I was shopping for some reasonably priced high frequency RF transistors and found a link to some TV tuners. The oscillator in the UHF section was interesting.
TR6 is a BF970 high frequency, high gain, low noise transistor. It is rated 1 GHz so it should serve us well in a .3 - 30 MHz set. Another search lead here.
Once again the transistor is a BF970 which is being used for a FM transmitter. He says a range of 200 meters with a 12 volt supply. This circuit would be perfect for a multi band set because the coils is not tapped and feed back is through the capacitor. Time for a simulation.
That little circuit is singing at 8 volt peak to peak. I don't have the model for the BF970 but I tried three transistors and they all worked. This was the strongest. So time for a build.
This is my scope trace for the circuit built with a Germanium transistor I had on hand. Just under 4 volts peak to peak and 489 KHz. I had three coils I tried which oscillated at differ frequencies.
I replaced the Germanium transistor with a Silicon transistor and used a smaller coil. I think the bias could be adjusted to improve it. My proof of concept worked with three coils and a Germanium and Silicon transistor so I think it is a keeper.
The good news on the transistor is you don't have to take your TV apart to get one. You can find them on E-Bay at a reasonable price.
$2.72 for 20 here:
BF970 on E-Bay
They will combine orders into one shipment so browse the store before hitting the buy button.
Post a Comment