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Odd Receiver Noise




 
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Author Topic: Odd Receiver Noise  (Read 2542 times)
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Carl WA1KPD
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« 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?
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Carl

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
WA1GFZ
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« Reply #1 on: November 30, 2007, 01:07:22 PM »

sounds like a stage is oscillating
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KF1Z
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Are FETs supposed to glow like that?


« Reply #2 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.
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Carl WA1KPD
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« Reply #3 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....
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Carl

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
Pete, WA2CWA
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« Reply #4 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.
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
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« Reply #5 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
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Carl WA1KPD
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« Reply #6 on: January 21, 2008, 03:54:05 PM »

Found it.
Image from up in the 4800 area
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Carl

"Okay, gang are you ready to play radio? Are you ready to shuffle off the mortal coil of mediocrity? I am if you are." Shepherd
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