On the bench: Zenith color roundie 25MC30

[Kamakiri]

Originally Posted by Electronic M

It is possible the seleniums on the convergence board are bad usually after caps are changed and resistors checked those are next on my list of suspects.

You want to replace them with shottkey diodes. Silicon diodes like the 1n4007 likely will not switch fast enough at horizontal frequency.

EDIT: have you checked the color circuit tubes? On my zenith roundy the color did not work because the tubes were simply dead out of cathode emission.

Tubes are good, and there are no seleniums on the convergence board. What's next I can check?

[Electronic M]

Originally Posted by Kamakiri

Tubes are good, and there are no seleniums on the convergence board. What's next I can check?

Does that mean you or someone else recently replaced the convergence diodes? The convergence diodes in all TVs into the 70's were selenium types, though after a while they packaged them to look nothing like the seleniums found in 40's and 50's power supplies that we are used to seeing. The newer packaging did not make them more reliable, and I have had sets as new as 1972 develop convergence problems from diode failure.

As for the color check to see it the 3.58Mhz reference oscillator is running. If so try adjusting the color killer and fine tuning* to see if you can get some color. Also many Zenith roundys had a switch on the color level control to turn off the color if that switch is bad it would explain no color reaching the screen. If all that is good and still no color then grab a scope and start following the color signal from the place the color splits off of the video to the CRT.
*I usually tune the same program on another set and tweak it so that saturated colors show a slight checker board on the monochrome limited set.

When one runs out of possible low hanging fruit trouble shooting, the next logical thing to do is usually signal tracing.