[R-390] 16 KHz filter oscillates

[2002tii]

Another possibility worth considering:  a bad ground somewhere.

Don


> >I have a R390/A on the bench. As I peak up the 16 KHz mechanical filter it
> >goes into oscillation. I get a nice audio tone out of the headphones. I can
> >change that tone with adjustment and drive it beyond the audio range. The
> >oscillation continues as it can be metered on the diode load. I can peak the
> >level with the trimmer caps. I can back the trimmers back off the 
> oscillation
> >point and get a signal through the filter at almost the same level as the
> >other filters. I do not know if the filter will ring at these trimmer
> >settings with real world signals.
>
>I suspect inadequate bypassing somewhere in the IF chain.  I'd start
>by tacking in .01 disk ceramics at the 455 kHz IF B+ feed points,
>starting with V501 and working forward.  I suppose it could be a
>problem with the filter itself, but I'd be surprised.  Do you have
>the facilities to sweep it?  You could substitute in a known good
>one, but if that cured the problem you'd also have to put the
>questionable filter into a known good IF strip to see if it oscillates there.
>
>Other possibilities that come to mind are weak AGC or IF gain set way
>too high or, as you suggest, an unwanted radiative coupling.
>
>Good luck!
>
>Don



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