Linux-Hams archive - September 1998: soundmodem ptt circuit

soundmodem ptt circuit

Hamish Moffatt (piizmxg@mailit.tunk.net)
Sat, 5 Sep 1998 20:24:03 +1000

Having trouble with the soundmodem & ptt here.
I have a Yaesu handheld radio which uses combined input & ptt on the
microphone signal. The circuit is a combination of the one off Thomas's
web page and one for the radio. Here's what I have:

Audio from SB
-------------------------------------||-------+
|
---------/\/\/\-----+------ Microphone tip
RTS(4) from serial /
--/\/\/\-----------+----|
| \
-- \
Gnd(7) from serial /\ \
--------------------+----+--------------------------- Microphone sleave
|
Ground from SB |
-------------------------+

Excuse the terrible diagram. That's a diode, 1N4148, between RTS and GND.
There's a 10k resistor in the RTS line, and a 2.2k on the collector.
The cap is a 0.22u polyster. The transistor is a BC548.

I see -10.9V on RTS. I see -0.61V on the base of the 548,
but 0V on the collector (no audio connected) and hence 0V on the tip
of the plug.

Any idea what might be wrong?

thanks,

Hamish vk3tyd (licensed for about 1.5 days now so go easy :-)

-- 
Hamish Moffatt, StudIEAust              hjfqc.dtvee@eschein.com, vinsxrzn@net-sweeper.com
Student, computer science & computer systems engineering.    4th year, RMIT.
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