Looks to me like interference at the color burst detector. The absence of
a valid signal from this circuit is supposed to turn off color and although
no stations now broadcast in black and white that function is, like the
human appendix, still there. It would naturally be sensitive to signals
in the 80 meter band. And if the phase detector is implemented digitally,
it would also be susceptible to signals at twice the color burst frequency.
If external video signals don't have problems with interference, this
pretty much nails it down. Somehow, the HF signal is getting into the IF.
This is unlikely to be via the normal mode antenna signal, It is more
likely to be a common-mode path. Common mode may be broken at the antenna,
by using back to back 300 ohm baluns and a balanced 300 ohm HPF between
them, It may also be broken at the AC power, with a sufficiently inductive
common-mode filter, or by resonant common mode filtering in the bands of
interest.
It MAY be due to direct pickup. Direct pickup on the PC board might be
treated by shielding what is receiving the signal. Internal wiring may be
the antenna for this, and RF chokes on audio, control and power wires
should help with THAT.
Cortland
> [Original Message]
> From: N6KJ <kelly@thejohnsons.ws>
>
> Someone suggested a few days ago that I try displaying one of the tv's
> menues instead of an input signal and see if the interference is there.
> Well, it isn't. I see no interference when the menu is on the display.
> Of course, I don't know what this tells me. I'm sure this rules
*something*
> out, but I'm not sure at what point in the RF/IF/etc chain the menu gets
> injected.
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