap back to full screen with a verical roll. This cycle would happen quite fast! I also recall hearing it "snap" everytime the picture would shrink to the line and then pop back.

I remember how amazed I was when the repair man came to the house, tipped the set over on it's side, removed the access panel on the bottom, pulled out his meter, and within 10 minutes had clipped out a defective capacitor, soldered in a new one, put the bottom back on the set and was out the door. It worked great from that day onward. Lucky for us the set was still under the manufacturers warranty.

I hope your set has a similar fairly simple problem to diagnose and repair.

Good luck with it.

Bob.

[3Guncolor]

Check the vertical output transformer for a primary to convergence winding short. I have had a few fail in the last few years just like your description. It seems the insulation fails and the high voltage from the vertical output plate punches through. I have found if you cut the convergence winding out of the circuit the vertical circuit will operate but there will be no dynamic convergence. The other device with three wires is the vertical integrator and they can fail as well and cause a flat line with no vertical sweep. Sorry to hear your set is in trouble so soon and please be careful with your CRT don’t burn a line in it.
Steve

[compucat]

I'm being very careful with the CRT as I just had it rebuilt. I hope it is not the vertical output transformer as I expect finding a replacement might be difficult. There was a slight odor coming from the set as the vertical was failing. I plan to open it up tonight to try and figure out what's going on. I really like this set, the color quality is amazing. I never knew early color sets could look so good. My set appears to have never had any under chassis work done. With the exception of horizontal sweep tubes, tuner tubes and a few others, the set looks to be original.

[rcaman]

why on earth would you leave a black beauty in the set when you are using it for a daily watcher. doesnt make sence to me. it would be fruitless to make any checks untill you get that bad black beauty out of there. steve

[rcaman]

also check your service switch. but get that black ugly outta there. or you might as well put one across the ac line input and burn your house down in the meantime. steve

[compucat]

Originally Posted by rcaman

why on earth would you leave a black beauty in the set when you are using it for a daily watcher. doesnt make sence to me. it would be fruitless to make any checks untill you get that bad black beauty out of there. steve

I only left it in because I did not have a replacement and I've never had a set that used these before. It was not a wax paper cap so I left it alone. Most of the stuff I work on is from the Forties and I always replace the paper caps. I thought this set was new enough that it would not be a problem.

I am going to order a replacement from Just Radios so I will not have to wonder about it. I can't wait to get into the set and find out what went wrong. I sincerely hope it is not the vertical output transformer.

[wa2ise]

Originally Posted by compucat

I only left it in because I did not have a replacement and I've never had a set that used these before. It was not a wax paper cap so I left it alone.

It is a wax paper cap encased in a plastic shroud. Guarenteed to be bad now.

As a quick test, use a few caps in series to get the cap rated at 0.015 uf, 1000V. The formula for ding series caps is C= 1/((1/c1)+(1/c2)+(1/c3)+etc), similar to parallel resistors. Best to use caps narly the same capacitence, so the voltage drop across them is close to even. The smaller cap will see higher voltage. A pair of 0.03 caps, or 3 0.047 caps would do it.

[andy]

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[rcaman]

you said you just had the crt rebuilt well one on your guns is not making contact at the cathode and it is causing more strain on the verticle output transformer if another cathode does not make good contact it will fry your output vertical transformer. sorry i couldt help it that cap may be your only problem. everybody that is a newbie in roundies needs a good joke on them.

[andy]

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[oldtvman]

In most cases problems like this can be traced down quickly 1st check all the power supplies and then get out the oscilloscope and trace signals.

[roundscreen]

Just guessing, look for a electrolytic cap in the vertical circuit. IIRC It is a can cap. 40 uf @450 v or something close to that. Also check the vertical lin control. I have seen the lin control burn up before.
Good luck
Ed