Less White Noise from FT-897D
This rig is a great little radio. However, I disliked the thick omnipresent white noise coming out of it. What was especially unpleasant that the white noise spreaded far above 3 kHz. This high-pitched white noise was very disturbing because it doesn't mix with the band noise coming from i.f. stages, either for SSB or CW.
I checked the a.f. amplifier at the end of the signal path. There is IC TDA2003 in use. I checked the schematics of FT-897D and the recommended circuitry for TDA2003. I noticed that the optional feedback suppressing high-pitched tones was not used in FT-897D.
schematics of TDA2003 in the tcvr (black)
and the added feedback (red).
I made this feedback with combination of 0.75 microF (1.5 and 1.5 microF in series) and 68 Ohms. According to the formula mentioned at the recommended circuitry of TDA2003 this should cut tones higher than about 3 kHz. These additional parts can be directly soldered onto pins 2 and 4 of TDA2003. Therefore there is no SMD soldering and this modification can be easily reverted.
I don't dare say this modification makes FT-897D noiseless. However, reception is much more pleasant now, either on the headphones or the speaker.
of TDA2003, after the top cover removed:
microF + 1.5 microF + 68 Ohms in series soldered
onto pin 2 and 4 of TDA2003:
Petr OK1FIG, January 2008
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