By G8MNY (Updated Nov 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
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³ \_/ ____ IC-735 ³
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Having just got going on this mode with an IBM Laptop & ICOM IC735. I was concerned about damaging either with a direct connection!

This applies to all modes like FT8 RTTY SSTV AMTOR etc, where a PC/RIG is connected to anything with medium to high DC current supply with a different earth or has RF currents!

So I have made an isolated barrier TO PROTECT THE LAPTOP's SOUND CARD FROM R.F.I. & getting ZAPPED & also the Rig from the Laptop's RF QRM. As well as doing that it also STOPS any HUM LOOPS & DAMAGING DC PATHS the can also burn out RIG's MIC/LS & the PC interfaces!

For this rig it gave a nice 1 plug solution...

LAPTOP ICOM IC735
PNP ______________ Pin 3
___ 1 OPTO 5 ³/e ³ PTT A
RTS 7 oÄÄ1KÄ)___)Ä2K2ÄÄÂÄÄÄÄ¿ ÚÄÄÄÄ´ _³_1N4001
RS232 9 pin ³ ³ 1N_³_ _³_ ³/ ³\ /_\ U
GND 5 oÄÄÄÄÄÙ ³ 4148/_\ \_/ð ´ ³ ³ Ground
ÀÄÄÄÄÄÄÁÄÂÄÄÙ ³\e______³___³_________ Pin 2 X
³ 2 3 ³ Screen
___ ³ ³ 1
LS Tip oÄ8RÄ)___)ÄÄÄÄÄÄÄÄ)ÄÄ¿ ÚÄÄÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Pin 4
Jack 3.5mm ³ ³ ³ )||( ³ Mod Input
Sleeve=ÄÄÄÙ ³ ³ )||( ³
³ ³ )||( ³ P
ÀÄÄÄÄÄÄÄÂÁÄÄÙ ÀÄÄÄÄ´
___ ³ ³ Rx AF L
LINE Tip oÄ1KÄ)___)ÄÄÄÄÄÄÄ)ÄÄÄ¿ ÚÄÄÄÄ)ÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄ Pin 5
Jack 3.5mm ³ ³ ³ )||( ³ ³ U
Sleeve=ÄÄÄÙ ³ ³ )||( ³ ³___ _ Other
³ ³ )||( ³ V ³_³ Rx Jack G
ÀÄÄÄÄÄÄÄÁÄÄÄÙ ÀÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÙ 3.5mm
small 1:1 signal
transformers

Of course for other Rigs with different pinouts etc. refer to handbooks.

TRANSFORMERS
They need to be low capacitance primary to secondary & could be from scrap modems/answering machines etc. about 1 cm cube in size. A 1:1 ratio is not that important as long as the Z presented does not attenuate the signal, & it has a flat enough responce at the Z used over the comms AF. If the transformer has tapings, these could be used to set the best signal level. On LS line drive, levels must be use not to saturate the transformer.

TX PTT
The RS232 port is initially protected by a series 1k in the plug to reduce the chance of zapping the laptop's RS232 port. (P.S. U will only do this once!)

The use of a PNP to follow the opto coupler increases the PTT current ability (so it could easily drive a 50mA rig PTT) as well as keeping down the RS232 load current. If you can't find a PNP then use a NPN like this..

___ 1 OPTO 5
RTS 7 oÄÄ1KÄ)___)Ä2K2ÄÄÂÄÄÄÄ¿ ÚÄÄÄÄÄÂÄÄÄÂÄ PTT
RS232 9 pin ³ ³ 1N_³_ _³_ ³/ ³ ³
GND 5 oÄÄÄÄÄÙ ³ 4148/_\ \_/ð ´ ³ ³ But the Opto & NPN
ÀÄÄÄÄÄÄÁÄÂÄÄÙ ³\e ³/ _³_ may be less RF immune
³ 2 3ÀÄÄÄ´ /_\ in this arrangement.
³\e ³ Gnd
ÀÄÄÄÁÄÄ

There are many types of opto isolators most should work OK once the pin outs are found.

SERIES Rs
A good idea but not essential is to copy the series R in the plug at the PC end if you can, as this does stop some RF & excessive LS currents if the leads get shorted etc.

MIC SOCKET
If you using a mic socket (rather than line) for the input either on the laptop or the rig, you will need an attenuator ideally in the mic plug so that RF+hum pickup are attenuated too.

High LS ÄÄÄÄ10KÄÂÄÄÄMic In a small 3.5mm plug
level 1K Plug components will need
Screen ÄÄÄÄÄÄÄÄÁÄÄÄGnd to be small.
Inside mic plug

BOXING UP
The whole lot is mounted on stripboard, & housed in a small plastic box making sure the 2 sets of leads are kept away from each other.

IN USE
With a local aerial, this simple isolation significantly reduces the QRM compared to a direct connection.

With a small nearby SW PLL SSB Rx with a small whip aerial it produces a good demonstration of the mode & waterfall display to analyse signals.

Steve, G1KQH @ GB7COV writes...
As well as using a similar circuit, I also use a Plug in USB Sound Card
which you can get NEW on ebay for a couple of quid each. Which again helps
to save the PCs internal guts from blowing!


See my Tech Bul on "Rig DC Power & RF Hazards".

Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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