[R-390] 60 Hz Hum Output from R-390A IF Module

[Jon Schlegel]

Hi Roger,

I've verified all four mech filters when I removed them from the 
module to sweep their responses.  Resistance-to-case for all eight 
transducer coils was greater than any of my ohmmeters could 
measure.  Also as Tim had suggested in his off-list email, I checked 
the AGC line resistance to ground with the filters in-circuit and 
found just under 500k ohms which looks about right from how I was 
able to interpret from the schematic.

Regards,
Jon


At 06:38 PM 5/20/2009 -0400, Flowertime01 at wmconnect.com wrote:
>Tim,
>
>You wrote,
> >
> > Can you use an ohmmeter to check for leakage between
> > filter can and terminals? The usual symptom is a false actuation
> > in AGC action between different filters, but this could also
> > I suppose manifest itself in terms of hum (maybe AGC action
> > is hiding a source of hum?)
> >
> > Some of us with still working, but leaky filter cans, have isolated
> > the can from ground to help alleviate this. But this is at best a
> > workaround.
> > The real solution is to open the can and clean out the foampucky.
> > (I'm presuming that is the correct technical term for decayed foam!)
> > The really adept have done this as well as repairing damaged filter
> > coil winding, e.g. http://www.delphelectronics.co.uk/filter390a/
> >
> >
>
>---------------
>I think yo are on to something here. Some of the foampucky could be doing a
>short to chassis. Not a dead short but enough to act as an input source for
>some hum.
>
>I have on easy way to test for such a condition. Unsoldering the filter
>inputs is a long grind to trouble shoot the problem. We do expect 
>the input and
>output to be essentially open from the coils of the filter to ground. Once
>you lift the leads it is easy to get a meter reading. But to undress all
>those leads on a witch hunt is work.
>
>At some point Jon may think it would be worth the effort.
>
>Roger AI4NI
>
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