Tired CRT symptoms that aren't a tired CRT?

[JohnCT]

Originally Posted by lnx64

Sorry for late reply, but I had to verify the filament voltage, and you are correct, the arcade monitor board is producing a higher voltage than that of the TV (and I did plug the TV into the tube as a test so it had a proper load as well).

Which one would be correct though, because both used the same CRT part number make and model, and both use monitor boards built by Samsung. So in that case, I'm not sure which is right or wrong.

Using a TRMS meter**, the correct voltage is 6.30VAC.

Often there is a dropping resistor (5W typically) that will be used to fine tune the filament winding of the flyback to the CRT filament impedance - perhaps one in either monitor was replaced and the value is off.

Also, make sure someone didn't add a filament winding to the flyback of either in order to "float" the filament from ground. We used to do this to kludge a heater to cathode short. Floating the filament with an external and isolated winding around the flyback's ferrite core would hide the short on screen, but a lot of guys didn't pay much attention to the turns or even placement of the winding on the core.

If the voltage is a couple of tenths bellow 6.3, the picture would look weak on a tube with a bunch of hours on it that otherwise would still be serviceable (a strong tube will run at 6.0 volts fine however). If the voltage is a couple of tenths above 6.3, the tube will look great but will have a shorter life.

**TRMS meter.. Since the AC voltage of the filament is sourced from the flyback, it's not a 60hz AC but a 15Khz AC, and a meter that isn't a TRMS will not read accurately at 15Khz. Another poster on VK used a TRMS meter and still got an erroneous reading, so apparently any TRMS meter may not be the solution. I do know the 80 series of Fluke works on TV scan derived AC voltages.

John