[R-390] 16 KHz filter oscillates

[2002tii]

Roger wrote:

>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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