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Title: Odd Receiver Noise Post by: Carl WA1KPD on November 30, 2007, 01:04:16 PM I have been working on a tube ham band only receiver that covers 160 75 and 40 in 3 different bands.
I have one odd problem that has me stumped. On just 75 meters there is a low level background noise that sounds like a short "swish". Maybe once per second. Almost like someone is rapidly and repeatedly tuning by with a VFO. Some observations: It is only on 75 & 80 It is weak I hear it with and without antenna connected I do not hear it in any other receiver, A strong signal will override it The BFO does modulate it. On 75 it is not frequency sensitive that is it does not change as I tune. IF peaking did not seem to affect it. The design is pretty simple, a step or two up from an AA5 using 455KHZ IF Any ideas where it might be coming from? Title: Re: Odd Receiver Noise Post by: WA1GFZ on November 30, 2007, 01:07:22 PM sounds like a stage is oscillating
Title: Re: Odd Receiver Noise Post by: KF1Z on November 30, 2007, 01:12:41 PM Bad capacitor in the audio stage, or maybe IF amp?
Or a toasted resistor? Seems like a "swish" would be caused by a cap , charging and discharging. Title: Re: Odd Receiver Noise Post by: Carl WA1KPD on November 30, 2007, 02:15:38 PM Bad capacitor in the audio stage, or maybe IF amp? Or a toasted resistor? Seems like a "swish" would be caused by a cap , charging and discharging. Thanks All caps have been replaced, all resistors are in spec. I'm wondering if I put a wrong cap value in place somewhere.... Title: Re: Odd Receiver Noise Post by: Pete, WA2CWA on November 30, 2007, 02:37:05 PM I would check each stage with a scope and isolate it that way. As Frank said, just sounds like the oscillator/mixer stage oscillation since it only happens on one band. It could even be front end oscillation. You could also try (carefully) a metal tipped probe held against each component in the area to see if you can change the oscillation. Could give you a clue to the problem area.
Title: Re: Odd Receiver Noise Post by: WZ1M on November 30, 2007, 04:38:49 PM Any test equipment turned on when this is happening? I scratched my head for hours and then noticed that the grid dip meter was on and laying about a foot from the receiver.
Regards, Gary Title: Re: Odd Receiver Noise Post by: Carl WA1KPD on January 21, 2008, 03:54:05 PM Found it. AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
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