Vincent wrote:
Hi all,
I've seached all the web but didn't found this information; long ago
this receiver was produced with full coverage 150 - 30000 kHz and
reduced versions like in Germany, whith coverage liìmited to 26100 kHz
(it didn't had the external antenna plug, too.

Does anyone here knows how to fix it?

Here's the info that I have (credit must go to a gentleman called
Stephan for these instructions).

To remove the 26.1 MHz limit, apparently all you need to do is
move R292 (22k 1/10W) located at one corner of IC202 (the
µP7508G on the LCD PCB) to the R259 position right next to it.
At this opportunity I found that all in all there are three
pins total on IC202 that are pulled to ground on the RF-B65DA:

* pin 52, marked "(P 72 )SW/2G", via R259 (pulling to VDD via
R292 results in 26.1 MHz limit)
* pin 2, marked "FM/J(P73)", via R258; presumably switches to
76-90 MHz FM (Japanese band) when pulled to VDD (vacant
position exists).
* pin 51, marked "(P 71) A I R", via R260 (vacant position
exists); that one's a mystery, AFAIK the RF-B65 never featured
air band (108...136 MHz) reception. Maybe shortwave reception
can be restricted to 3850 kHz upwards here, as found with
(Sony) models for Italy.

Something which might be problematic is that the DA uses a
different antenna coil than the D, so sensitivity may not be
perfect above 26 MHz. Also different IF transformers are used
and the values of a few other parts are changed (I'd attribute
most of these to normal improvements, but you never know).
Something really odd: The D model for Germany used a different
second IF of 459 instead of 450 kHz.

If you plan on doing a modification, I'd really recommend
getting the service manual. (For the RF-B65DA in this case,
which includes the RF-B65D manual plus changes from D to DA.)
Disassembly does not seem entirely trivial.