Zenith 25MC33 HV Trouble

[dieseljeep]

Originally Posted by mrjukebox160

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.

[Electronic M]

Originally Posted by dieseljeep

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.

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).

[dieseljeep]

Originally Posted by Electronic M

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).

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.

[old_coot88]

Originally Posted by dieseljeep

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.

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.

[compucat]

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.

[old_tv_nut]

Originally Posted by compucat

... 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.

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!