TopBand: Noise from 160m preamp QRMS 220 mHz

[AA1K Jon Zaimes]

I have used for years a homebrew 160m preamp that uses a pair of MPF102
FETs. The popular circuit has been published many times over the years.

A few days ago I started experiencing "white noise" that was a steady S7 on
the 220 mhz rig used on my packetcluster node backbone frequency, covering
more than 2 mHz of the band. So I went mobile with a 220 mHz rig yesterday
to hopefully track down the source. Found it dropped off in all directions
as I got away from the house. Put a battery on the mobile rig and walked
around the house; noise strongest in the shack! Started disconnecting power
to equipment one at a time. When I pulled the 12v power cord going to the
160m preamp, the noise disappeared; re-connected and it came back.

The FETs had blown some months ago and I hadn't bothered to replace them yet
since I had other preamps available. I just left the preamp in the "out"
position, but the power was still connected. Somewhere along the line the
FETs turned into oscillators. I found adjusting the gain control on the
preamp also made the noise go away. Power is now disconnected!

73/Jon AA1K 



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