Zenith 25MC33 HV Trouble
 

I was watching my Zenith tonight for about two and a half hours with no problems and I leave the room for a moment and I hear this loud hissing sound. I check on the set to find no picture, and a slight burning smell. I pop the back off the set, open the HV cage and the flyback is hot but I can keep my finger on it. I turn the set back on and find the high voltage rectifier red plating. I have not swapped any tubes as it is late but I hope it is not the flyback. I restored this set eleven years ago, replacing all the sweep and High Voltage tubes in the process. I set the cathode current per the Sams and have never had high voltage problems with this set. Can a shorted HV rectifier cause the red plating or might I be looking at a bad flyback?

I'd first suspect a bad HV rect or more likely a short in the HV wiring (say the HV filament winding or another place it gets close to grounded metal) or excessive load from a miss-biased or gassy CRT or HV regulator tube.

Good diagnostic methods: disconnect the top cap from the HV rectifier and see if you can draw an arc 1/4"+ from the top cap lead or fly doughnut with an ungrounded screwdriver...If yes your fly and its drive circuits are fine. If you have a neon bulb or CFL you can hold it near the fly and H output tube and the ambient RF emitted by a good horizontal system will light the bulb.

The arc will be substantially longer than a quarter-inch with a color set.

It's well to heed the old admonition: never, ever leave a vintage TV (especially color) running unattended, even for a moment.

I disconnected the hv rect plate cap and it did arc but swapping rectifiers produced the same result, red plating. I took the cap off of the shunt regulator but that made no change either. I wonder if there is a problem with the fly back high voltage winding. When it failed, there was this loud hissing sound like I have heard before from failed flybacks and that telltale smell of burning windings. I suppose 53 years is a good run to get out of an original flyback. When my wife finds out it is not working she will be after me to get rid of it. She does not share my passion for vintage electronics, especially big items like a round tube console tv.

I wouldn't power it up anymore until a better investigation is made inside the H/V cage.
If the weather is anything like it has been around here, with high humidity, it's the perfect time for insulation breakdown problems. A strong possibility that the H/V rect filament winding punctured. This overcurrent condition can easily waste the horiz output tube or maybe the flyback.
Long stretches of this kind weather meant a lot of flyback failures, especially RCA's.

Tubes redplate when excessive current flows through them. The only flyback failure that could cause excessive current through the HV rectifier tube would be a short/breakdown of the rectifier filament winding (the 1-2 turn loop around the core).

Try running the set briefly with the anode lead disconnected from the CRT, and the 6BK4 pulled out. Measure the HV at the anode lead. The failure HAS TO be downstream of the HV rectifier. Fortunately, there isn't all that much circuitry there. The excessive current through the tube is going to ground :

A.) Right at the rectifier filament because of a carbon tracked socket/mounting cup or a failed filament winding on the flyback.

B.) Through the 6BK4, because of a bad tube, or a failure in the HV regulator circuitry that is turning the tube on hard in an attempt to pull the HV down.

C.) Through the CRT, due to gas or an internal short between the internal dag coating and some part of the gun.

I can practically guarantee that the flyback is fine if you are still able to draw a healthy arc off the HV rect plate cap lead.

There is going to be some kind of short or excessive loading on the cathode end of the HV rectifier.
Open the cage up and examine things carefully. Remove the rect socket from the cup and the insulator cup from the cage, clean and examine the cup for arc through damage (I've seen insulator cups fail) follow the wiring to the HV rect socket around and look for pinholes where it could have arced through.
One of the more likely culprits is the HV rect filament winding. The filament winding typically consists of the same type of wire that the HV lead is made of wrapped loosely 1-3 turns around the frame of the flyback....Get some fresh (40KV rated) HV wire and replace it if it looks bad.

What you were hearing is probably arcing/corona from the short. What you smelled was probably burning HV wire insulation or HV cup...Electrical arcing consists of a tiny path of superheated plasma...Arcing is hot enough to melt metal and easily hot enough to burn plastic which is probably what you are smelling.


