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 Post subject: Re: Which oscillator should I use?
PostPosted: Apr Tue 07, 2020 4:26 am 
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Location: SoCal, 91387
(fifties ducks osc coil lobbed in his direction)

Check the voltage on the control grid of the 12SA7, I think it's pin 4, to determine if it's in the negative range. If so, don't hesitate to turn the osc core all the way, down and up. Reset to about the center if nothing.

If still nothing you can try reversing the connections to pins 1 and 3 of the osc coil, and then check again for negative voltage, and adjusting the core.

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 Post subject: Re: Which oscillator should I use?
PostPosted: Apr Tue 07, 2020 5:40 am 
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Joined: Aug Mon 08, 2011 4:19 am
Posts: 468
Location: Indianapolis, Indiana, USA
Thanks for info fifties,

I have turned the coil from all the in (half way inside), to even with bottom of coil- still no osc. I will check voltage on the control grid of the 12SA7. if nothing fixed after that, I will try reversing pins 1 and 3 on the osc coil.

I did check wiring again also. My gut feeling is that the mica cap in the osc circuit is bad. I may put my cap checker on that.

Joe


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 Post subject: Re: Which oscillator should I use?
PostPosted: Apr Tue 07, 2020 6:13 am 
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Location: Indianapolis, Indiana, USA
Here is link to tube data sheet for 12sa7:
https://frank.pocnet.net/sheets/127/1/12SA7.pdf
I removed the 12sa7 tube (metal tube).
I squeezed lightly the pins socket connectors on the 12sa7 and reseated it.

Then I took vdc on these pins:
pin 4 82.0 vdc should be 70vdc
pin 5 0 vdc
pin 6 11.3vdc

Joe


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 Post subject: Re: Which oscillator should I use?
PostPosted: Apr Tue 07, 2020 7:04 am 
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Joined: Aug Mon 08, 2011 4:19 am
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Yeeeaaaa! It is oscillating! I swapped pins 1 and 3. 12sa7 pin5 -12.6vdc. Now for alignment.

thanks for help fifties

Joe


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 Post subject: Re: Which oscillator should I use?
PostPosted: Apr Tue 07, 2020 7:31 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 34880
Location: SoCal, 91387
Congrats! Always look for a decent size negative voltage on the control grid as a sign that the osc is functioning. Osc coils need their winding going in a specific direction as well.

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