No Horiz Oscillator on Zenith Porthole- Solved!

[DavGoodlin]

Eric,
Seeing only positive supply voltages to these stages, I used to wonder how there could be a negative DC voltage on the grid G1 of the HOT or in your case, the oscillator. Normally when you measure an AC sinewave (linear with no DC offset component) with a DC meter, you will get zero because there is an equal amount of voltage above and below zero reference.

The negative voltage is the average DC voltage of the non-linear sawtooth. A majority of this voltage is below the zero crossing, a clue that the oscillator is running if you don't have an oscilloscope.

With all the parts we replace, its no wonder this happens.

Congratulations!

[old_coot88]

Originally Posted by DavGoodlin

Eric,
Seeing only positive supply voltages to these stages, I used to wonder how there could be a negative DC voltage on the grid G1 of the HOT or in your case, the oscillator. Normally when you measure an AC sinewave (linear with no DC offset component) with a DC meter, you will get zero because there is an equal amount of voltage above and below zero reference.

The negative voltage is the average DC voltage of the non-linear sawtooth. A majority of this voltage is below the zero crossing, a clue that the oscillator is running if you don't have an oscilloscope.

Not to be a fuddyduddy, but you're overlooking the cathode/G1 diode action which also generates negative grid voltage (as in grid leak detection).
A sine wave on the grid will generate it (provided the grid's at high impedance and the signal is capacity-coupled. A superhet local oscillator will make negative voltage on the grid.