Today's Mystery Guest (RCA CTC-4)

[Phil Nelson]

I have begun to investigate the red-plating condition on the 6CB5 horizontal output tube. I pulled the HOT, VOT, and damper tubes in hopes that I can safely check voltages & waveforms. The yoke and convergence coils are connected.

The screen grid voltage (pin 1-8) on the HOT is MUCH too high, about double the expected voltage. The resistors between that point and the +350V supply (R140, R141) check out OK. (These are Sams numbers.) The cathode voltage (pin 3) is in the ballpark, measuring -25V where -23V is expected.

Moving back to the 6SN7 horizontal oscillator tube, its resistances look reasonable but its voltages are off. Instead of -85V at the grid (pin 4), I measure -1.5V. The expected waveform (W13) at point L is not correct. It is a muddy slope similar to W14 instead of the classic W13 waveform with the rounded shoulders and nice peaks that I saw the first time I powered up the TV:



The horizontal has always been rock-solid on this set, so it's disappointing to see it degrade. Subbing oscillator tubes had no effect on voltages or that waveform.

I hope I'm not barking up the wrong tree in checking these things. I hope even more that the flyback hasn't failed. Besides checking components around the oscillator, I'll see what I can learn by testing resistances on the flyback.

Phil Nelson