Finally, I found the problem: it was a capacitor in the neutralization
network (between cold side of driver coils and ground) which was 44pF
instead of 250pF.
It is not indicated in FL-200B schematic diagram, but it is indicated in
the schematic diagram of the FL-400DX transmitter, which is almost
identical.
The schematic diagram of that part is here:
https://drive.google.com/open?id=1uAWvr5I0DIXvSy6ZB5lsyjtszEvcSHeo
The capacitor is C78 and it is used on 20 and 40m
On 80m it is C76 and the value is 5000pF, which I measured and it is
correct.
I don't explain why I had problems on 80m, maybe the problem was the
plate variable capacitor, because I heard some arcings while tuning the
plate on this band.
After I tried to clean the capacitor moving contact, the 80m stopped the
arcings (at least with 60W) and now the TX tunes OK on all bands.
Normally the capacitor in the neutralization network is only one, I
don't understand why the used different capacitors values for 10, 15,
20+40 and 80m...
Thanks to everybody for the halp and suggestions
73 de Guido, ik2bcp / k2bcp
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