Hi all, I would like to share my technical experience with my FT980. After many many years of no use I wished to make a deep check and precise alignment of the equipment. I found three blocking failures:
- fail contact of the coax connector J02 on RF UNIT (symptom: very poor rx sensitivity and no output pwr in tx)
- output PA UNIT coax shield disconnection (instability in output spectrum and output pwr, in particular on low bands)
- open capacitor C3067 on AF UNIT (tx spurs of 240 Hz and harmonics in SSB, AM, FM)
After these mayor fixing I investigated on the frontend HAM bandpass filters: after a little retuning of the inductances the resulting behaviour was not so good due to frequency regrowth for frequencies lower than the bandpass regions. Probably this is due to the aging of the electrolytic bypass capacitors used in the pin diode switching band circuit. I added two new tantalum smd 47uF 16V cap and a 330 nF smd ceramic cap in parallel to C212, C130, C125, C120, C114, C109, C104, C97, C91. The results were very good: the spectral regrowths disappeared as you can see from the attached pictures. Only four bands are present on the pictures, but all the bands gain benefits from the additional bypass caps. It is important to use very low ESR capacitors, those used in switching power supplies generally fit the scope. Best if you can trust on specific caps datasheets for the ESR value. No significant difference in the response were noted in the bandpass region nor in the higher band frequencies region after the use of additional bypass caps.?
The 10m bandpass filters is a bit worse in terms of insertion loss and form factor, I'm investigating on it because I plan to develop a multiband transverter that will use the FT980 10m band.
The responses were carried out with AGC off, gain RF max,? input rx -30 dBm, output at TP1 on RF UNIT with a 450 ohm resistor in series to the spectrum analyzer input (it results in an additional loss of 20 dB in measurement, but it will not load significantly the circuit).?
The same check was carried out for the GEN (wide) band pass filters, but in this case adding low ESR caps in parallel of the existing bypass caps didn't result in significant improvements, so I don't recommend it.
Next steps will be a new 11m band pass filter and the complete check and retuning. Also 12.5 KHz FM channel spacing and CTCSS encoder are in my plans.?
Thanks for hosting me, best regards
Enrico IK2XEI
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