Icom 775 Mods - Courtesy of SV3EAO - Fotis Visilias
(Enlargable Photos At Bottom Of Page)
First of all the mod takes place only on
the component's side of pcb.
The diode in the black image is 1N 4148 and
the parallel capacitor is a 10nf (use plastic or polyester).
The switch is on-off type. The relay is a
small type PCB 12V like Omron or anything with the same contacts.
This is useful if switching between 2.kHz
and 4.5Khz needed.
If you want to fix the BW at 4.5khz without
switching, you must connect the P4 with P3 with a piece of wire.
You must cut the PCB points like the photo,
but leave the P1,2,5,6, without connections.
This mod bypasses the Hi-cut and Low cut
of the DSP unit, no matter what crystal filters you use, because
the Icom's PSN modulation take place outside of them when the
radio is on TX.
On RX, the received signal passes through
the crystal filters, so the BW depends on them first.
If you are using the standard factory 2.8 kHz crystal filters,
there will be no difference on receive after the mod.
But on TX the difference is fantastic!!!
On the menu settings, the DSP and PSN must
be turned "ON".
The DSP unit is on the upper side of the
radio. Is a vertical PCB at right of the internal speaker.
If someone wants to experiment or try something
better, I have the radio's service manual and will entertain any
ideas with pleasure. I believe that the radio can TX out to 6 kHz
BW, but I haven't found free time from my QRL to check it.The overall
BW depends is dependent on the DSP unit only and the block diagrams
is- [AD converter, to limiter, to lowcut, to hicut, to line pre,
to DA converter.]
For further information or details:
73, Fotis Visilias - SV3EAO