Yaesu FT-990 Manual

Audio hiss reduction

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Yaesu FT-990 Receiver Audio Hiss Reduction

I purchased a Yaesu FT-990 several weeks ago, primarily for the fine ergonomic design. The rig

worked well, but then I discovered the infamous receiver audio hiss discussed in various forums.

Turning on the SCAF filter changed the sound of the audio hiss, but the hiss was still there.

The tail-end filtering approach used in W7ZOI's receiver "An Integrated Contest-Grade CW

Station" in "Solid State Design For The Radio Amateur" specifically dealt with a noisy receiver

IF. I have tried this approach in several receivers. It did in fact reduce broadband noise.

This implementation described here for the FT-990 is not quite tail-end filtering, rather the filter

is inserted just before Q2033, the last stage of the 455 kHz IF amplifier chain on the IF board.

Only three parts are needed:

A 6 kHz wide ceramic filter with a 2 kHz input and output impedance (Murata

•

CFUM455H or similar)

2000 Ohm ¼ Watt resistor

•

1000 Ohm ¼ Watt resistor

•

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Page 1 Yaesu FT-990 Receiver Audio Hiss Reduction I purchased a Yaesu FT-990 several weeks ago, primarily for the fine ergonomic design. The rig worked well, but then I discovered the infamous receiver audio hiss discussed in various forums. Turning on the SCAF filter changed the sound of the audio hiss, but the hiss was still there.
Page 2 A well lit work location and magnifying glass will be helpful. Do a web search to find a copy of the FT-990 Technical Supplement manual that shows the board layout and schematics for the rig.
Page 3 Now remove the signal source, connect an antenna and listen for signals, while adjusting • the AF gain of the FT-990 to a comfortable listening level. Remove the antenna connection and listen to the audio hiss level. I found it to be •...
Page 4 Spectrogram or PowerSDR. Use the audio out from the Phones connection on the FT- 990 front panel to the PC sound card to view the audio signal from the FT-990. Hint: one of the best kept secrets is that PowerSDR from the Flex website can be used as a no cost outboard audio filter, including band width selection, an auto notch filter, fabulous tracking notch filters, 10 band audio equalizer, and noise reduction.