Alternative Way of Adjusting G2s (25EC58)

[Electronic M]

Originally Posted by TinCanAlley

I know it doesn't take much advancement of the G2s to bring up color in a B&W image, so I'm assuming the CRT is working well. Why that same advancement doesn't produce a line is beyond me.

A tiny tweak of G2 should adjust relative color level a bit on normal viewing. It's absolutely possible to still have that effect on a normal picture but no setup line...that happens when the gun bias voltages (cathode, G1,G2) are wrong or when the CRT is weak. If the cathode to grid voltage difference has the grid too negative or G2 isn't able to reach the B+ boost voltage feeding the one end, or the HV is below rated level that all could cause it.

I'm not sure how the schematic snippits you show relate to the service switch. You show the brightness limiter hooked to the Red video transistor but it probably interacts with all 3 color video transistors through paths that can't be fully seen in the snippits.

If you have a DMM and a CRT tester that's known working check the CRT and check the voltages feeding it. Maybe write them down and report back on the voltages VS schematic.

I used to have a Zenith 4-tube hybrid (basically a CCII flat chassis chassis but with tube horizontal sweep) with a bad CRT previous owner got it used in the mid 70s and it was his main daily watcher TV until 4 years ago. Two of the guns couldn't produce a line and I think I just turned off color and did what I could with color at minimum and a full screen grey scale pattern. It looked ok as long as adjusted several minutes after warm up after gun emission drift stopped.

The service switch isn't a rare part and I have seen dead ones completely kill raster in both positions...I just replaced the switch to cure the problem.