Dumont RA113 "Burlingame" restoration

[HighHopes]

I never heard of a cathode-to-grid diode action. If there is a diode action, then wouldn't the results be the video (plus some weird sound on the FM audio). Were would the IF frequency come from if the RF is detected?

Is the sweep input so strong that the tube is driven into Class C operation?

Where can I read about this circuit?

-Steve

[old_coot88]

Originally Posted by HighHopes

I never heard of a cathode-to-grid diode action.

Google 'grid leak detection'. It's the classical AM detection method used in the earliest tube radios. It utilizes the natural diode action that occurs between a tube's filament (cathode) and grid ('anode') to rectify the amplitude-modulated carrier.

In the case under discussion here, the sweep generator's carrier, rectified and displayed on the scope, denotes the bandpass of the TV tuner.

If there is a diode action, then wouldn't the results be the video (plus some weird sound on the FM audio).

The two fixed-frequency markers or 'blips' on the trace are the sound and video carriers.
And yes, the TV would probably display some pretty weird stuff just from the genny's 60-hz sweep rate (though i never tried looking at it during an alignment).

Where would the IF frequency come from if the RF is detected?

The IF is not derived from detected RF.
It is the 'beat' or difference frequency between the local oscillator (LO) and the
incoming carrier.
Or in the case here, there are two IF strips. The first one is tuned to the difference frequency between the LO and the video carrier.
The second strip is tuned to the difference frequency between the LO and the sound carrier.
The video and sound carriers are 4.5 mhz apart.
Hence the two IF strips are tuned 4.5 mhz apart.

This system was superceded by the intercarrier method, in which the sound carrier is included in the video IF, and picked off right after the video detector at 4.5 mhz, amplified and detected. This simplified things and greatly reduced parts count

[HighHopes]

Hi 88,

I think I am getting this. With a capacitor in the grid circuit, the charge is held to a level depending upon the amplitude of the ac signal, charging only on the positive swings. So you get the envelop / detected waveform on the grid.

AND / BUT, the mixed radio frequencies still appear on the plate.

Horray! I got it.

Thanks for your patience.

Steve

[old_coot88]

BOOYAH !!