> Date: Sat, 5 Sep 1998 20:24:03 +1000
> From: Hamish Moffatt <znlgap.pcznnwjajy@ktuo.cz>
> To: nqhpgbcg.ehgsnm@kerailya.tunkki.fi
> Subject: soundmodem ptt circuit
>
> 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,
Congratulations on the new licence.
Your circuit:
I presume that the 1N4148 is across the base - emitter junction of the
BC 548, i.e between the 10k (from RTS) and ground as per the circuit.
Presuming that the BC 548 is NPN (I think it is) I would suggest the
following in order of possibility:
1 There is no +ve supply on the tip of the plug (check for volts at the
junction of the 0.22 uF and 2k2) to ground. This could be caused by a
short in the plug/wiring or in the handheld.
2 If there is a +ve supply (0.22/2k2) then it is likely that the BC548
is collector-emmitter short. This may be the device or again the
wiring.
The -0.6 V at the base of the BC 548 indicates that the diode is clamping
OK. What happens when RTS is high? Does the base voltage rise much above
+0.6 V ( > +1 V or so). If so the BC 548 is probably at fault.
Geoff
--
Geoff Blake terhi.victor@logonet.com linux 2.0.31
Chelmsford ktcq.nzoxleppu@kerailya.tunkki.fi i586
Intel create faster processors - Microsoft create slower processes