AF Filter for FT-897D
The modification with the optional feedback on the TDA2003 helped to supress the white noise but I still was not quite happy with the result. I looked for even a better solution. I revealed that not only the high-pitched white noise needed to suppress, but also the low-pitched hum. Such a hum is clearly audible when you rotate the AF-gain knob and listen on the good quality headphones (such as Yaesu YH-77STA), for example.
I have made a small AF filter for this purpose. It consists of HPF suppressing tones lower than about 300Hz and LPF suppressing tones higher than about 2900Hz. The filter schematics looks like this:
|C1, C2: 6n8
C3: 10uF (tantalum)
C4: 11n5 (10n+1n5)
C5: 5n9 (4n7+1n2)
C6: 2u2 (ceramic)
The filter had to fit into the space about 11x31mm. Therefore SMD was the only option. The PCB of the filter looks like this:
|SMD was the
only option to fit the filter into the space about
There is a hole of the diameter 2.5mm for the screw in the middle of the PCB. The screw brings in also GND.
There are three outlets at the bottom. One goes to the pin #5 of TDA2003 and brings in the supply voltage for the filter. The other two go to the pin #1 that was cut in the middle.
The filter is mounted above the a.f. final stage, the integrated circuit TDA2003:
|There is an
8mm high metal spacer between the filter PCB and TDA2003
integrated circuit. It makes space enough for the parts
soldered beween pins #2 and #4 (see the other modification) and also for the gray coaxial cable.
I had to find a suitable replacement for the original screw holding TDA2003. The new screw must have the same diameter, the same rise, but it must be about 10mm longer.
Outlet #1 of TDA2003 was cut in the middle to get tapped into the signal path without soldering on the main 897's PCB.
The supply voltage 13.8V is taken from the pin #5 of TDA2003.
The previous modification was kept because TDA2003 produces some little noise itself.
Petr OK1FIG, April 2008
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