I have owned Yaesu FT 897D rig that they seem to have to much audio hiss or more correct description : »Noise in the back ground audio«. This hiss-type noise is a little irritating if you use sensitive HI-FI headphones. After I have added recommended feedback circuit to power audio amplifier TDA 2003 ; see my Blog: ( FT 920, FT 897D and FT1000MP audio white noise reduction) I have found, that some hiss is still present. So I decided that the most effective way of addressing the rig noise was to modify the AF passband. You can simply get rid of it to roll off the highs with some paralleled caps added on the audio chain.(See picture 1)
Picture 1: FT 897D audio preamplifier Q1120
Picture 1: FT 897D audio preamplifier Q1120
Preparing the FT 897D for adding some elements.
- Carefully remove the top cover and gently remove the speaker cable.
- Locate the work area on PC board ( see picture 2)
Picture 2: Preamplifier Q1120 location
- Solder 390 pF SMD capacitor parallel to C 1456( 100pF)
- Solder 560pF SMD capacitor parallel to C 1609 (100pF)
- Connect the speaker back and close the top cover.
Before adding some elements simulations were done in EWB program which is a fast utility for these kind small changes on this circuit. I decided on a 2,3 kHz corner frequency target because I often do SSB. The best way is to parallel the needed value with the stock caps. In this way, no components need to be removed. The cap sizes are in the SMD 0603 category.
My observation after this mod is that the hiss level is far less with the cap changes. I also made some measurements (see picture 3)
Picture 3 : New 6dB audio spectrum taken at headphone AF Output - Mode FM
Warning !!! Changing any hardware inside FT 897D will void your warranty.
I am not responsible for any hardware failure. You made this mods on your own risk. ! This modification requires a high level of soldering skill, possibly beyond that normally possessed by the average HAM. Professional assistance is advised if you are not confident that you have this ability.
