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
The plan is simple:
- Replace thermistor TH2002 on the IF board with these three parts.
- The filter limits the broad band noise in the IF
- The 2000 Ohm resistor terminates the filter on the input side.
- The 1000 Ohm resistor connected to the top of the 1K IF gain pot, properly terminates the output of the ceramic filter.
- The IF gain pot will have to be tweaked to add some gain due to the loss of the filter.
The idea is to reduce the noise while still passing AM signals. A number of 6 kHz wide filters could be used. I used one from my spare parts bin.
Note, proceed at your own risk. There are static sensitive parts on the IF board. Too much heat from the soldering iron can damage nearby parts on the board. An improperly grounded soldering iron can damage sensitive parts. 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.
Here is the procedure:
- First get a stable signal source that will provide a signal somewhere in the HF bands. It needs to be rock steady in frequency and amplitude. I used an old tube type signal generator tuned to around 3.5 MHz. After a warmup of several hours, it was plenty stable.
- Disconnect the mic and the key or paddle. While the rig is in receive mode, feed the signal from your signal generator into the SO-239 or the Receive antenna input (the latter is safer). Set the signal level while in LSB mode to get a convenient reverence level, say S6, S7, or S9. After installing the filter, the IF gain will need to cranked up to exactly this initial level, overcoming the 6 dB or so of loss from the filter installation.
- Disconnect power, and all other connections to the FT-990. Take off the top cover of the radio. Store the screws and handle in a plastic bag for safe keeping.
- Remove the screws from the IF board (second plug-in board from the left. Unplug the coax cable going to this board. Remember to replace this cable before reinstalling the board.
- Remove thermistor TH2022 that connects the Q2032 IF amp stage to the Q2033 stage. Clean out the holes in the board with solder wick.
- Solder one end of the 2000 Ohm resistor to the the T2022 metal case toward the top side of the board. This provides a ground point for the resistor. The other end of the resistor goes in the leftmost hole left by removal of the thermistor. This puts a 2000 Ohm load across the secondary of T2022. Leave some lead length so you can solder another wire to each end of this resistor.
- Place the filter on the board dead bug style with the leads sticking up. Using a small piece of wire, such as a clipped lead from a resistor, solder the In lead of the ceramic filter to the side of the 2000 resistor that goes into the leftmost thermistor hole. The Ground side of the filter is soldered to the other end of the resistor, that is grounded to the metal case of T2022.
- Solder the 1000 Ohm resistor to the Out lead of the ceramic filter. Solder the other end of the 1000 Ohm resistor into the right most thermistor hole. This puts this resistor in series with the 1K IF gain pot, providing a 2000 Ohm termination for the ceramic filter.
- Check your wiring, plug the small coax cable back into the IF board, seat the IF board in the rig and screw it down.
- Reconnect the power source cable to the back of the radio and connect the input signal source as before.
- Using an insulated tuning wand, adjust the IF gain pot so the S meter reads exactly the same level that it did before the filter was installed. The IF gain pot can be reached with the IF board installed in the radio.
- 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 dramatically better, approximately equivalent to the audio hiss from my Kenwood TS-850S set at the same volume level.
For a visual perspective of what is happening with these changes and the effect of the SCAF filter, try 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. Use PowerSDR v2.3.5, and put it in the SoftRock 40 mode.
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