|[AMRadio] Ghost from the 30's|
ka4inm at tampabay.rr.com
Thu Jun 2 14:57:59 EDT 2011
Rob Atkinson wrote:
> I have been working on an Admiral AA5 receiver built in 1951 or 52 and
> has the larger octal tubes. I have it working FB but it, and other
> AM bc band receivers at my QTH have a curious modulated hum problem
> below around 900 to 1000 kc. There is 120 cycle hum pickup that seems
> to mix with the local oscillator and received station into the 455 kc
> IF. I know it is not any one rx because I get it even with my newer
> battery powered GE Superadio. I am guessing there is a ground loop
> somewhere in the house but if anyone else has had this problem I'd
> like to know what the fix was as it might save me a lot of time by not
> pursing dead end leads.
Remember the second harmonic of 455 kc is 910 kc and incomplete
bypassing of the power buss in the receiver can result in enough
feedback to cause a squeal, but before you get to that point you have a
regenerative effect going on in the vicinity of 910 kc.
Connect a capacitor box across the B+ line between the IF amplifier/s
and the mixer (and RF amplifier if any) and slowly increase the capacity
to see if the noise is reduced or eliminated.
Look at the B+ line at the IF amplifier with a scope and measure the
Ron KA4INM - Every HAM should have at least one desk MIC,
it's the perfect place to hang the cans when
CW is "arm chair copy."
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