[R-390] BFO Drift?

[Jon Schlegel]

Tony mentioned Dallas' "filament regulator" idea.  If that's the 
Dallas article I think it is, I recall that he showed that the 3TF7 
ballast wasn't really that effective at stabilizing heater current 
and therefore oscillator stability.  Don't remember the details but I 
think he powered the PTO (and BFO?) filaments from a separate 
external source and then wiggled the AC line input to the 
receiver.  I recall that this showed that the freq stability of the 
receiver was considerably better than when the PTO - BFO - 3TF7 
string was allowed to wiggle along with the rest of the receiver.

Bottom Line - Most of the freq instability due to line voltage 
variation is due to the resultant current change in the PTO - BFO - 
3TF7 string.  I THINK this is what I remember of Dallas' findings.

Jon - WA3MVM



At 03:43 PM 12/11/2008 -0600, Barry wrote:
>>You can take a look at the heater/filament voltages to see if you 
>>can spot any variation over time.  Another thought along the same 
>>lines is to operate the BFO heater on a separate supply.  This is a 
>>complicated test though since I seem to recall that the BFO and PTO 
>>heaters are in series along with the 3TF7.  Try to isolate which 
>>oscillator is doing the dance before getting into his kind of thing.
>>
>>Regards,
>>Jon WA3MVM
>
>Wouldn't it take considerable fluctuation in the AC supply to cause 
>a significant shift in these oscillator frequencies?  I ran one from 
>the 12.6V supply from the power transformer and bypassed the 3TF7 
>altogether.  I never noticed any drift, etc.
>
>Barry - N4BUQ
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