Just for fun I built the Rf generator in the Feb. 2001 edition of the OTB. I can get it to make a RF signal, but the AF oscillator used to modulate the RF signal won't work. I have tried everything I can think of but it still won't work. Does anyone have any ideas?

Hi Chris

Do you have a copy of the schematic or is it on the internet?

AF should be an amplifer to modulate the RF oscillator. If you are using a tube grid #1 may be used for oscllator and grid #3 audio input. You will need several volts of audio to modulate this type of circuit.

Norm

:Just for fun I built the Rf generator in the Feb. 2001 edition of the OTB. I can get it to make a RF signal, but the AF oscillator used to modulate the RF signal won't work. I have tried everything I can think of but it still won't work. Does anyone have any ideas?

I have posted a schematic at http://home.rica.net/cfprops/mvc-879f.jpg It uses a 12AU7. One half of the tube is the RF oscillator and the other half is the AF oscillator. Would there be any problems with the AF half oscillating in this circuit? I can't get the AF modulation side to oscillate.

:Hi Chris
:
: Do you have a copy of the schematic or is it on the internet?
:
: AF should be an amplifer to modulate the RF oscillator. If you are using a tube grid #1 may be used for oscllator and grid #3 audio input. You will need several volts of audio to modulate this type of circuit.
:
:Norm
:
::Just for fun I built the Rf generator in the Feb. 2001 edition of the OTB. I can get it to make a RF signal, but the AF oscillator used to modulate the RF signal won't work. I have tried everything I can think of but it still won't work. Does anyone have any ideas?

Hi Chris

A couple things to try. Reverse one set of wires on the audio transformer. It takes positive feedback to cause oscillation. If you don't have that it won't operate.

Don't know why they used a 1 meg resistor to couple to the transformer. Try replacing this resistor with a .01-.1 mfd cap. A .1 mfd cap across the audio transformer will work but is large in value. Try a smaller cap if tone is too low.

If the cathode resistor of the audio oscillator was bypassed with a cap output would be greater. Strange way they take a signal off this unit.

Norm

:I have posted a schematic at http://home.rica.net/cfprops/mvc-879f.jpg It uses a 12AU7. One half of the tube is the RF oscillator and the other half is the AF oscillator. Would there be any problems with the AF half oscillating in this circuit? I can't get the AF modulation side to oscillate.
:
::Hi Chris
::
:: Do you have a copy of the schematic or is it on the internet?
::
:: AF should be an amplifer to modulate the RF oscillator. If you are using a tube grid #1 may be used for oscllator and grid #3 audio input. You will need several volts of audio to modulate this type of circuit.
::
::Norm
::
:::Just for fun I built the Rf generator in the Feb. 2001 edition of the OTB. I can get it to make a RF signal, but the AF oscillator used to modulate the RF signal won't work. I have tried everything I can think of but it still won't work. Does anyone have any ideas?

I reversed one side of the transformer before I tried anything else. Now I get a nice tone. I have posted a picture of it here http://home.rica.net/cfprops/MVC-881F.JPG The controls from left to right are modulation on/off, frequency, a switch to switch between 455kHz and variable frequency, output level, and on/off. The two jacks at the bottom are RF output and ground. Thanks for your help.

:Hi Chris
:
: A couple things to try. Reverse one set of wires on the audio transformer. It takes positive feedback to cause oscillation. If you don't have that it won't operate.
:
: Don't know why they used a 1 meg resistor to couple to the transformer. Try replacing this resistor with a .01-.1 mfd cap. A .1 mfd cap across the audio transformer will work but is large in value. Try a smaller cap if tone is too low.
:
: If the cathode resistor of the audio oscillator was bypassed with a cap output would be greater. Strange way they take a signal off this unit.
:
:Norm
:
::I have posted a schematic at http://home.rica.net/cfprops/mvc-879f.jpg It uses a 12AU7. One half of the tube is the RF oscillator and the other half is the AF oscillator. Would there be any problems with the AF half oscillating in this circuit? I can't get the AF modulation side to oscillate.
::
:::Hi Chris
:::
::: Do you have a copy of the schematic or is it on the internet?
:::
::: AF should be an amplifer to modulate the RF oscillator. If you are using a tube grid #1 may be used for oscllator and grid #3 audio input. You will need several volts of audio to modulate this type of circuit.
:::
:::Norm
:::
::::Just for fun I built the Rf generator in the Feb. 2001 edition of the OTB. I can get it to make a RF signal, but the AF oscillator used to modulate the RF signal won't work. I have tried everything I can think of but it still won't work. Does anyone have any ideas?