1.Adding an extra RX-only
port in the Icom IC706.
The receive sensitivity of the IC706 at 50MHz is not
exceptional good.
Using the IC706 with an external PA you might need a
low-noise pre-amp.
But this is not so easy as it looks at first hand because
without extra protection relays.
You can (will
sure) destroy it by the tx power of the IC706.
A solution is having a receive–only port like most
bigger tranceivers have.
This gives the possibility of using an extra outside preamp (and / or
bandpass filter) placed direct behind the first antenna relay preferable close
to the antenna.
The output of this preamp is connected to IC706 <60MHz RX input not through the
IC706 t/r switch but
over the “backdoor” behind this internal relay.
The normal RX route over the internal T/R relay is
still there unconnected but the
external preamp
output is overruling the little noise from this open
piece of line.
Adding this RX
port is an easy and clean modification.
There is allready a spare opening in the back panel
where an extra BNC connector fits using
some extra rings because the hole is to big for the BNC (type single hole)
Then this BNC must be connected
to J1 (rx) point At the filter unit through a short piece of thin 50ohm coax.
The filterunit is situated top-backside. After
removing 5 screws the board can be moved up at the backside just enough to
mount the BNC underneath it.
It is not needed to remove it completely
that would be a problem because of all connections to it.
2. The HM103 microphone.
At the modification
site mods.dk some things are said about the modulation. I did follow up the
recommendations but it did not realy improved the modulation index.
This is a picture of the HM103. One
would expect that the microphone element is the big metal thing you see.
But this is a passive metal plate
with a rubber gasket at the speak site. The actual electret microphone element
is very small and placed at the edge of this
“voice reflector” underneath the printed circuit.
There is a small air channel to it
that’s where all your voice has to go 
through! Removing the gasket did not improve the situation in my case.
Without this gasket the modulated air
is no longer forced to lead through this small channel this must be resulting
in a lower output level.
Someone told me: this HM103 is no
good, use a better mike!
But it is hard to believe that a
microphone these days can be so bad that it is not good enough to produce
300Hz-3kHz audio ??
Possible its audio responce is
optimised for Japanese voice?
I tried a cheap PC microphone and
indeed it gave a better modulation! So the bad modulation of my IC706 was at
least partly caused by the HM103. I
decided to change the element.
Therefore remove the small PCB un-lead the two element wires.
Replace the element with a “better” one
(diameter. 9-10mm).
Electret type microphones needs a
supply voltage to work. The red wire is the plus pole. The polarity must be
correct.
The HM103 schematic diagram shows a 3 wire element but this is actual a 2 wire type.
The element signal and supply use a
common wire. Like with the PC microphones
audio cards supply 5V at the microphone input for electret of course with internal current limit
resistor.
To
increase the modulation index i did change the gain of
the microphone amplifier IC22 located at the reverse side of the
mainboard.
This mod is at modification site mods.dk
R234
27k->120k C210 + extra 220pF
This is a very
difficult operation the SMD parts are realy tiny! 1/20 inch but this is
the best modification you can do.
