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Old 05-27-2017, 07:57 PM
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Notimetolooz Notimetolooz is offline
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Join Date: Mar 2014
Location: Dallas, TX
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Quote:
Originally Posted by jr_tech View Post
Mine does not produce a good sine wave even at 455 kHz, and modulation is quite low, but it seems to be an adequate signal source for simple AM radio alignment. Is this typical, or does mine need some attention?
The frequency counter in my 'scope seems to measure it ok, but I suspect that some would have a problem with that waveform.

jr
If it works OK for you then you could just leave it.
That IS a pretty bad waveform. It could be that the DC bias on a section of the 12AU7 is off somehow.
One thing I noticed from the schematic is that the output level control pot wiper doesn't have a DC blocking capacitor. That means that the changing that pot changes the DC load on the output section of the 12AU7. I added a 0.05 uF there. My SG-8 wasn't mint, the ON-Off switch was replaced with a toggle switch, so I wasn't concerned about keeping it stock. I did quite a bit of experimenting on it and modified the circuit. I got it to work better but it still is less than ideal. I just use it for AM BC adjustment, 540-1600KHz. I made a separate solid state 455KHz source with modulation.
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