Do NOT allow your wife to throw it out! If the CRT is good that alone is a $100-300 part that is becoming scarse, the rest of the set also has value and I suspect if it were on my bench with the problems you describe I could have it working again in under an hour.

I think the CRT lost vacuum.

God---We hope not!
The best way to check that is to remove the horiz output tube and observe the CRT neck, to check the status of the heaters of each gun.
If the CRT went to air, all three heaters would be burnt out.
The set uses an all glass CRT, 21FB or 21FJ. :scratch2:

I hope not too, but it is not impossible...I put a VERY gassy (Zenith badged)CRT into one of these Zenith chassis and got to watch internal neck arcing and HV rect arcing both accompinied with glow, and spectacular amount of acoustic protest from the horizontal circuits...Set worked fine before and after with a different CRT (I did not run it long enough to cause damage).

In some cases where the CRT has gone to air, the heaters still have continuity but will not glow because thery're immersed in a heat-conductive fluid (air). But the whole neck gets real hot real quick due to heat conduction.

The heaters are glowing just fine in the CRT. The tube is a 2007 Hawkeye rebuild with low hours on it. The set was working just fine until this happened. I really think it is an HV problem. I will need to pull the chassis to troubleshoot it properly. I did look at the HV rectifier socket and cup. There appears to be no damage to either. The 1500 ohm resistor across the HV rectifier socket is pretty cooked. It measures 2K and looks like a ferrite bead. The body is black and none of the color bands are visible. The inside of the HV cage is fairly clean as I cleaned everything when I restored the set eleven years ago. This set has performed well while I have had it nothing beats that Zenith color and build quality. For now I will have to put my 1956 RCA 8" B&W set on top of it until I can get it fixed.:yes:

Ah! The famous Zenith 21FJ. I took it out of a 24NC37 because it really looked sick. Then it went to Zenithfan1, intended to be a dud, then Miniman82 got it, then you lucked up on it. :D

My first principle of trouble shooting: fix the obvious things first. If a new resistor fixes it, you are done. If a new resistor cooks also, it will lead you to the real problem. Good luck!

That resistor probably cooked because the short is drawing to much current through it.

If you disconnect the anode from the CRT and get hi voltage, the CRT is bad.

I have the same set and was lucky enough to grab an extra FBT. Check the boost rectifier diode. It sounds like time to break out the schematic and do some troubleshooting.

I took the second anode lead off the CRT and still no HV. At least I know the CRT is no loading it down. I do have the schematic and when I get time, I will pull the chassis and really dig into it, hopefully this weekend.

I pulled the chassis today. After partially dismantling the high voltage cage, I found the trouble. A hole had burned into the hv rectifier filament wire. I found another wire in a box of Zenith roundie parts and installed it. I also replaced the 1.5k resistor with a 1k since it is all I had. The high voltage came right back up with no arcing. I put the set back together and checked the high voltage. It was a 30kv presumably because of the resistor replacement. I turned it down to 25kv as recommended in the Sams. Now for a new problem. The horizontal hold will not lock. I suspect the afc diodes because it will operate throughout the range of the horizontal hold control but at center it will not lock in. I have been having trouble up to this point where the horizontal hold would drift especially after being on for a half hour or more and would have to be set further toward the end of the control range to bring it in. I don’t have any diodes to replace them with so I will have to wait until I can order them. I did replace the 6U10 horizontal osc, afc and discharge tube to no affect. I am just glad the flyback did not fail.

There may be more problems lurking around the flyback transformer. Generally a pulse winding on the fly back feeds the phase detector and if it is open, you will lose lock.

Can you post the schematic focussed on the horizontal afc?

I got it fixed. It turned out it was a dirty tube socket causing the horizontal hold not to lock. I touched up the convergence and a few other adjustments and now it is back as great as ever. These old sets will keep you guessing sometimes.

If the CRT lost vacuum you will see a blue purple glow in the neck of the tube.

That burned resistor on the 3AT2 socket was a common problem, even when those sets were relatively new.