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[wxsig] Tx problem fixed

William Beals will at beals5.com
Wed May 13 01:23:35 UTC 2009



The purpose of the capacitor is to provide DC isolation between the modem
and your radio.  Most handhelds use a DC offset to signal PTT, but even the
ones that don't mind an isolated output.  Where we have run into trouble
before is that if the radio input impedance is low enough, the cap ends up
being more of a filter than just a DC isolator.  The symptom is usually low
audio, not NO audio though.  Hence, not sure if it was a bad cap or just so
far off on radio impedance that no audio at all was able to pass.  In any
event, I would suggest a larger capacitor over no DC isolation at all.  The
0.1uf cap suggested by John would be fine.  Also, since you are solving it
for just your radio, what you can do is get an electrolytic cap.  You need
to measure the DC bias on both the T238 and your radio to determine the
correct polarity.  Since I was making a generic interface I needed to stick
with a non-polarized capacitor.  


-----Original Message-----
From: wxsig-bounces at tapr.org [mailto:wxsig-bounces at tapr.org] On Behalf Of
John Koster
Sent: Tuesday, May 12, 2009 4:05 PM
To: pedro.motilla at ono.com; TAPR Weather Station SIG Mailing List
Subject: Re: [wxsig] Tx problem fixed

Please refer to my message of 4-28 where I suggested changing it to     
0.1 mF.

On Tue, 12 May 2009, pedro.motilla at ono.com wrote:

> Hi, today I isolated the problem, it was the capacitor 111, I do not know
why, but it does not permit the signal to reach pin8 on P100.
> The procedure was to follow the signal with the scope. Finally it was
clear that this cap was causing the problem, so I jumped the cap, and,
voilá¡¡¡¡¡ the signal was on the air.
> Now I have one question, what is the function of this capacitor?, should I
change the value of the cap? or can I leave it without the cap?
> On my last comunication I made some mistakes with the measurements of the
scope. Today it has been my second time on using a scope, I am not still
used to work with the oscilloscope. The pin 7 on U100, has a sine wave
signal with 500 Hz, not 50 Khz, as I said. I used a loudspeaker from an old
toy. So, I could assure that from pin 7 were going out signal. Also on pins
1 and 2 voltage is not 30 mv and 20 mv, but 3 and 2 mv.
> So I put the signal on the air, but a little bit worried because I do not
know if the absence of the cap 111 could affect to the modem's performance
or the transceiver.
> Cable is good, the wiring works ok.
> I do sorry if I confused some of you with my measurements.
> Thanks to everyone, hope anybody can tell me something about the c111.
> Best Regards.
> EA5BFT, Pedrolo._______________________________________________wxsig
listwxsig at tapr.orghttps://www.tapr.org/cgi-bin/mailman/listinfo/wxsig

John, W9DDD

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