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Eddie,
I've got three words for you: printed, circuit, and board.
The layout of the traces was not conducive to eliminating
spurious RF, simple as that. Can a kludge-fix be made?
Probably. I'd have to see a unit that has the trouble
in order to make a proper diagnosis, but general experience
says, put a very small cap (about 10pf) across the input
stage grid. If that doesn't do it, then move the cap the
grid of the recovery/mixer stage. Place the cap as close
to the socket pins as possible, or even right on them,
if you can.
Why does this work (most of the time)? Because the tubes
are all called "radio receiving" tubes for a reason -
they can work at radio frequencies just as easily as audio.
A poorly laid out circuit, whether hand-wired or a printed
circuit board, can add a "stray" capacitance to the circuit.
Coupled with a resistance, or even a coiled up wire that
becomes an inductor, a 'tuner' of sorts is created, and
if it happens to hit on a radio station (or it's multiple
(harmonic)), then the tube will amplify the signal, and
the rest of the circuit is just like any other radio.
Adding a small cap at the tube socket should bypass RF to
ground, but should not affect the audio.
unlunf
--- In , John McCorvey <eddiekatcher@y...> wrote:
>
> Ah Ha! Maybe you know why the RI units are so prone
> to picking up RFI as in local FM radio stations.......
> I've had two that did it. intermittently but often
> enough to cause me to just go out and buy a real '65
>
> ed
>
